OpenPIC timer usage?

[Nick Withers]

Hullo all,

I've finally looked into this one again and thought I'd follow up for
the archives... Long story short, https://git.rtems.org/rtems/commit/?h
=4.11&id=90ce69b0f19ab410d502fa93a64f02de55cf4a53 fixed it :-)

On Thu, 2014-08-14 at 10:39 +1000, Nick Withers wrote:
> On Wed, 2014-08-13 at 10:31 -0400, Gedare Bloom wrote:
> > I'm not a powerpc expert, but I have some questions for you to
> > consider.
> 
> Thanks for taking the time, much appreciated.
> 
> > How long does it take the openpic_settimer() to take effect?
> > If the isr was delayed, could another timer interrupt fire before
> > setting the inhibit bit? Will both interrupts be received? This is
> > hw-dependent, and I don't know the answer for powerpc.
> 
> Interestingly, my interpretation of the MPIC manual (2.3, from points
> 5
> particularly) seems to suggest I should see two interrupts if I break
> at
> my interrupt handler (after IACK, before End-of-Interrupt) before the
> openpic_settimer(), if the timer would fire again. I don't... But
> there
> are a few layers between my handler and the hardware and no doubt I
> just
> dont understand it properly.

Yeah, I'm full of it here.

"Timer interrupts will function essentially as edge-triggered events.
If the timer period expires while a previous interrupt from the same
timer is pending, the subsequent interrupt will be lost". The
interrupt's pending until EOI.

> > Perhaps the
> > powerpc ISR_Handler clears the interrupt source before calling the
> > isr, so there might be time for another timer to fire? Or perhaps
> > the
> > hardware buffers multiple interrupts received from the same source.
> > If
> > so, then you would need to acknowledge the interrupt after clearing
> > the inhibit bit, so another interrupt that was queued from the same
> > source doesn't fire.

powerpc/shared/irq/openpic_i8529_irq.c:C_dispatch_irq_handler() calls
 openpic_eoi() after bsp_irq_dispatch_list(), so I think I'm in the
clear here.

Post https://git.rtems.org/rtems/commit/?h=4.11&id=90ce69b0f19ab410d502
fa93a64f02de55cf4a53 , things look good on the mvme3100 (updated test
program attached).

PSim still doesn't work as expected, but I'm reasonably convinced
that's a PSim thing... As per below, the reported OpenPIC timer
frequency's not valid and certainly not being honoured in terms of
actual timings.

