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Date:   Mon, 4 Sep 2023 09:36:10 +0200
From:   Maxime Ripard <mripard@kernel.org>
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc:     Douglas Anderson <dianders@chromium.org>,
        dri-devel@lists.freedesktop.org, airlied@gmail.com,
        daniel@ffwll.ch, linux-kernel@vger.kernel.org
Subject: Re: [RFT PATCH 01/15] drm/armada: Call drm_atomic_helper_shutdown()
 at shutdown time
Message-ID: <wi2j43vzk3t55xc6ylvbekxqybnc62fqpx7v277d5htcqxrtxf@qya4mbky2y7l>
References: <20230901234202.566951-1-dianders@chromium.org>
 <20230901164111.RFT.1.I3d5598bd73a59b5ded71430736c93f67dc5dea61@changeid>
 <ZPSsBhbekKY7VyDg@shell.armlinux.org.uk>
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On Sun, Sep 03, 2023 at 04:53:42PM +0100, Russell King (Oracle) wrote:
> On Fri, Sep 01, 2023 at 04:41:12PM -0700, Douglas Anderson wrote:
> > Based on grepping through the source code this driver appears to be
> > missing a call to drm_atomic_helper_shutdown() at system shutdown
> > time. Among other things, this means that if a panel is in use that it
> > won't be cleanly powered off at system shutdown time.
> >=20
> > The fact that we should call drm_atomic_helper_shutdown() in the case
> > of OS shutdown/restart comes straight out of the kernel doc "driver
> > instance overview" in drm_drv.c.
> >=20
> > This driver was fairly easy to update. The drm_device is stored in the
> > drvdata so we just have to make sure the drvdata is NULL whenever the
> > device is not bound.
>=20
> ... and there I think you have a misunderstanding of the driver model.
> Please have a look at device_unbind_cleanup() which will be called if
> probe fails, or when the device is removed (in other words, when it is
> not bound to a driver.)
>=20
> Also, devices which aren't bound to a driver won't have their shutdown
> method called (because there is no driver currently bound to that
> device.) So, ->probe must have completed successfully, and ->remove
> must not have been called for that device.
>=20
> So, I think that all these dev_set_drvdata(dev, NULL) that you're
> adding are just asking for a kernel janitor to come along later and
> remove them because they serve no purpose... so best not introduce
> them in the first place.

What would that hypothetical janitor clean up exactly? Code making sure
that there's no dangling pointer? Doesn't look very wise to me.

Maxime