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Date:   Wed, 29 Apr 2020 18:24:34 +0200
From:   Thierry Reding <thierry.reding@gmail.com>
To:     Dmitry Osipenko <digetx@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
Subject: Re: [PATCH v2 1/2] i2c: tegra: Better handle case where CPU0 is busy
 for a long time
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On Wed, Apr 29, 2020 at 05:46:46PM +0300, Dmitry Osipenko wrote:
> 29.04.2020 16:57, Jon Hunter =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> >=20
> > On 29/04/2020 13:35, Dmitry Osipenko wrote:
> >> 29.04.2020 11:55, Thierry Reding =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> >> ...
> >>>>> 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 n=
ot
> >>>>> support atomic transfers at all, that proper solution won't be back=
portable.
> >>>>
> >>>> 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/2019121=
3134417.222720-1-thierry.reding@gmail.com/
> >>>>
> >>>> I also ran that through our test farm and I don't see any other issu=
es.
> >>>> 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 exactl=
y 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 susp=
end
> >>>> 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 fir=
st
> >>> 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.
> >=20
> > 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.
>=20
> 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.

Been there, done that:

	http://patchwork.ozlabs.org/project/linux-tegra/patch/20191128160314.23812=
49-2-thierry.reding@gmail.com/

Thierry

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