Re: [PATCH] i2c: rk3x: Increase wait timeout to 1 second

[Doug Anderson <dianders@chromium.org>]

Hi,

On Mon, May 4, 2015 at 8:24 AM, Uwe Kleine-König
<u.kleine-koenig@pengutronix.de> wrote:
>> Thank you for looking at this!  I will clarify by giving explicit CPU
>> numbers (this issue can only happen in SMP, I think):
>>
>> 1. CPU1 is running rk3x_i2c_xfer()
>>
>> 2. CPU0 calls vprintk_emit(), which disables all IRQs on CPU0.
>>
>> 3. I2C interrupt is ready but is set to only run on CPU0, where IRQs
>> are disabled.
> Why does this irq only trigger on cpu0?

So I've never done the research myself on the interrupt architecture
on ARM, but certainly on all ARM boards I've worked on recently all
IRQs (except those local to just one core) get routed to CPU0.  I've
been told that there are some userspace programs that try to balance
things out a little bit, but even in those cases each IRQ is assigned
a single CPU and if that CPU has its interrupts off then the IRQ won't
be dynamically rerouted.  I think I remember someone telling me that
there was also extra complexity around what happens when CPUs get
taken offline...

A quick search shows some discussion from 2011 at
<http://comments.gmane.org/gmane.linux.ports.arm.kernel/102251>.

Given that lots of smart people have looked at this and our interrupts
are still all going to CPU0, it's not something I'm going to try to
solve right now...


> Assuming this is correct, the
> more robust change would be to detect this situation after 200ms instead
> of waiting 1s to work around this issue.

Detect in what way?  You mean add code to detect that the CPU that's
assigned our interrupt has been stalled for 200ms?  ...and what do I
do in that case?

I suppose I could add code that reads the I2C interrupt status and
notices that although the I2C controller claims that it should have an
interrupt by now but we never saw it go off.  I could then give it
more time.  Is that what you're looking for?  We'd still want to
timeout eventually since there could be some other bug in the system
that's causing the interrupt not to ever go off...

That adds a bunch of extra complexity, though.  Is there a use case
where a timeout of 1 second poses a problem for you that would justify
the extra code?  I'd presume you're thinking of a case where a timeout
would be expected in a case other than a bug in the i2c driver or a
hardware bug in the i2c controller where 200ms is an acceptable
timeout but 1 second is far too long.  Note: if we are using the
timeout to detect a bug of some sort then I'd imagine that 1 second
might be actually better (a slightly longer delay makes it more
obvious that something bad is happening).


One other thing occurs to me: having a longer than 200ms delay may
actually be a correctness thing anyway.  Technically i2c devices are
allowed to clock stretch "indefinitely", so transfers be quite long
and still be "legal".  In practice a REALLY long clock stretch signals
a problem somewhere so we really do need some timeout, but 200ms may
be too short.  For instance, some docs of the bq27541 battery gas
gauge claim that it can clock stretch (in extreme cases) for 144ms.
While this is still less than 200ms, it does seem prudent to give a
little more leeway before seeing a timeout.

-Doug