Smooth as a baby's bum

Well I didn't end up getting out, but I had a break for a while. I also started thinking about what I want to do now I have interrupts working and decided I couldn't be bothered doing too much with the vblank demo. But since there was nothing on TV for a bit I tidied it up and committed it as is.

It's moving smoothly, honest.

Yes, not much of a screenshot, but I thought the page needed some colour and a break from all the text of late. It just smoothly scrolls from one screen to the other and back again, drawing one box on the lower screen every time it's fully hidden. Demo in irq-scroll.c, with the interrupt code from yesterday in exceptions.S.

So what to do next. Well there's still the idea of a little game or demo or so, but i'm lacking a bit of inspiration, and besides, a bit of support to actually run the code in, for things like sound. So i'm starting to think about working on a little real-time micro-kernel.

Many months ago I had been writing one for x86 but got a bit pissed off with all the PC crappyness, and found something more interesting to do instead. So i'll probably start with that, although i'm not sure how much i'll end up using (not that I can remember the state I left it in anyway). For example, currently it does all the VM and process management inside the kernel (it made it simpler at the time), but I want to move that to a process instead so the kernel can work without any pre-emption or locks. Basically the goal is to make the kernel as simple as possible (nano-kernel?) without the simplification getting in the way. What I had been working towards was something like a mix of Minix 3 and AmigaOS; taking features like memory protection and processes from Minix, in addition to the asynchronous non-copying message passing, shared libraries, tasks, and the device mechanism from AmigaOS, with a bit of a reworking to make it all fit. Ahh well, maybe a tad on the optimistic side so I wouldn't hold my breath, on the other hand apart from device drivers there's not all that much to the core of such a design.

Not sure if i'll put it in PuppyBits or another project, but for now I still need to work out some basic routines like context switching and so on so i'll definitely put that stuff in PuppyBits at the least.

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.