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Subject: Re: [PATCH v2 1/2] i2c: tegra: Better handle case where CPU0 is busy
 for a long time
To:     Jon Hunter <jonathanh@nvidia.com>,
        Thierry Reding <thierry.reding@gmail.com>
Cc:     Wolfram Sang <wsa@the-dreams.de>,
        Laxman Dewangan <ldewangan@nvidia.com>,
        Manikanta Maddireddy <mmaddireddy@nvidia.com>,
        Vidya Sagar <vidyas@nvidia.com>, linux-i2c@vger.kernel.org,
        linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org
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From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <5863e364-480e-7839-c42b-73a7f6990a30@gmail.com>
Date:   Wed, 29 Apr 2020 17:46:46 +0300
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29.04.2020 16:57, Jon Hunter пишет:
> 
> On 29/04/2020 13:35, 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.
> 
> 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.

I'm curious whether there is a way to tell RPM that it's okay to do it
for a particular device, like I2C that uses IRQ-safe RPM + doesn't have
parent devices that need to be resumed.