Re: I2C PM overhaul needed? (Re: [PATCH 1/2] i2c: sprd: Prevent i2c accesses after suspend is called)

[Grygorii Strashko]

On 05/08/2018 11:32 AM, Wolfram Sang wrote:
> Grygorii,
>
> thanks a lot for your input. Much appreciated!
>
> > That would be great, but note:
> > 1) only i2c_transfer() operations are locked, so if driver is doing
> > i2c_transfer(1)
> > i2c_transfer(2) <- suspend in the middle
> > <- suspend in between
> > i2c_transfer(3)
> > It will not help.
>
> Will it not improve the situation by ensuring that at least the transfer
> with its (potenitally) multiple messages got completed? That we are at
> least in a bus-free state (assuming single-master here) before
> suspending?
>
> > Everything depends on timings :( - in my practice 10000 suspend iteration tests
> > where required to run many times to catch 3 buggy I2C client drivers.
>
> Matches my experiences that creating a reliable test case for that is
> not that easy as I thought. Or I am missing something obvious.
>
> > 2) It's normal to abort suspend if system is busy, so if I2C core will be able
> > to catch active I2C operation - it should abort, but again I do not see how it
> >   can be detected 100% with current I2C core design or without reworking huge number of drivers.
>
> I agree. After second thought, waiting for i2c_transfer to finish maybe
> won't be enough, I am afraid. We don't know if STOP has been put on the
> wires yet. My best bet now is that we implement such a
> 'is-transfer-ongoing'-check in the suspend function of the master
> driver? That check should be optional, but recommended.
>
> > 3) So, only one thing I2C core potentially can do - catch invalid access and
> > report it. "wait for transfer to finish" wouldn't work as for me.
>
> And we do this in suspend_noirq function of the i2c core.
>
> > > I at least know of some Renesas boards which needed the I2C connected
> > > PMIC to do a system reset (not sure about suspend, need to recheck
> > > that). That still today causes problems because interrupts are disabled
> > > then.
> >
> > this was triggered few times already (sry, don't have links), as of now,
> >   and as I know, the only ways to W/A this is:
> > - to create barametal platform driver (some time in ASM)
> > - or delegate final suspend operation to another system controller (co-processor),
> >    as example TI am335x SoCs,
> > - or implement I2C driver in hw - TI AVS/SmartReflex.
>
> Yes. Please note that this is only needed for reset, not suspend. So, it
> is a bit easier. Still, it might make more sense to use a platform based
> solution. I'll think about that.
>
> > Sry, but 99% percent of I2C client drivers *should not* access I2C bus after
> > .suspend_noirq() stage it's BUG-BUG!! Any W/A will just hide real problems.
>
> I do believe you, still is there documentation about such things? I like
> to understand more but didn't dig up something up to now. E.g. I grepped
> for "noirq" in Documentation/power.
>
> > "master_xfer_irqless" might be a not bad idea, but, in my opinion, it
> > should be used explicitly by platform code only, and each usage should
> > be proved to exist.
>
> Yes, we can think about it once it is really needed.
>
> > Some additional info:
>
> Thanks a lot for that!
>
> > I'm attaching some very old patch (don't ask me why it was not sent :()
> > I did for Android system - which likes suspend very much. Some
> > part of below diff are obsolete now (like omap_i2c_suspend()),
> > but .noirq() callback are still valid and can show over all idea.
> > Really helped to catch min 3 buggy client drivers with timers, delayed
> > or periodic works.
>
> Ok, so what do you think about my plan to:
>
> 1) encourage drivers to check if there is still an ongoing transfer in
> their .suspend function (or the core can do that, too, if we agree that
> checking for a taken adapter lock is sufficient)
>
> -> to ensure transfers don't get interrupted in the middle

It probably should be part of .suspend_noirq() also.

> 2) use a .suspend_noirq callback in i2c_bus_type.pm to reject and WARN
> about transfers still going on in that phase
>
> -> this ensures that buggy drivers are caught
>
> 3) write some documentation about our findings / assumptions /
> recommendations to a file in Documentation/i2c/
>
> -> this ensures we won't forget why we did things like they are ;)
>
> ?

Sry, for delayed reply. It sounds good.

--
regards,
-grygorii