linux-kernel - Re: [PATCH v1 2/3] media: cedrus: Set the platform driver data earlier

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Date:   Sun, 21 Aug 2022 23:40:21 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Jernej Škrabec <jernej.skrabec@...il.com>,
        linux-media@...r.kernel.org, Maxime Ripard <mripard@...nel.org>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Samuel Holland <samuel@...lland.org>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>
Cc:     kernel@...labora.com, stable@...r.kernel.org,
        linux-staging@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/3] media: cedrus: Set the platform driver data
 earlier

On 8/20/22 11:25, Jernej Škrabec wrote:
> Dne petek, 19. avgust 2022 ob 17:37:20 CEST je Nicolas Dufresne napisal(a):
>> Le vendredi 19 août 2022 à 06:17 +0200, Jernej Škrabec a écrit :
>>> Dne četrtek, 18. avgust 2022 ob 22:33:07 CEST je Nicolas Dufresne 
> napisal(a):
>>>> From: Dmitry Osipenko <dmitry.osipenko@...labora.com>
>>>>
>>>> The cedrus_hw_resume() crashes with NULL deference on driver probe if
>>>> runtime PM is disabled because it uses platform data that hasn't been
>>>> set up yet. Fix this by setting the platform data earlier during probe.
>>>
>>> Does it even work without PM? Maybe it would be better if Cedrus would
>>> select PM in Kconfig.
>>
>> I cannot comment myself on this, but it does not seem to invalidate this
>> Dmitry's fix.
> 
> If NULL pointer dereference happens only when PM is disabled, then it does. I 
> have PM always enabled and I never experienced above issue.

Originally this fix was needed when I changed cedrus_hw_probe() to do
the rpm-resume while was debugging the hang issue and then also noticed
that the !PM should be broken. It's a good common practice for all
drivers to have the drv data set early to avoid such problems, hence it
won't hurt to make this change anyways.

In practice nobody disables PM other than for debugging purposes and !PM
handling is often broken in drivers. I assume that it might be even
better to enable PM for all Allwiner SoCs and remove !PM handling from
all the affected drivers, like it was done for Tegra some time ago.

-- 
Best regards,
Dmitry