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

[Uwe Kleine-König]

Hello,

On Mon, May 04, 2015 at 09:38:28AM -0700, Doug Anderson wrote:
> On Mon, May 4, 2015 at 8:24 AM, Uwe Kleine-König
> <u.kleine-koenig@xxxxxxxxxxxxxx> 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...
researching a bit myself it seems you're right here. It would be nice to
mention this in the commit log though.
 
> > 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...
Hmm, probably that's overkill. (Better spend the time fixing printk not
to disable irqs that long :-)

> 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.
If you rework your commit log to describe the failing situation more
accurately that should be fine.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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