Interrupt progress

Had another poke at interrupts late last night. And finally got a simple interrupt handler working. As usual a couple of simple mistakes initially thwarted my efforts.

1. To start with I was trying to use the FRAMEDONE interrupt from the display controller. But it seems I needed to use VSYNC for HDMI out, and the EVSYNC_ODD/_EVEN for S-Video.
2. I was using the rfe instruction, but neglected the ! on the register, so it wasn't fixing the stack pointer properly on exit. Somehow the application code managed to run ok for a few seconds with a constantly changing stack!
3. I forgot to fix lr before saving it on the stack using the srs instruction (subtract 4). Again, rfe was thus skipping an a application instruction every interrupt, and again somehow the code didn't crash immediately either.

Other than using the ARMv6 instructions above, the code is basically straight out of the OMAP TRM § 10.5.3 MPU INTC Preemptive Processing Sequence, the step numbers below relate to that section. I haven't implemented the priority stuff yet (I was hoping it wasn't necessary for such a simple bit of code since I don't need priorities, but it seems it is for other reasons), so it doesn't actually implement re-entrant interrupts, but I might try to get that working before committing it.

        .set    MODE_SUPERVISOR, 0x13

ex_irq:
        // 1. save critical registers
        sub     lr,lr,#4
        srsdb   #MODE_SUPERVISOR!
        cps     #MODE_SUPERVISOR
        push    { r0-r3, r12, lr }

        ldr     r3,=INTCPS_BASE

        // 2,3 save and set priority threshold (not done)

        // 4. find interrupt source
        ldr     r0,[r3,#0x40]

        // 5. allow new interrupts
        mov     r1,#1
        str     r1,[r3,#INTCPS_CONTROL]

        // 6. data sync barrier for reg writes before enable irq
        dsb                              // not sure what options it should use
        
        // 7. enable irq

        // 8. jump to handler
        ldr     r2,=irq_vectors
        and     r0,r0,#0x7f
        ldr     lr,=ex_irq_done
        ldr     pc, [r2, r0, lsl #2]
        
ex_irq_done:
        // 1. disable irq

        // 2. restore threshold level (not done)

        // 3. restore critical registers
        pop     { r0-r3, r12, lr }
        rfeia   sp!

        .data
        .balign 4
        .global irq_vectors
irq_vectors:
        .word   exception_irq, exception_irq, ...
        .word   ... total of 96 vectors

The srs instruction and cps instructions are used to run everything on the supervisor stack/in supervisor mode. On entry the code is executing in irq mode, so it first saves lr_irq and spsr_irq onto the supervisor stack (after fixing the return address in lr!), and then switches to supervisor mode. Without the srs instruction (pre ARMv6) things are pretty messy since you either have to muck about with the irq stack first (and last), or have to switch between modes a few times to get everything sorted (see the links at the end of this post).I also implement a simple vectored interrupt table to simplify the C side of things, although I think I can just use a simple mov lr,pc before jumping to the vector rather than a literal load.

The ARM ARM actually recommends using the system mode for re-entrant interrupts (you can't use the interrupt mode itself because lr could be clobbered), so why am I using the supervisor stack? Partially historically because at first I couldn't work out how to save the state without clobbering some system stack registers (they're shared with user state). But I also have other plans where this scheme might work better, and if nothing else it stops broken code in user-mode crashing interrupts by breaking the stack pointer.

And finally one more thing I noticed whilst reading bits and pieces is that the AAPCS (EABI) specifies that the stack pointer should remain double-word (8-byte) aligned for entry points. I probably read it before but didn't take notice. This just normally means you always need to push an even number of registers onto the stack before calling other functions. Fortunately this just falls out with this code ... but with interrupt handlers which can be invoked at any time, we don't know what the alignment of the stack is so a specific check is needed too, according to the ARM Info Centre (damn, and I definitely know i've read that before just looking it up now - and it has some other important other bits too!).

