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:   Mon, 27 Apr 2020 18:18:34 +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

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.