[PATCH] drm/doc: Document legacy_cursor_update better

[Daniel Vetter]

Also cc Sean, who reviewed the msm patch to double down on the
not-so-awesome async solution (I think at least, I'm still not
entirely sure what's all going on there):

commit 2d99ced787e3d0f251fa370d2aae83cf2085a8d9
Author: Rob Clark <robdclark at chromium.org>
Date:   Thu Aug 29 09:45:16 2019 -0700

   drm/msm: async commit support

On Tue, Oct 20, 2020 at 4:39 PM Daniel Vetter <daniel.vetter at ffwll.ch> 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;
>         bool async_update : 1;
>         /**
> --
> 2.28.0
>


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