Re: [PATCH v2 1/2] drm/vc4: ->x_scaling[1] should never be set to VC4_SCALING_NONE

[Boris Brezillon <boris.brezillon@bootlin.com>]

On Fri,  9 Nov 2018 11:26:32 +0100
Boris Brezillon <boris.brezillon@bootlin.com> wrote:

> For the YUV conversion to work properly, ->x_scaling[1] should never
> be set to VC4_SCALING_NONE, but vc4_get_scaling_mode() might return
> VC4_SCALING_NONE if the horizontal scaling ratio exactly matches the
> horizontal subsampling factor. Add a test to turn VC4_SCALING_NONE
> into VC4_SCALING_PPF when that happens.
> 
> The old ->x_scaling[0] adjustment is dropped as I couldn't find any
> mention to this constraint in the spec and it's proven to be
> unnecessary (I tested various multi-planar YUV formats with scaling
> disabled, and all of them worked fine without this adjustment).
> 
> Fixes: fc04023fafec ("drm/vc4: Add support for YUV planes.")
> Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
> Reviewed-by: Eric Anholt <eric@anholt.net>

Series queued to drm-misc-next.

> ---
> Changes in v2:
> - Remove the the ->x_scaling[0] = VC4_SCALING_PPF
>   assignment when ->is_unity is true and update the comment
> - Add Eric's R-b
> ---
>  drivers/gpu/drm/vc4/vc4_plane.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vc4/vc4_plane.c b/drivers/gpu/drm/vc4/vc4_plane.c
> index 98fae4daa08c..6caa678edcff 100644
> --- a/drivers/gpu/drm/vc4/vc4_plane.c
> +++ b/drivers/gpu/drm/vc4/vc4_plane.c
> @@ -341,12 +341,14 @@ static int vc4_plane_setup_clipping_and_scaling(struct drm_plane_state *state)
>  			vc4_get_scaling_mode(vc4_state->src_h[1],
>  					     vc4_state->crtc_h);
>  
> -		/* YUV conversion requires that horizontal scaling be enabled,
> -		 * even on a plane that's otherwise 1:1. Looks like only PPF
> -		 * works in that case, so let's pick that one.
> +		/* YUV conversion requires that horizontal scaling be enabled
> +		 * on the UV plane even if vc4_get_scaling_mode() returned
> +		 * VC4_SCALING_NONE (which can happen when the down-scaling
> +		 * ratio is 0.5). Let's force it to VC4_SCALING_PPF in this
> +		 * case.
>  		 */
> -		if (vc4_state->is_unity)
> -			vc4_state->x_scaling[0] = VC4_SCALING_PPF;
> +		if (vc4_state->x_scaling[1] == VC4_SCALING_NONE)
> +			vc4_state->x_scaling[1] = VC4_SCALING_PPF;
>  	} else {
>  		vc4_state->is_yuv = false;
>  		vc4_state->x_scaling[1] = VC4_SCALING_NONE;

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