Thanks all

> > On Wed, Aug 13, 2014 at 3:58 AM, Nick Withers <
> > nick.withers at anu.edu.au> wrote:
> > > Sorry, hit "Send" a bit quick...
> > > 
> > > On Wed, 2014-08-13 at 16:42 +1000, Nick Withers wrote:
> > > > On Tue, 2014-08-12 at 12:48 +1000, Nick Withers wrote:
> > > > > Hullo all,
> > > > > 
> > > > > Has anyone used an OpenPIC timer (see
> > > > > "c/src/lib/libbsp/powerpc/shared/openpic/") in single-shot
> > > > > mode before?
> > > > > I seem to be having some trouble with them, in particular
> > > > > ensuring that
> > > > > an ISR fires only once.
> > > > > 
> > > > > According to "c/src/lib/libbsp/powerpc/mvme3100/README", "The
> > > > > setup
> > > > > routine allows for specifying single-shot or periodic mode",
> > > > > but I can't
> > > > > seem to see how to achieve the former.
> > > > > 
> > > > > Going off
> > > > > "c/src/lib/libcpu/powerpc/new
> > > > > -exceptions/bspsupport/nested_irq_test.c" I
> > > > > should at least openpic_maptimer(..., 0) and
> > > > > openpic_inittimer(..., ...,
> > > > > 0) to stop the timer. I've tried that in the ISR and the
> > > > > openpic_inittimer() call can stall indefinitely in
> > > > > openpic_safe_writefield() on "while
> > > > > (openpic_read(addr=0xe1041120) &
> > > > > OPENPIC_ACTIVITY);".
> > > > 
> > > > Turns out the "OpenPIC" on at least the MVME3100 is actually an
> > > > IBM
> > > > MPIC, which has documentation[1]! Huzzah!
> > > > 
> > > > So, to follow up, neigher openpic_inittimer() and
> > > > openpic_maptimer()
> > > > need to be called to halt the timer and prevent further
> > > > interrupts (but
> > > > need to be done to set the timer up before first use).
> > > > 
> > > > An openpic_settimer(..., OPENPIC_TIMER_COUNT_INHIBIT, 0) will
> > > > do it.
> > > > 
> > > > Also note that for the hardware to latch a new time programmed
> > > > through
> > > > openpic_settimer(), the OPENPIC_TIMER_COUNT_INHIBIT bit has to
> > > > transition from a 1 to a 0.
> > > 
> > > So I think I'm driving the timer correctly (but if anyone else's
> > > ever
> > > used 'em, a double-check'd be loverly :-P).
> > > 
> > > Any yet, it seems I'm potentially getting a double-ISR-hit,
> > > though I
> > > haven't been able to trigger it reliably; it feels like some kind
> > > of fun
> > > race condition.
> > > 
> > > I've attached a minimised code sample (build with, e.g., "[g]make
> > > RTEMS_ROOT=/home/nick/rtems/rtems-4.11 BSP=mvme3100") which has
> > > been
> > > tested successfully (i.e., it shows the failure) through PSim and
> > > on
> > > MVME3100 hardware (though it's timing-dependent, so YMMV!). When
> > > mvme3100_timer_set()'s called, it:
> > >   - Asserts that a semaphore's locked*
> > >   - Asserts that the timer's currently stopped
> > >   - printk()s 's'
> > >   - Starts the timer
> > >   - printk()s 'S'
> > >   - Blocks on the semaphore
> > >   - Asserts that the semaphore's locked
> > >   - printk()s '_'
> > >   - returns
> > > 
> > > The timer's vectored to mvme3100_timer_interrupt() and when this
> > > is
> > > called, it:
> > >   - Stops the timer (notionally, anyway! I've stepped through it
> > > in GDB
> > > and it seems to work as expected), through openpic_settimer(...,
> > > OPENPIC_TIMER_COUNT_INHIBIT, 0)
> > >   - printk()s 'p'
> > >   - Increments the semaphore
> > >   - printk()s 'P'
> > > 
> > > So I'm expecting, for each '_'-delimited block of text, to see
> > > one set
> > > of "s(...)S" for each "p(...)P".
> > > 
> > > Here's part of a run in PSim:
> > > ____
> > > 
> > > psim-cmds:5: Error in sourced command file:
> > > No symbol "the_source" in current context.
> > > (gdb) run
> > > Starting program: /usr/home/nick/projects/sem_isr/sem_isr.debug
> > > warning: failed to reevaluate condition for breakpoint 1: No
> > > symbol "the_source" in current context.
> > > OpenPIC Version 1.2 (2 CPUs and 17 IRQ sources) at 0x0C130000
> > > OpenPIC Vendor 0 (Unknown), Device 0 (Unknown), Stepping 0
> > > Overriding NumSources (17) from configuration with 16
> > > OpenPIC timer frequency is 1 Hz
> > > Starting...
> > > sSpP_sSpP_sSpP_sSpP_sSpP_sSpP_sSpP_sSpP_sSpPpP_assertion
> > > "rtems_semaphore_count(timer->semaphore) == 0" failed: file
> > > "mvme3100_timer.c", line 207, function: mvme3100_timer_wait
> > > ____
> > > 
> > > [The reported OpenPIC timer frequency's not valid and certainly
> > > not
> > > being honoured in terms of actual timings - this is expected with
> > > PSim,
> > > isn't it?]
> > > 
> > > That's consistent with what I see when running it on MVME3100
> > > hardware
> > > (albeit typically with different interleavings of the ISR /
> > > task),
> > > except that there's no obvious double-up (e.g., the "pPpP" just
> > > before
> > > the assertion failure, above) - I presume because printk()'s
> > > output
> > > isn't guaranteed to make it out to a serial console before the
> > > assertion
> > > crashes it...?
> > > 
> > > I guess it'd be nice for me to provide code that perhaps
> > > generates
> > > interrupts in another manner. Would anyone have any suggestions
> > > here? is
> > > there an rtems_interrupt_cause()-like that's implemented? Even if
> > > there
> > > is it might be hard to get the timings right without a higher
> > > -precision
> > > timer.
> > > 
> > > > > Any ideas? Perhaps I'm going about this the wrong way to
> > > > > begin with...
> > > > > What I'm after is a microsecond-precision-ish one-shot timer
> > > > > that'll be
> > > > > used thousands of times per second.
> > > > > 
> > > > > Ta all!
> > > > 
> > > > [1]
> > > > https://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/F2797
> > > > 1551C9EED8E8525774A0048770A/$file/mpic_db_05_16_2011.pdf
> > > 
> > > * Possibly dodgily: I implemented an rtems_semaphore_count
> > > kludge, but
> > > can't claim to actually understand what I did properly. I have
> > > observed
> > > the same symptoms using a POSIX semaphore (which to my knowledge
> > > isn't
> > > explicitly specified as being allowed in an RTEMS ISR, though
> > > ISTR
> > > seeing it somewhere before)
> > > --
> > > Nick Withers
> > > 
> > > Embedded Systems Programmer
> > > Department of Nuclear Physics, Research School of Physics and
> > > Engineering
> > > The Australian National University (CRICOS: 00120C)
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