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Subject: Re: [PATCH v2 1/2] i2c: tegra: Better handle case where CPU0 is busy
 for a long time
To:     Thierry Reding <thierry.reding@gmail.com>
Cc:     Jon Hunter <jonathanh@nvidia.com>,
        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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 <fd1ca178-1ea3-851f-20a6-10bf00453ce3@nvidia.com>
 <a5734f19-254e-b6bc-e791-fa1ac63f11a4@gmail.com>
 <79f6560e-dbb5-0ae1-49f8-cf1cd95396ec@nvidia.com>
 <20200427074837.GC3451400@ulmo>
 <c1190858-eaea-8e94-b4d1-1cf28076c330@gmail.com>
 <20200427110033.GC3464906@ulmo>
 <3a06811c-02dc-ce72-ebef-78c3fc3f4f7c@gmail.com>
 <20200427151234.GE3464906@ulmo>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <1ab276cf-c2b0-e085-49d8-b8ce3dba8fbe@gmail.com>
Date:   Mon, 27 Apr 2020 18:18:34 +0300
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27.04.2020 18:12, Thierry Reding пишет:
> On Mon, Apr 27, 2020 at 05:21:30PM +0300, Dmitry Osipenko wrote:
>> 27.04.2020 14:00, Thierry Reding пишет:
>>> On Mon, Apr 27, 2020 at 12:52:10PM +0300, Dmitry Osipenko wrote:
>>>> 27.04.2020 10:48, Thierry Reding пишет:
>>>> ...
>>>>>> Maybe but all these other problems appear to have existed for sometime
>>>>>> now. We need to fix all, but for the moment we need to figure out what's
>>>>>> best for v5.7.
>>>>>
>>>>> To me it doesn't sound like we have a good handle on what exactly is
>>>>> going on here and we're mostly just poking around.
>>>>>
>>>>> And even if things weren't working quite properly before, it sounds to
>>>>> me like this patch actually made things worse.
>>>>
>>>> There is a plenty of time to work on the proper fix now. To me it sounds
>>>> like you're giving up on fixing the root of the problem, sorry.
>>>
>>> We're at -rc3 now and I haven't seen any promising progress in the last
>>> week. All the while suspend/resume is now broken on at least one board
>>> and that may end up hiding any other issues that could creep in in the
>>> meantime.
>>>
>>> Furthermore we seem to have a preexisting issue that may very well
>>> interfere with this patch, so I think the cautious thing is to revert
>>> for now and then fix the original issue first. We can always come back
>>> to this once everything is back to normal.
>>>
>>> Also, people are now looking at backporting this to v5.6. Unless we
>>> revert this from v5.7 it may get picked up for backports to other
>>> kernels and then I have to notify stable kernel maintainers that they
>>> shouldn't and they have to back things out again. That's going to cause
>>> a lot of wasted time for a lot of people.
>>>
>>> So, sorry, I disagree. I don't think we have "plenty of time".
>>
>> There is about a month now before the 5.7 release. It's a bit too early
>> to start the panic, IMO :)
> 
> There's no panic. A patch got merged and it broken something, so we
> revert it and try again. It's very much standard procedure.
> 
>> Jon already proposed a reasonable simple solution: to keep PCIe
>> regulators always-ON. In a longer run we may want to have I2C atomic
>> transfers supported for a late suspend phase.
> 
> That's not really a solution, though, is it? It's just papering over
> an issue that this patch introduced or uncovered. I'm much more in
> favour of fixing problems at the root rather than keep papering over
> until we loose track of what the actual problems are.

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.