[PATCH] drm: Don't force all planes to be added to the state due to zpos

[Daniel Vetter]

On Mon, Oct 10, 2016 at 2:19 PM,  <ville.syrjala at linux.intel.com> wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
>
> We don't want all planes to be added to the state whenever a
> plane with fixed zpos gets enabled/disabled. This is true
> especially for eg. cursor planes on i915, as we want cursor
> updates to go through w/o throttling. Same holds for drivers
> that don't support zpos at all (i915 actually falls into this
> category right now since we've not yet added zpos support).
>
> Allow drivers more freedom by letting them deal with zpos
> themselves instead of doing it in drm_atomic_helper_check_planes()
> unconditionally. Easiest solution seems to be to move the call
> up to drm_atomic_helper_check(). But as some drivers might want
> to use that function without the zpos handling, let's provide
> two variants: the normal one, and one that deals with zpos.
>
> Cc: Marek Szyprowski <m.szyprowski at samsung.com>
> Cc: Benjamin Gaignard <benjamin.gaignard at linaro.org>
> Cc: Vincent Abriou <vincent.abriou at st.com>
> Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Cc: Inki Dae <inki.dae at samsung.com>
> Cc: Joonyoung Shim <jy0922.shim at samsung.com>
> Cc: Seung-Woo Kim <sw0312.kim at samsung.com>
> Cc: Kyungmin Park <kyungmin.park at samsung.com>
> Cc: Lyude <cpaul at redhat.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: stable at vger.kernel.org
> Fixes: 44d1240d006c ("drm: add generic zpos property")
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Seems a bit fragile, and then drivers still need to not overshot when
they do zpos (which we want eventually in i915 too). I think the
proper way is to keep track of a per-plane zpos changed (or compute
that ad-hoc, we have both states). And only grab more planes if a zpos
value changed.

That would fix the issue at the source, also work for us in the
future, and it should be contained to just the helper function itself.
Win all around ;-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch