linux-kernel - Re: [PATCH v2 1/2] i2c: tegra: Better handle case where CPU0 is busy for a long time

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Date:   Wed, 29 Apr 2020 19:54:49 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Jon Hunter <jonathanh@...dia.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Manikanta Maddireddy <mmaddireddy@...dia.com>,
        Vidya Sagar <vidyas@...dia.com>, linux-i2c@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] i2c: tegra: Better handle case where CPU0 is busy
 for a long time

29.04.2020 19:30, Thierry Reding пишет:
> On Wed, Apr 29, 2020 at 03:35:26PM +0300, Dmitry Osipenko wrote:
>> 29.04.2020 11:55, Thierry Reding пишет:
>> ...
>>>>> It's not "papering over an issue". The bug can't be fixed properly
>>>>> without introducing I2C atomic transfers support for a late suspend
>>>>> phase, I don't see any other solutions for now. Stable kernels do not
>>>>> support atomic transfers at all, that proper solution won't be backportable.
>>>>
>>>> Hm... on a hunch I tried something and, lo and behold, it worked. I can
>>>> get Cardhu to properly suspend/resume on top of v5.7-rc3 with the
>>>> following sequence:
>>>>
>>>> 	revert 9f42de8d4ec2 i2c: tegra: Fix suspending in active runtime PM state
>>>> 	apply http://patchwork.ozlabs.org/project/linux-tegra/patch/20191213134417.222720-1-thierry.reding@gmail.com/
>>>>
>>>> I also ran that through our test farm and I don't see any other issues.
>>>> At the time I was already skeptical about pm_runtime_force_suspend() and
>>>> pm_runtime_force_resume() and while I'm not fully certain why exactly it
>>>> doesn't work, the above on top of v5.7-rc3 seems like a good option.
>>>>
>>>> I'll try to do some digging if I can find out why exactly force suspend
>>>> and resume doesn't work.
>>>
>>> Ah... so it looks like pm_runtime_force_resume() never actually does
>>> anything in this case and then disable_depth remains at 1 and the first
>>> tegra_i2c_xfer() will then fail to runtime resume the controller.
>>
>> That's the exactly expected behaviour of the RPM force suspend/resume.
>> The only unexpected part for me is that the tegra_i2c_xfer() runtime
>> resume then fails in the NOIRQ phase.
>>
>> Anyways, again, today it's wrong to use I2C in the NOIRQ phase becasue
>> I2C interrupt is disabled. It's the PCIe driver that should be fixed.
> 
> I don't think so. Everything works perfectly fine if we fix system
> suspend/resume not to rely on pm_runtime_force_{suspend,resume}() and
> directly call the runtime suspend/resume implementations.

It should "work" only in conjunction with my I2C patch, otherwise you'll
get a spurious I2C timeout error. And it will "work" only because
interrupt is handled manually after the timeout, meaning that yours
suspending time will take few hundreds ms more.

> So can we please stop deflecting and fix the damn I2C driver. From my
> perspective we have two choices:
> 
>   1) do what I suggested above and revert the force suspend/resume patch
>      and apply the "manual" suspend/resume patch instead
> 
>   2) revert this patch and go back to the drawing board
> 
> I suspect that with 2) we'd end up back where we started and have to do
> 1) anyway.
> 
> An alternative that I'd prefer even more would be to do 2) now for v5.7
> and then we do 1) for v5.8 and give this some more soaking time.

I2C driver isn't broken, PCIe driver is. IMO.

Both yours variants are not going to be a backportable fix for the
stable kernels, they won't fix the suspended interrupt problem. What I'm
missing?