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

[Daniel Vetter]

On Tue, Jul 18, 2017 at 01:14:12PM +0300, Laurent Pinchart wrote:
> Hi Maxime,
> 
> On Tuesday 18 Jul 2017 09:05:22 Maxime Ripard wrote:
> > On Fri, Jul 14, 2017 at 02:43:12AM +0300, Laurent Pinchart wrote:
> > > 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@xxxxxxxxxxxxxxxxxx>
> > >> ---
> > >> 
> > >>  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?
> 
> I think that would make sense.
> 
> There are a few options that result in too many combinations for separate 
> commit tail helpers to be provided in my opinion:
> 
> - disable/enable/planes vs. disable/planes/enable
> - DRM_PLANE_COMMIT_ACTIVE_ONLY vs. all CRTCs
> - drm_atomic_helper_wait_for_vblanks vs drm_atomic_helper_wait_for_flip_done
> 
> Maybe we could add a few CRTC commit helper flags along the line of 
> DRM_PLANE_COMMIT_ACTIVE_ONLY, add a field to the drm_crtc structure to store 
> them, and have drm_atomic_helper_commit_tail() use those flags to control the 
> sequence of operations.

Why not write your own? Yes it's a bit of copypaste, but imo that's really
not horrible. I'm already not happy with the flags for commit_planes
because the docs for them are not great and it's hard to know when to use
them and when not to.

->commit_tail was specifically done to allow drivers to overwrite the hw
commit stage without having to reinvent all the other commit tracking. I
expect most non-simple drivers to have their own commit_tail function.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch