Re: [PATCH v2 15/19] drm/sun4i: backend: Check for the number of alpha planes

[Chen-Yu Tsai]

On Mon, Jan 22, 2018 at 6:35 PM, Maxime Ripard
<maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote:
> Due to the way the composition is done in hardware, we can only have a
> single alpha-enabled plane active at a time, placed in the second (highest
> priority) pipe.
>
> Make sure of that in our atomic_check to not end up in an impossible
> scenario.
>
> Signed-off-by: Maxime Ripard <maxime.ripard@xxxxxxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/sun4i/sun4i_backend.c | 50 ++++++++++++++++++++++++++++-
>  drivers/gpu/drm/sun4i/sun4i_backend.h |  2 +-
>  drivers/gpu/drm/sun4i/sun4i_layer.c   | 23 +-------------
>  3 files changed, 53 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/gpu/drm/sun4i/sun4i_backend.c b/drivers/gpu/drm/sun4i/sun4i_backend.c
> index c4986054909b..eb1749d2c0d5 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_backend.c
> +++ b/drivers/gpu/drm/sun4i/sun4i_backend.c
> @@ -329,6 +329,8 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
>         struct drm_atomic_state *state = crtc_state->state;
>         struct drm_device *drm = state->dev;
>         struct drm_plane *plane;
> +       unsigned int num_planes = 0;
> +       unsigned int num_alpha_planes = 0;
>         unsigned int num_frontend_planes = 0;
>
>         DRM_DEBUG_DRIVER("Starting checking our planes\n");
> @@ -341,6 +343,7 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
>                         drm_atomic_get_plane_state(state, plane);
>                 struct sun4i_layer_state *layer_state =
>                         state_to_sun4i_layer_state(plane_state);
> +               struct drm_framebuffer *fb = plane_state->fb;
>
>                 if (sun4i_backend_plane_uses_frontend(plane_state)) {
>                         DRM_DEBUG_DRIVER("Using the frontend for plane %d\n",
> @@ -351,6 +354,50 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
>                 } else {
>                         layer_state->uses_frontend = false;
>                 }
> +
> +               DRM_DEBUG_DRIVER("Plane FB format is %s\n",
> +                                drm_get_format_name(fb->format->format,
> +                                                    &format_name));
> +               if (fb->format->has_alpha)
> +                       num_alpha_planes++;
> +
> +               num_planes++;
> +       }
> +
> +       /*
> +        * The hardware is a bit unusual here.
> +        *
> +        * Even though it supports 4 layers, it does the composition
> +        * in two separate steps.
> +        *
> +        * The first one is assigning a layer to one of its two
> +        * pipes. If more that 1 layer is assigned to the same pipe,
> +        * and if pixels overlaps, the pipe will take the pixel from
> +        * the layer with the highest priority.
> +        *
> +        * The second step is the actual alpha blending, that takes
> +        * the two pipes as input, and uses the eventual alpha
> +        * component to do the transparency between the two.
> +        *
> +        * This two steps scenario makes us unable to guarantee a
> +        * robust alpha blending between the 4 layers in all
> +        * situations, since this means that we need to have one layer
> +        * with alpha at the lowest position of our two pipes.
> +        *
> +        * However, we cannot even do that, since the hardware has a
> +        * bug where the lowest plane of the lowest pipe (pipe 0,
> +        * priority 0), if it has any alpha, will discard the pixel
> +        * entirely and just display the pixels in the background
> +        * color (black by default).
> +        *
> +        * Since means that we effectively have only three valid

             ^^^^^ This

> +        * configurations with alpha, all of them with the alpha being
> +        * on pipe1 with the lowest position, which can be 1, 2 or 3
> +        * depending on the number of planes and their zpos.
> +        */
> +       if (num_alpha_planes > SUN4I_BACKEND_NUM_ALPHA_LAYERS) {
> +               DRM_DEBUG_DRIVER("Too many planes with alpha, rejecting...\n");
> +               return -EINVAL;
>         }
>
>         if (num_frontend_planes > SUN4I_BACKEND_NUM_FRONTEND_LAYERS) {
> @@ -358,6 +405,9 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
>                 return -EINVAL;
>         }
>
> +       DRM_DEBUG_DRIVER("State valid with %u planes, %u alpha, %u video\n",
> +                        num_planes, num_alpha_planes, num_frontend_planes);
> +
>         return 0;
>  }
>
> diff --git a/drivers/gpu/drm/sun4i/sun4i_backend.h b/drivers/gpu/drm/sun4i/sun4i_backend.h
> index 04a4f11b87a8..52e77591186a 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_backend.h
> +++ b/drivers/gpu/drm/sun4i/sun4i_backend.h
> @@ -146,6 +146,8 @@
>  #define SUN4I_BACKEND_HWCCOLORTAB_OFF          0x4c00
>  #define SUN4I_BACKEND_PIPE_OFF(p)              (0x5000 + (0x400 * (p)))
>
> +#define SUN4I_BACKEND_NUM_LAYERS               4
> +#define SUN4I_BACKEND_NUM_ALPHA_LAYERS         1
>  #define SUN4I_BACKEND_NUM_FRONTEND_LAYERS      1
>
>  struct sun4i_backend {
> diff --git a/drivers/gpu/drm/sun4i/sun4i_layer.c b/drivers/gpu/drm/sun4i/sun4i_layer.c
> index fbf25d59cf88..900e716443b8 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_layer.c
> +++ b/drivers/gpu/drm/sun4i/sun4i_layer.c
> @@ -201,32 +201,11 @@ struct drm_plane **sun4i_layers_init(struct drm_device *drm,
>         struct sun4i_backend *backend = engine_to_sun4i_backend(engine);
>         int i;
>
> -       planes = devm_kcalloc(drm->dev, ARRAY_SIZE(sun4i_backend_planes) + 1,
> +       planes = devm_kcalloc(drm->dev, SUN4I_BACKEND_NUM_LAYERS,
>                               sizeof(*planes), GFP_KERNEL);

The returned "planes" array has to have a sentinel at the end, as we
never return the actual number of layers created. That is what the +1
was for in the original code.

Otherwise,

Reviewed-by: Chen-Yu Tsai <wens@xxxxxxxx>

>         if (!planes)
>                 return ERR_PTR(-ENOMEM);
>
> -       /*
> -        * The hardware is a bit unusual here.
> -        *
> -        * Even though it supports 4 layers, it does the composition
> -        * in two separate steps.
> -        *
> -        * The first one is assigning a layer to one of its two
> -        * pipes. If more that 1 layer is assigned to the same pipe,
> -        * and if pixels overlaps, the pipe will take the pixel from
> -        * the layer with the highest priority.
> -        *
> -        * The second step is the actual alpha blending, that takes
> -        * the two pipes as input, and uses the eventual alpha
> -        * component to do the transparency between the two.
> -        *
> -        * This two steps scenario makes us unable to guarantee a
> -        * robust alpha blending between the 4 layers in all
> -        * situations. So we just expose two layers, one per pipe. On
> -        * SoCs that support it, sprites could fill the need for more
> -        * layers.
> -        */
>         for (i = 0; i < ARRAY_SIZE(sun4i_backend_planes); i++) {
>                 const struct sun4i_plane_desc *plane = &sun4i_backend_planes[i];
>                 struct sun4i_layer *layer;
> --
> git-series 0.9.1