[Intel-gfx] [PATCH] drm/doc: Document legacy_cursor_update better

[Daniel Vetter]

On Tue, Oct 20, 2020 at 4:55 PM Thomas Zimmermann <tzimmermann at suse.de> wrote:
>
> Hi
>
> On 20.10.20 16:39, Daniel Vetter wrote:
> > It's the horror and shouldn't be used. Realized we're not clear on
> > this in a discussion with Rob about what msm is doing to better
> > support async commits.
> >
> > Cc: Rob Clark <robdclark at gmail.com>
> > 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 linux.ie>
> > Cc: Daniel Vetter <daniel at ffwll.ch>
> > ---
> >  include/drm/drm_atomic.h | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> > index d07c851d255b..413fd0ca56a8 100644
> > --- a/include/drm/drm_atomic.h
> > +++ b/include/drm/drm_atomic.h
> > @@ -308,7 +308,6 @@ struct __drm_private_objs_state {
> >   * struct drm_atomic_state - the global state object for atomic updates
> >   * @ref: count of all references to this state (will not be freed until zero)
> >   * @dev: parent DRM device
> > - * @legacy_cursor_update: hint to enforce legacy cursor IOCTL semantics
> >   * @async_update: hint for asynchronous plane update
> >   * @planes: pointer to array of structures with per-plane data
> >   * @crtcs: pointer to array of CRTC pointers
> > @@ -336,6 +335,17 @@ struct drm_atomic_state {
> >        * drm_atomic_crtc_needs_modeset().
> >        */
> >       bool allow_modeset : 1;
> > +     /**
> > +      * @legacy_cursor_update:
> > +      *
> > +      * Hint to enforce legacy cursor IOCTL semantics.
> > +      *
> > +      * WARNING: This is thoroughly broken and pretty much impossible to
> > +      * implement correctly. Drivers must ignore this and should instead
> > +      * implement &drm_plane_helper_funcs.atomic_async_check and
> > +      * &drm_plane_helper_funcs.atomic_async_commit hooks. New users of this
> > +      * flag are not allowed.
> > +      */
> >       bool legacy_cursor_update : 1;
>
> The text could also say why the field exists in the first place. If it's
> supposed to go away, you could add an item to the TODO list.

"Hint to enforce legacy cursor IOCTL semantics." is the purpose of
this thing. It was used way back as pretty shoddy approach to handling
cursor updates, but is slowly dying.

Good point on adding a todo item for this, I'll respin with that added.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch