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 10:14:48 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Dmitry Osipenko <digetx@...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

On Mon, Apr 27, 2020 at 06:18:34PM +0300, Dmitry Osipenko wrote:
> 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.

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.

Thierry

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