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

[Jon Hunter]

On 29/04/2020 13:35, Dmitry Osipenko wrote:
> 29.04.2020 11:55, Thierry Reding ÐÐÑÐÑ:
> ...
>>>> 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@xxxxxxxxx/
>>>
>>> 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.

>From reading the changelog for commit 1e2ef05bb8cf ("PM: Limit race
conditions between runtime PM and system sleep (v2))", this is the
expected behaviour for runtime resume in the noirq phase.

Jon

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