[PATCH] drm/atomic: clarify the rules around drm_atomic_state->allow_modeset

[Simona Vetter]

On Sun, Dec 22, 2024 at 06:47:18AM +0200, Dmitry Baryshkov wrote:
> Hello,
> 
> On Wed, 11 Oct 2023 at 12:20, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> >
> > msm is automagically upgrading normal commits to full modesets, and
> > that's a big no-no:
> >
> > - for one this results in full on->off->on transitions on all these
> >   crtc, at least if you're using the usual helpers. Which seems to be
> >   the case, and is breaking uapi
> >
> > - further even if the ctm change itself would not result in flicker,
> >   this can hide modesets for other reasons. Which again breaks the
> >   uapi
> >
> > v2: I forgot the case of adding unrelated crtc state. Add that case
> > and link to the existing kerneldoc explainers. This has come up in an
> > irc discussion with Manasi and Ville about intel's bigjoiner mode.
> > Also cc everyone involved in the msm irc discussion, more people
> > joined after I sent out v1.
> 
> I have been looking at the drm_atomic_helper_check_modeset() issue, I
> couldn't help but notice that this hasn't been committed.
> Maybe it's time for v3?

Oops. Working on it now.
-Sima

> 
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Cc: Maxime Ripard <mripard at kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann at suse.de>
> > Cc: David Airlie <airlied at gmail.com>
> > Cc: Daniel Vetter <daniel at ffwll.ch>
> > Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
> > Cc: Rob Clark <robdclark at gmail.com>
> > Cc: Simon Ser <contact at emersion.fr>
> > Cc: Manasi Navare <navaremanasi at google.com>
> > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > Cc: Abhinav Kumar <quic_abhinavk at quicinc.com>
> > Cc: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
> > Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> > ---
> >  include/drm/drm_atomic.h | 23 +++++++++++++++++++++--
> >  1 file changed, 21 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> > index cf8e1220a4ac..545c48545402 100644
> > --- a/include/drm/drm_atomic.h
> > +++ b/include/drm/drm_atomic.h
> > @@ -372,8 +372,27 @@ struct drm_atomic_state {
> >          *
> >          * Allow full modeset. This is used by the ATOMIC IOCTL handler to
> >          * implement the DRM_MODE_ATOMIC_ALLOW_MODESET flag. Drivers should
> > -        * never consult this flag, instead looking at the output of
> > -        * drm_atomic_crtc_needs_modeset().
> > +        * not consult this flag, instead looking at the output of
> > +        * drm_atomic_crtc_needs_modeset(). The detailed rules are:
> > +        *
> > +        * - Drivers must not consult @allow_modeset in the atomic commit path,
> > +        *   and instead use drm_atomic_crtc_needs_modeset().
> > +        *
> > +        * - Drivers may consult @allow_modeset in the atomic check path, if
> > +        *   they have the choice between an optimal hardware configuration
> > +        *   which requires a modeset, and a less optimal configuration which
> > +        *   can be committed without a modeset. An example would be suboptimal
> > +        *   scanout FIFO allocation resulting in increased idle power
> > +        *   consumption. This allows userspace to avoid flickering and delays
> > +        *   for the normal composition loop at reasonable cost.
> > +        *
> > +        * - Drivers must consult @allow_modeset before adding unrelated struct
> > +        *   drm_crtc_state to this commit by calling
> > +        *   drm_atomic_get_crtc_state(). See also the warning in the
> > +        *   documentation for that function.
> > +        *
> > +        * - Drivers must never change this flag, it is only under the control
> > +        *   of userspace.
> >          */
> >         bool allow_modeset : 1;
> >         /**
> > --
> > 2.40.1
> >
> 
> 
> -- 
> With best wishes
> Dmitry

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch