linux-kernel - Re: [PATCH v2 8/8] drm/amdgpu: Call drm_atomic_helper

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Date: Tue, 18 Jun 2024 17:59:42 -0400
From: Alex Deucher <alexdeucher@...il.com>
To: Doug Anderson <dianders@...omium.org>
Cc: dri-devel@...ts.freedesktop.org, Maxime Ripard <mripard@...nel.org>, 
	Alex Deucher <alexander.deucher@....com>, Christian König <christian.koenig@....com>, 
	Xinhui Pan <Xinhui.Pan@....com>, André Almeida <andrealmeid@...lia.com>, 
	Aurabindo Pillai <aurabindo.pillai@....com>, Candice Li <candice.li@....com>, 
	Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>, 
	Hamza Mahfooz <hamza.mahfooz@....com>, Hawking Zhang <Hawking.Zhang@....com>, Le Ma <le.ma@....com>, 
	Lijo Lazar <lijo.lazar@....com>, Ma Jun <Jun.Ma2@....com>, 
	Mario Limonciello <mario.limonciello@....com>, Shashank Sharma <shashank.sharma@....com>, 
	Srinivasan Shanmugam <srinivasan.shanmugam@....com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	Victor Lu <victorchengchi.lu@....com>, amd-gfx@...ts.freedesktop.org, 
	chenxuebing <chenxb_99091@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 8/8] drm/amdgpu: Call drm_atomic_helper_shutdown() at
 shutdown time

On Tue, Jun 18, 2024 at 5:40 PM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
>
> On Mon, Jun 17, 2024 at 8:01 AM Alex Deucher <alexdeucher@...il.com> wrote:
> >
> > On Wed, Jun 12, 2024 at 6:37 PM Douglas Anderson <dianders@...omium.org> 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.
> > >
> > > 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.
> > >
> > > Suggested-by: Maxime Ripard <mripard@...nel.org>
> > > Cc: Alex Deucher <alexander.deucher@....com>
> > > Cc: Christian König <christian.koenig@....com>
> > > Cc: Xinhui Pan <Xinhui.Pan@....com>
> > > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > > ---
> > > This commit is only compile-time tested.
> > >
> > > ...and further, I'd say that this patch is more of a plea for help
> > > than a patch I think is actually right. I'm _fairly_ certain that
> > > drm/amdgpu needs this call at shutdown time but the logic is a bit
> > > hard for me to follow. I'd appreciate if anyone who actually knows
> > > what this should look like could illuminate me, or perhaps even just
> > > post a patch themselves!
> >
> > I'm not sure this patch makes sense or not.  The driver doesn't really
> > do a formal tear down in its shutdown routine, it just quiesces the
> > hardware.  What are the actual requirements of the shutdown function?
> > In the past when we did a full driver tear down in shutdown, it
> > delayed the shutdown sequence and users complained.
>
> The "inspiration" for this patch is to handle panels properly.
> Specifically, panels often have several power/enable signals going to
> them and often have requirements that these signals are powered off in
> the proper order with the proper delays between them. While we can't
> always do so when the system crashes / reboots in an uncontrolled way,
> panel manufacturers / HW Engineers get upset if we don't power things
> off properly during an orderly shutdown/reboot. When panels are
> powered off badly it can cause garbage on the screen and, so I've been
> told, can even cause long term damage to the panels over time.
>
> In Linux, some panel drivers have tried to ensure a proper poweroff of
> the panel by handling the shutdown() call themselves. However, this is
> ugly and panel maintainers want panel drivers to stop doing it. We
> have removed the code doing this from most panels now [1]. Instead the
> assumption is that the DRM modeset drivers should be calling
> drm_atomic_helper_shutdown() which will make sure panels get an
> orderly shutdown.
>
> For a lot more details, see the cover letter [2] which then contains
> links to even more discussions about the topic.
>
> [1] https://lore.kernel.org/r/20240605002401.2848541-1-dianders@chromium.org
> [2] https://lore.kernel.org/r/20240612222435.3188234-1-dianders@chromium.org

I don't think it's an issue.  We quiesce the hardware as if we were
about to suspend the system (e.g., S3).  For the display hardware we
call drm_atomic_helper_suspend() as part of that sequence.

Alex