Hmm, now i'm thinking about it ... i'm not sure I even need re-entrant interrupts at all. I'm thinking of working towards something along the lines of a microkernel architecture similar to AmigaOS or Minix 3, where device drivers are just high priority unprivileged tasks - the Cortex-A8 should be more than fast enough for this to work. All interrupt handlers will need to do is post events to these tasks, and the software will handle the priorities and whatnot. I suspect re-entrant interrupts are much more important in an embedded system where you just leave most of the work to the interrupt handlers, where DMA isn't available for everything, or the CPU speed is a limiting factor.

Specific Handler

The next step after the interrupt handler is the interrupt vector code itself. This is just a plain function call since the entry point has handled all the nitty gritty. But it still has to deal with the hardware - to identify which interrupt caused it to be invoked, and to clear it. Even with 96 interrupts in the interrupt controller, most of them map to multiple physical events.In the case of the video subsystem, there is a single interrupt DSS_IRQ (25) which can be triggered from 29 different events in either the DISPC module or the DSS module (actually I just noticed there are many more from the DSI module). § 15.3.2.2 Interrupt Requests has a pretty good overview. Fortunately there is a couple of bits in the DSS_IRQSTATUS which lets the code determine which are asserted to simplify processing. After that test is made, each bit needs to be checked in turn and processed accordingly. And finally the interrupt bits must be reset by writing a 1 to each bit in the DISPC_IRQSTATUS or DSI_IRQSTATUS register - otherwise it will go into an infinite loop re-invoking the interrupt as soon as it exits.

void dispc_handler(int id) {
        uint32_t dssirq = reg32r(DSS_BASE, DSS_IRQSTATUS);

        // see if we have any dispc interrupts
        if (dssirq & DSS_DISPC_IRQ) {
                uint32_t irqstatus = reg32r(DISPC_BASE, DISPC_IRQSTATUS);

                if (irqstatus & DISPC_VSYNC) {
                        ... do vsync code ...
                }

                // clear all interrupt status bits set
                reg32w(DISPC_BASE, DISPC_IRQSTATUS, irqstatus);
        }

        // check for dsi ints (to clear them)
        if (dssirq & DSS_DSI_IRQ) {
                // not expecting this, just clear everything
                reg32w(DSI_BASE, DSI_IRQSTATUS, ~0);
        }
}

This is basically the same process that all interrupt handlers need to go through. Identify the source, handle it, clear the assertion.

There are lots of 'gotchas' with interrupt handler writing at first, but the main thing is to not call any functions which share state with non-interrupt code. e.g. anything non-reentrant, or using hardware registers. Oh, and they should always run as fast as possible - all the `real work' your cpu could be doing is halted the entire time the interrupt is executing, and you could be processing thousands per second in a busy system.

The last piece of the puzzle is the interrupt enable masks. You don't just get all interrupts possible in the system all the time, you can mask (or enable) which ones you want to receive. This is all set-up before interrupts are enabled but after the hardware in question is setup. Here I clear all the status bits as well, just to make sure I don't get an unexpected surprise when I enable CPU interrupts later.

        // disable all but vsync
        reg32w(DISPC_BASE, DISPC_IRQENABLE, DISPC_VSYNC);
        reg32w(DISPC_BASE, DISPC_IRQSTATUS, ~0);
        // dss intterrupt can also receive DSI, so disable those too
        reg32w(DSI_BASE, DSI_IRQENABLE, 0);
        reg32w(DSI_BASE, DSI_IRQSTATUS, ~0);

I think I have some sort of set-up bug because I think that i'm sometimes getting interrupts when no event i'm testing is asserted. I will have to check the extra DSI interrupts I just noticed whilst writing this - they should all be masked off (should be reset condition anyway, but ...).

My little demo code right now just does a vsync'd smooth-scroll by changing the video dma base registers. The TRM states that the register is a `Shadow register, updated on VFP start period or EVSYNC.' There is another little trick though, it looks like all DISPC registers themselves are shadowed again, so you always have to set the GOLCD bit in DISPC_CONTROL whenever you make changes for them to make their way to the hardware. I guess I realised that anyway, but initially forgot.

