[Intel-gfx] [PATCH] drm/atomic: refuse changing CRTC for planes directly

[Rob Clark]

On Wed, Aug 26, 2015 at 3:49 PM, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> Very strictly speaking this is possible if you have special hw and
> genlocked CRTCs. In general switching a plane between two active CRTC
> just won't work so well and is probably not tested at all. Just forbid
> it.
>
> I've put this into the core since right now no helper or driver copes
> with it, no userspace has code for it and no one asks for it. Yes
> there's piles of corner-cases where this would be possible to do this
> like:
> - switch from inactive crtc to active crtc
> - swithc from active crtc to inactive crtc
> - genlocked display
> - invisible plane (to do whatever)
> - idle plane hw due to dsi cmd mode/psr
> - whatever
> but looking at details it's not that easy to implement this correctly.
> Hence just put it into the core and add a comment, since the only
> userspace we have right now for atomic (weston) doesn't want to use
> direct plane switching either.

I suspect that eventually we'll want to make this a helper exposed to
drivers and push this check down into the drivers.. perhaps that is
not until weston (and/or atomic based hwc) grows driver specific
userspace plugin type API to make smarter hw specific decisions..
until then, this is probably the most reasonable thing to do.. so

(w/ s/swihc/switch/ fix smashed in above)

Reviewed-by: Rob Clark <robdclark at gmail.com>


> v2: don't bother with complexity and just outright disallow plane
> switching without the intermediate OFF state. Simplifies drivers, we
> don't have any hw that could do it anyway and current atomic userspace
> (weston) works like this already anyway.
>
> v3: Bikeshed function name (Ville) and add comment (Rob).
>
> v4: Also bikeshed commit message (Rob).
>
> Cc: Thierry Reding <thierry.reding at gmail.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Daniel Stone <daniels at collabora.com>
> Acked-by: Daniel Stone <daniels at collabora.com>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
>
> ---
>
> Imo this is bikeshedded enough, so either someone takes this or
> someone else can mangle this patch more.
> -Daniel
> ---
>  drivers/gpu/drm/drm_atomic.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index 1066e4b658cf..40ddd6aa100f 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -663,6 +663,27 @@ drm_atomic_plane_get_property(struct drm_plane *plane,
>         return 0;
>  }
>
> +static bool
> +plane_switching_crtc(struct drm_atomic_state *state,
> +                    struct drm_plane *plane,
> +                    struct drm_plane_state *plane_state)
> +{
> +       struct drm_crtc_state *crtc_state, *curr_crtc_state;
> +
> +       if (!plane->state->crtc || !plane_state->crtc)
> +               return false;
> +
> +       if (plane->state->crtc == plane_state->crtc)
> +               return false;
> +
> +       /* This could be refined, but currently there's no helper or driver code
> +        * to implement direct switching of active planes nor userspace to take
> +        * advantage of more direct plane switching without the intermediate
> +        * full OFF state.
> +        */
> +       return true;
> +}
> +
>  /**
>   * drm_atomic_plane_check - check plane state
>   * @plane: plane to check
> @@ -734,6 +755,12 @@ static int drm_atomic_plane_check(struct drm_plane *plane,
>                 return -ENOSPC;
>         }
>
> +       if (plane_switching_crtc(state->state, plane, state)) {
> +               DRM_DEBUG_ATOMIC("[PLANE:%d] switching CRTC directly\n",
> +                                plane->base.id);
> +               return -EINVAL;
> +       }
> +
>         return 0;
>  }
>
> --
> 2.5.0
>
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