Re: [PATCH 1/4] drm/atomic: implement drm_atomic_helper_commit_tail for runtime_pm users

[Maxime Ripard <maxime.ripard@free-electrons.com>]

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Hi Laurent,

On Fri, Jul 14, 2017 at 02:43:12AM +0300, Laurent Pinchart wrote:
> Hi Maxime,
> 
> Thank you for the patch.
> 
> On Thursday 13 Jul 2017 16:41:13 Maxime Ripard wrote:
> > The current drm_atomic_helper_commit_tail helper works only if the CRTC is
> > accessible, and documents an alternative implementation that is supposed to
> > be used if that happens.
> > 
> > That implementation is then duplicated by some drivers. Instead of
> > documenting it, let's implement an helper that all the relevant users can
> > use directly.
> > 
> > Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
> > ---
> >  drivers/gpu/drm/drm_atomic_helper.c        | 47 +++++++++++++++--------
> >  drivers/gpu/drm/exynos/exynos_drm_fb.c     | 27 +-------------
> >  drivers/gpu/drm/rcar-du/rcar_du_kms.c      | 18 +---------
> 
> I've submitted "[PATCH] drm: rcar-du: Setup planes before enabling CRTC to 
> avoid flicker" that changes the rcar-du implementation to the standard 
> disable/update planes/enable order, so I'd appreciate if you could drop the 
> rcar-du part of this patch to avoid conflicts.

I will.

> This being said, the reason why I switched back from the "runtime PM" to the 
> "standard" order is probably of interest to you. Quoting the commit message,
> 
> > Commit 52055bafa1ff ("drm: rcar-du: Move plane commit code from CRTC
> > start to CRTC resume") changed the order of the plane commit and CRTC
> > enable operations to accommodate the runtime PM requirements. However,
> > this introduced corruption in the first displayed frame, as the CRTC is
> > now enabled without any plane configured. On Gen2 hardware the first
> > frame will be black and likely unnoticed, but on Gen3 hardware we end up
> > starting the display before the VSP compositor, which is more
> > noticeable.
> > 
> > To fix this, revert the order of the commit operations back, and handle
> > runtime PM requirements in the CRTC .atomic_begin() and .atomic_enable()
> > helper operation handlers.
> 
> I believe that the "runtime PM" order is problematic in most drivers. The 
> problem usually goes unnoticed as most monitors will not even display the 
> first frame, and I assume many devices will just output it black, but it's an 
> issue nonetheless.
> 
> Note that my driver hasn't lost the "runtime PM" requirements, so I had to 
> support them with the "standard" order. The best way I've found was to runtime 
> resume in the one of .atomic_begin() and .enable() that is run first. Not very 
> neat, as similar code would be needed in most drivers. I wonder whether it 
> wouldn't be useful to add resume/suspend helper callbacks for the CRTC.

I'm not sure it would apply. Our driver doesn't use runtime_pm at all,
but in order for the commits to happen, we need to have the CRTC
active, but it will remain powered up the whole time. I'm not sure if
we'll ever see such a frame.

But since this seems to be a pretty generic, maybe we should address
it in the helper itself?

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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