        // update the graphic layer 0 address (video out) to scroll it
        reg32w(DISPC_BASE, DISPC_GFX_BA0, addr);
        reg32w(DISPC_BASE, DISPC_GFX_BA1, addr);
        reg32s(DISPC_BASE, DISPC_CONTROL, DISPC_GOLCD, ~0);

I might come up with a more impressive demo before committing though. Actually now I have interrupts working it opens up a lot of possibilities, such as a real sound driver, serial driver, and proper timing events (in a very odd twist, sometimes my delay loops seem to run twice as fast as other times ...).

Links

I came across a couple of links on the internet about bare-metal ARM coding, some of it doesn't apply/wont work on OMAP3, but the general ideas are the same.

• Simplest bare metal program for ARM;
• And a related article on Embedded.com Building Bare-Metal ARM Systems with GNU. or a much easier to read PDF version I just found.

Oh, I finally got out in the yard yesterday - if only for a couple of hours. More or less finished the trench for the main retaining wall foundation. Now I just need to get off my lazy bum and order some road-base and sand. Can't say I felt the fittest - easily out of breath, although I'm sure that has something to do with the sleep apnoea, my particularly poor sleep the night before (i let the cat stay in and he was wandering around all night), as well as my bum sitting. Glorious day today, and no meeting organised yet about work, so I should probably get out on a bike. Maybe I can scan a few pawn shops in the extremely unlikely event any have C64's lying around.

The cult of stupid

I've been thinking about writing about this for some time, and even written a couple of posts, but I was never happy with how they ended up.

Is it just me, or does it seem as though a new religion has started to gain hold, at least amongst the west. And the religion I speak of is a religion of stupidity and ignorance. All religion relies on a certain level of ignorance in order to maintain the integrity of their flock, but this new one is taking the idea to a whole new non-denominational level.

One only has to see any discussions that arise when climate change is mentioned, or recently almost any science-related topic.

The discussion quickly devolves into a slanging match against science in general. All sorts of people pop out of the wood-work in an incessant and boorish tirade of willful ignorance and stupidity. That people can mis-understand science to such a level in an age of universal education and information access is simply astounding. Unless every argument is framed in the purely black and white, good vs evil terms of an undeveloped mind, they cannot grasp it (or at least, this is the impression they wish to give). Science of course does not work this way, even scientific `facts' are not solid. Science only works because of informed scepticism (e.g. don't believe what you're told, without reason), but these fools are not sceptics. They are deniers.

So I wonder, just from where is this stupidity springing forth? Or more importantly, how is it able to gain a hold in such educated societies as Australia and the UK?

I have some ideas, but for now, the following is the article which finally prompted me to publish my thoughts. It is very sickening reading. It is the first part of a five part series discussing this new religion against science, and has already attracted over 500 comments at this time. Whilst the abusive e-mails in the article are alarming, the numerous comments are simply depressing - it seems that Australians really are that stupid.

I await the follow-on parts with interest.

Bullying, lies and the rise of right-wing climate denial.

Well so much for that.

Well that was an odd week. I did a lot of nothing ... instead of getting out and about or digging in the garden I spent most of it reading about the world's woes and getting worked up about it.

Maybe I should've stuck with the coding, but my mind did need a little rest anyway.

I'm pretty much resigned to the fact that I will have to get USB working ... but boy is it a lot of work. The *BSD and Linux implementations are massive - although I don't need anywhere near that sophistication. The Haiku one is about the only other public free implementation I've been able to find (and in-fact the only with a suitable license), and thankfully it is much simpler, although in C++. Most other free operating systems just don't implement USB. Maybe I should just shelve that whole idea and try and get Haiku working instead ... but my last patch hasn't gone anywhere so I lost some interest in that.

I did have a little play with trying to get interrupts working ... but no real progress on that front yet.

Another side-track was that I ended up with an old casio electronic keyboard to play with (for nothing). Given I have so much spare time I thought i'd try and learn a little piano, or at least see if I can drum up enough interest to want to learn it properly. Still not sure yet, my fingers seem to seize up pretty quickly, but it passes the time in the sort of cathartic way that reading the news or programming doesn't.

It gave me other ideas too, like hooking it up to a beagleboard, since I have a spare one still sitting in it's box. There are enough GPIO pins to hook up the matrix scan directly, although the 1.8v level logic adds a twist. Could make a fun little synth, even if I can't play it properly. Alternative is to use a smaller part like a PIC or AVR to decode the keyboard and ship out USB, serial, or even midi. Haven't played with hardware for ages.Somehow that got me onto another site (through hack-a-day) that had some guys remarkable efforts with old Commodore 64's. I could just use the keyboard and box to put a beagleboard in to make a usable computer and not have to worry so much about USB and the like (and even if I just ran some version of linux on it, it would make a nice box to put everything in, particularly the C64-C or Amiga 600 cases).

My brother still has a few old computers at home, so I might try and get one

(shit, he threw them all out!), or ask around.

All good things must come to an end.

Looks like my extended holiday will soon be over - the work i've had lined up for some time is finally in the last stages of it's paperwork. Damn! Hmmm, working again will be tough, although the work will be more interesting than last time (or, at least it bloody well better be!).

Might take a bit of a break from (or at least, a significant reduction of) the beagle board hacking whilst I catch up on house work, yard work, sleep, exercise, get a hair cut, and/or just to go to the beach in the few days I have left in this hot weather. Damn i'm so far behind on sleep. I never catch up on that. Sleep sleep sleep. Sigh, repeating it doesn't help either. Sleep sleep. Worth a try.

I felt i'd hit a bit of a wall anyway, so it's probably a good time to take a break, and making progress on sound was a nice milestone.

Mary had a ...

Had a go out sound in - and couldn't get it to work at first, so I went for a nice ride. Just a casual 58km with a long beer and pizza stop at a mate's on the way. I was a little sore and tired by the time I got home (mostly from the saddle; haven't ridden that bike much for ages), but after a decent sleep, not a hint of soreness.

After I got home and became bored with TV I decided to have a search on the net about sound again - and the vital clue - aux-in is line-level and not mic-level (well I should've known that but it all goes through the same amps). So the code was working after-all, I plugged a laptop out into line-in and got some results.

Actually I worked out that if I use a speaker (or headphones) as the line-in, I can get enough signal as well, but I left the gain at 0dB for a line-level signal.

Then I wrote a very simple synthesiser loop which plays 'mary had a little lamb' (badly) with a triangle-wave and simple ADSR envelope (in true SID-chip style!), and then jumps to a rather annoying 440Hz sound which phases from side to side, whilst showing the line-in waveform on the screen (as in the screenshot).

Source in audo-beep.c, which includes all of the initialisation code as well.There's a commented line in the init:

aregw(AV_ATX2ARXPGA, (7 << 3) | 7);

which can send the digital signal from line-in directly to the output. But the signal is really rather awful, and doesn't seem to match the data received, with lots of clipping and nastiness. Probably some bad interaction with the synethsiser data, and not biasing things properly.

This code doesn't use DMA, so is about as simple as you can get whilst still making noises.

Hmm, I wonder what to do next. Having USB for keyboard and mouse is a real pain, it's pretty hard to do much interesting `computer' stuff when you only have a serial port for communications.

Now to get 'mary had a little lamb' out of my head ...

Beagle beeps

Well, some semblance of a beep, it sounds more like an alarm going off. Another frustrating tasks of mis-understanding and reboots.

To start with I had it 'click' when it turned the audio codec on or off, so at least I knew I was writing to the right registers. And changing the volume even changed the magnitude of the click.

Then after setting all the registers to something that seemed to make sense, I simply forgot to turn on the audio interface, so I spent a lot of time wondering why the McBSP end of things wasn't sending any data out (I was copying the `MP3 Out' use-case too closely which neglected to mention it ... although I read about the enable bit numerous times and knew I needed it, I simply forgot to check I was setting it). Then once I got that working I still had no sound - I couldn't get the routing to work. I knew the serial port frame signaling was at least working (no idea on the data) as changing the sample rate had an effect on how often XDRDY was asserted.

But nothing I tried worked. The audio device has quite a few registers spread all over the place, and the manual is formatted in a way which makes them hard to look up or follow. It only has a TOC entry for the whole lot, and with Evince, that makes it pretty tricky to navigate. Not to mention that Evince wants to select links half the time when you click on them just to make things more painful. I think I missed something in the manual about which serial sources receive data from the port, and data only comes in via serial 2 for non-TDM signaling. And I was trying to use serial input 1 as the source, then switching that to audio 1 internally - so I guess there was no sound from the start.

In the end I cheated a bit - I got a dump of the registers on a running GNU/Linux system which was playing sound at the time. Then I compared them one by one with those I had - on the way found some bugs anyway - and eventually just set the routing up the same way.

After all the hassles and straining to hear anything from a set of headphones I was surprised when it worked - fortunately I had the headphones off otherwise I might've got a rude shock at 3 in the morning. I'm still not using DMA to write to it, but that shouldn't be too difficult. I want to try to receive sound first before I try DMA or drop the code, but hopefully that should be simpler, since I don't have much choice in what goes where now.

Oh there was one other sting in the tail. I was playing with one of the gain controls to see if I could turn the volume down a bit: AV_ARXL2PGA. It's supposed to have a range from 0x00 to 0x3f (i.e. from mute to 0dB in 1dB steps; the master volume control), but if I set any value below about 0x30 it simply mutes the output. Which was another value I 'got wrong' when testing, so who knows, I might've had the routing correct at one point and not heard anything because of this. I'll have to have another poke at that too.

So mistakes in summary:

• Forget to turn on the AUDIO_IF bit to receive data.
• Didn't make sure the master volume (ARXL2PGA, ARXR2PGA) is over 0x30 (why this is an issue I don't know).
• For I2S mode, it seems audio only comes in through SDRL2 and SDRR2.

For the McBSP2 there was some issues too, to do with signaling polarities mostly - although I don't know if I have them right yet. Requires looking at the raw signal diagrams more closely.

Hmm, looks like a nice day for a ride.

Searching for myself

No i'm not about to go all deep and meaningful on you, although I was probably drunk enough earlier to consider it. Neither have I lost track of my person. Nor was I vanity-surfing.

First I was looking at how easy this blog was to find when looking for beagle board coding info, and then I was looking to see how easy info about me was to find for those that might be looking.

For the first I found a `BeagleBoard' user on facebook who has been noting my posts - but for that I had to dig, as I did for this site. And for the second I found a real gem of a memory on the first page of Google search results:

PC Backups/bugs etc.
Michael Zucchi (cismpz@cis.unisa.edu.au)
Tue, 10 Sep 1996 13:00:20 +0930 (CST)

    * Messages sorted by: [ date ][ thread ][ subject ][ author ]
    * Next message: Ken Laprade: "BUG: "cannot create" error file"
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    * Next in thread: Gregory T. Notch: "Re: PC Backups/bugs etc." 

Howdy,

I'm an assistant sysadmin in a small computer department at the Uni
of South Australia. We have automated backup systems for unix
(using amanda), and mac systems (using retrospect), however its
currently upto the users of pc systems to backup the files they
use regularly, with whatever means they have at thier disposal.

Now, the recent versions of 'samba' include an smbclient which can
create tar files from pc shares. Obviously the idea here is to
use the recent tar support of Amanda to automagially do our PC
backups too, based on WFWG shares on each of the systems to be
backed up. Essentially, i'm trying to 'bolt on' remote pc share
'tar' support into the gnu-tar files. e.g. if the disklist
contains:

hostname //pchost/password nocomp-user-tar

then the 'hostname' host has samba installed (in my case i use
the master backup host - load to this host could be a problem
when a large number of remote pc disks are being backed up through it),
and when a backup takes place it connects to \\pchost\backup with
the password 'password', and tars to stdout. The gnutar code
recognises a remote pc host by the disk name syntax. I also modified
the smbclient software slightly to produce a 'totals' line from
stderr. I'm not sure if i'll ever get it finished, but i've managed
to get it to tar a remote NT workstation share and record it in
/etc/amandates, and incremental backups should work (properly - using
the dos archive bit) too.

However i seem to have found a bug in Amanda in the process.

client-src/sendsize.c calls start_amandates(0) in line 89.
start_amandates runs ok, but calls enter_record (from
common-src/amandates.c) which, on error (when the recorded
dump date doesn't match the /etc/amandates dumpdate) it
tries to use the 'log' function. This in turn tries to access
the config files - which aren't stored on the client side of
an amanda setup. Consequently sendsize bombs out with:
"sendsize: could not open log file (null): Bad address"

As this seems the only place to report bugs, i'm leaving it
here.

Regards,

Michael Zucchi

-- 
     ///   `... thinking is an exercise to which all too few brains
    ///     are accustomed.' - First Lensman, E.E. `Doc' Smith
\\\///  Michael Zucchi B.E.                      M.Zucchi@UniSA.edu.au
 \\\/   CIS, Assistant Systems Administrator, UniSA     +61 8 302 3033

    * Next message: Ken Laprade: "BUG: "cannot create" error file"
    * Previous message: Martin Espinoza: "I need someone's, ANYONE's ...
    * Next in thread: Gregory T. Notch: "Re: PC Backups/bugs etc." 

Wow. From my first job. I had written and released free-ware before for AmigaOS in isolation (an accelerated GIF decoding system component for example), but IIRC this was pretty much my first post to a free software mailing list, and it was followed not long after by my first patch.

I was so polite back then, I wonder what happened. And wrote overly verbose email messages as I continue to do to this day. Although I had a somewhat pretentious signature (which I kept for many years), I still think it speaks volumes.

But after 15 years of crapping around the internet, you might have expected something like that would've dropped from the first page by now ...

What I find interesting is that if you search in 'bing', that is about the only sort of thing you CAN easily find with my name on it. It's like the whole internet after 1998 simply doesn't exist as far as I'm concerned (oddly enough, that's about when I started on GNOME).

Hmm, maybe that wouldn't be such a bad thing either.

A couple of bits and pieces

Wow this really turned into a monster post, I guess I had a lot of crap on my mind. So much for going for a ride today as i'd hoped - the wind really picked up anyway so it wouldn't have been much fun.

On Software Engineering ...

First, a couple of interesting posts on a blog run by a guy from Insomniac (Ratchet & Clank, Resistance, etc). I've read bits of his before because he has some posts about CELL coding, but I came across his revamped site recently whilst looking up issues about C99's un-ANSI-C-ed-ness.Three Big Lies of Software Development. I think they are pretty fundamental things that every programmer needs to keep in mind. The first is the scourge of pretty much all modern software - and why software seems to stay about the same speed even though hardware has jumped in leaps and bounds. The second I suggest could be squarely aimed at every introductory course to `object oriented design' and is exacerbated by the first lie. And the third is probably the most important; it's the data, stupid.Sketches on on concurrency, data design, and performance is also a must read in this day and age of multi-processors. The post-it notes are a particularly nice touch although the PDF versions are much nicer to read than the web gallery. I particularly like his rant against typical C++, and how he explains that ideas such as a 'lock free doubly-linked list' are nonsensical in a concurrent environment. e.g. with no 'after' or 'before', how can you insert 'after' or 'before', and without an insert 'after' or 'before', it is no longer the same ADT.And finally the aforementioned reference to C99's weirdness in understanding strict aliasing. I see why they added this feature - to enable optimisations that a compiler couldn't otherwise do (but looked obvious to an author), but to me it just isn't `C' any more, it's a slightly different language (and worse, it conflicts explicitly with the sort of stuff you need to do to make normal C faster or do operating-system type tasks). Anyway, I think this particular comment really tells the story, with my own emphasis:

The above source when compiled with GCC 3.4.1 or GCC 4.0 with the -Wstrict-aliasing=2 flag enabled will NOT generate a warning. This should serve as an example to always check the generated code . Warnings are often helpful hints, but they are by no means exaustive and do not always detect when a programmer makes an error. Like any peice of software, a compiler has limits. Knowing them can only be helpful.

I'm not sure that even I would be terrible comfortable with a language where that was a requirement - and I suspect i'd be far more comfortable with it than the majority. I was looking this stuff up because Jeff was chatting to me about some of my code in Evolution that a recent gcc was just silently dropping because it aliased some pointers. Funny, I thought that's what casts were for - and it's not like you can't still do it, you can just use a union to do exactly same thing; it's just a lot messier for humans to read. Anyway, that post is a really good explanation of the issue and how to change your code to fit the newer C variant so the compiler can make it run (potentially quite a bit) faster.

Anyway, a very interesting few posts on Mr Acton's site - it's a pity he doesn't update it a little more often (there's more on Insomniacs R&D site too, much more I still need to read myself).

On Menus ...

Odd information I feel the need to share: IceCat (Firefox) gets a little unwieldy with 180 tabs open, I think this is the fourth copy of A Hacker's Craic open because I can't be bothered to find the other ones. It still runs though.

Which leads me on to partly why ... scrolling @#@#ing menus. What idiot decided they belonged in any modern toolkit (let me guess, they're in Cocoa)? I remember seeing some GUI toolkit implement them in the early 90's and thought `that looked pretty cool' - but they aren't. They suck. They don't scale, and they're difficult to use. Even a small list on a button at the top or bottom of the screen will put on tiny hard-to-hit and hard-to-use up/down arrows if you've previously selected an item at the wrong part of the list. Try using one with a G-spot under your finger.

This my friend, is a G-spot.

So here's a perfect example in Evince, which I have been using heavily recently (more on that later). It looks pretty and all, nice and simple and obvious and `easy to use'.

Now look what happens when you access it from Evince running in full-screen mode. And this is only one of the possibilities, you get various, quite-different results depending on exactly where within the button you click and if the mouse moves whilst you press on it with your finger e.g. if you click near the top it actually selects `Best Fit' for you and removes the scrolling buttons since the mouse is suddenly over the 'up scroll region' and not over the current item.

Hmm, nice one. It looks like arse, is completely unnecessary, and those silly little `scroll region/buttons' are too hard to use; apart from being too small, they're actually a whole new type of button which aren't used anywhere else in any GUI - a `click-less button', or `hover button' if you will. I'm using a fairly old Evince, so these might be bugs specific to the version, but the whole idea stinks and it's going to always have these sorts of serious usability issues - a menu that can look different every time you use it isn't going to aid muscle memory for starters. Not to mention the stupid `hover buttons' that you need to put your mouse over and wait ... wait ... wait for it to show the desired item.

The problem is it's mixing two access modes, a 'click' and a 'hover mode' - once you click on the button you're in 'hover mode' - whatever the mouse is over is selected or activated. So it has to attempt to ensure the mouse is over the right location when you click, and since it wont warp the mouse pointer, it warps the menu instead. Even mouse pointer warping would be (much) better than this, but there are alternatives like staying in a 'click' mode and just requiring another click to actually select an option. It sort of does this if you don't move when you 'click', but I don't know many mice that don't move most of the time when you physically push part of them, so it's even worse - you could get either behaviour depending on how steady your hand is. In the Firefox case with 180 items it would just never work anyway, it's just the wrong UI element to use.

As for Evince, I think i'm stuck using the wrong tool for the job. It's not really much of a document reader once you get a document with over few dozen pages. The search is very slow too, on my workstation it takes about 70 seconds (depending greatly on the search string) to scan the OMAP3 TRM (~3500 pages); although I realise this has a lot to do with the PDF format. fgrep scans the whole 22MB raw file in 0.035s. Add no find all, and the fact it forgets the last search as soon as you click somewhere, and it's pretty painful. Not to mention the lack of navigation stack and other basics which get in the way. Ok perhaps it is intended to be a simple viewer for 'users' - but developers are friggan `users' too .

Now, there was something else I wanted to mention ... what was it. Damn, completely gone. Oh oh, no, here it comes. It's about `distributions'.

On Distributions ...

On the Haiku lists there's been some discussions about package management. And that naturally leads to the idea of a `distribution' - which the Haiku guys are fundamentally against, and I think with good reason.

What exactly is a `distribution'? It's a collection of software which has been thrown together and presumably validated to some extent in such a way as to work together. In a way it is a kind of `neat' thing, and one that can only exist because all the software is free software; every single piece of third software can (potentially) be compiled and validated specifically for the target platform. This is a pretty fundamental change to the way software is distributed, and can lead to fundamentally different outcomes, for example with security, or platform support. Even a lowly IRC client is under complete vendor control, so can be prevented from becoming a penetration attack vector because of some sloppy code or accidents. Or a vendor can rebuild everything for a different hardware platform without having to wait for every ISV to support it. And upgrading can be a bit fun if your computer is a playground - discovering all the new features of every application (until you're a jaded old prick like me, where half the changes are frustratingly stupid).

So, there are many benefits from `distribution' based deployment.

However there are problems as well. We all used to scowl at how M$ forces everyone onto the upgrade treadmill, but although different in nature, is quite a problem with `distribution' based software too. In some ways it is worse - you actually have to upgrade all of your software at once; and this is not always a good thing. New versions means new bugs, and with the propensity for the most active developers being rewarded with having their projects anointed for inclusion, often stable mature applications are replaced with unstable shinier newer ones. Some previously existing software may not make the cut at all, so you can't even keep using it if you wanted to. And although it's more of a serious problem for proprietary software (and thus not particularly important), the upgrade mill affects free software too - it can be extremely tough going compiling older software to a usable state - and probably drag in a whole pile of tools and libraries not used elsewhere. In effect old software bit-rots faster than it needs to, and is effectively locked out and thrown away. This is particularly acute with the modern scourge of using massive and complex frameworks which change in incompatible ways from version to version in time-frames of 5 years or less.

The strength of a Unix-like `open platform' is also it's major weakness. For example, by having the option to use different toolkits in X, every dog and his man will choose a different toolkit for their apps. And this just multiplies all of the potential issues that large-scale code faces; project goals, consistent design philosophy, bugs and maintenance. Not only do you have these problems within a given software `platform', it is multiplied across all of them.

Just look at the completely fucked-up-mess that sound is. Just imagine the state we'd be in if the kernel didn't implement TCP/IP but we had 4 or 5 different, incompatible, and competing implementations running in user-land. Sound is just as fundamentally a hardware issue as networking is, so why isn't it all just in the kernel the same way? And i'm talking about the full audio stack here, not just a buffer to a DAC which is all Linux will allow, throwing the guts of modern sound off to user-land. After all, we don't have user-land switching of the ethernet packets do we?

So I think what the Haiku devs are really trying to avoid is such a mess infiltrating their system. Without this crap to deal with there just isn't any need for a `distribution' in the first place. And even the fact of having a single GUI toolkit with tighter focus will take a lot less effort to maintain than a dozen (regardless of the matter of spreading the load), and more importantly provide the user with a more consistent and pleasant experience. And developers too for that matter; having a choice just means the wrong one can be taken.

Hmm, I like the sound of that, and can't find any reference in a web search, so with a bit of outright ego-tism, I shall claim it to be my own.

Zucchi's Law

“Having a choice always leads to

the wrong option being taken

at least some of the time.”

Somewhat analagous to Murphy's Law I guess, but the key difference is this is about the limitations of informed choice and the fallibility of humans, not of their environment.

There's nothing technical stopping GNU/Linux doing the same thing, but there's too much `wet ware' in the way, so the politics make it completely impossible. It will take another operating system to do it. That's all Android or ChromeOS are; a single toolkit and application environment with less choice for the developer, but still built on the same base operating system.