Re: [PATCH v2] drm/amd/display: don't ignore alpha property on pre-multiplied mode

[Rodrigo Siqueira Jordao <rjordrigo@amd.com>]

On 2022-03-29 16:18, Melissa Wen wrote:
> "Pre-multiplied" is the default pixel blend mode for KMS/DRM, as
> documented in supported_modes of drm_plane_create_blend_mode_property():
> https://cgit.freedesktop.org/drm/drm-misc/tree/drivers/gpu/drm/drm_blend.c
> 
> In this mode, both 'pixel alpha' and 'plane alpha' participate in the
> calculation, as described by the pixel blend mode formula in KMS/DRM
> documentation:
> 
> out.rgb = plane_alpha * fg.rgb +
>            (1 - (plane_alpha * fg.alpha)) * bg.rgb
> 
> Considering the blend config mechanisms we have in the driver so far,
> the alpha mode that better fits this blend mode is the
> _PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN, where the value for global_gain
> is the plane alpha (global_alpha).
> 
> With this change, alpha property stops to be ignored. It also addresses
> Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1734
> 
> v2:
>   * keep the 8-bit value for global_alpha_value (Nicholas)
>   * correct the logical ordering for combined global gain (Nicholas)
>   * apply to dcn10 too (Nicholas)
> 
> Signed-off-by: Melissa Wen <mwen@igalia.com>
> ---
>   .../drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c  | 14 +++++++++-----
>   drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c | 14 +++++++++-----
>   2 files changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c b/drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c
> index ad757b59e00e..b1034e6258c8 100644
> --- a/drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c
> +++ b/drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c
> @@ -2528,14 +2528,18 @@ void dcn10_update_mpcc(struct dc *dc, struct pipe_ctx *pipe_ctx)
>   	struct mpc *mpc = dc->res_pool->mpc;
>   	struct mpc_tree *mpc_tree_params = &(pipe_ctx->stream_res.opp->mpc_tree_params);
>   
> -	if (per_pixel_alpha)
> -		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA;
> -	else
> -		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA;
> -
>   	blnd_cfg.overlap_only = false;
>   	blnd_cfg.global_gain = 0xff;
>   
> +	if (per_pixel_alpha && pipe_ctx->plane_state->global_alpha) {
> +		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN;
> +		blnd_cfg.global_gain = pipe_ctx->plane_state->global_alpha_value;
> +	} else if (per_pixel_alpha) {
> +		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA;
> +	} else {
> +		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA;
> +	}
> +
>   	if (pipe_ctx->plane_state->global_alpha)
>   		blnd_cfg.global_alpha = pipe_ctx->plane_state->global_alpha_value;
>   	else
> diff --git a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
> index 4290eaf11a04..b627c41713cc 100644
> --- a/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
> +++ b/drivers/gpu/drm/amd/display/dc/dcn20/dcn20_hwseq.c
> @@ -2344,14 +2344,18 @@ void dcn20_update_mpcc(struct dc *dc, struct pipe_ctx *pipe_ctx)
>   	struct mpc *mpc = dc->res_pool->mpc;
>   	struct mpc_tree *mpc_tree_params = &(pipe_ctx->stream_res.opp->mpc_tree_params);
>   
> -	if (per_pixel_alpha)
> -		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA;
> -	else
> -		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA;
> -
>   	blnd_cfg.overlap_only = false;
>   	blnd_cfg.global_gain = 0xff;
>   
> +	if (per_pixel_alpha && pipe_ctx->plane_state->global_alpha) {
> +		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA_COMBINED_GLOBAL_GAIN;
> +		blnd_cfg.global_gain = pipe_ctx->plane_state->global_alpha_value;
> +	} else if (per_pixel_alpha) {
> +		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_PER_PIXEL_ALPHA;
> +	} else {
> +		blnd_cfg.alpha_mode = MPCC_ALPHA_BLEND_MODE_GLOBAL_ALPHA;
> +	}
> +

Hi Melissa,

Thanks a lot for this patch. I run your patch in our CI, and everything 
looks good from the IGT test result.

In this sense:
Tested-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>

However, I think it is still necessary to have someone else review.

Harry, Nick, or Zhan, what do you think about this change?

Thanks
Siqueira

>   	if (pipe_ctx->plane_state->global_alpha)
>   		blnd_cfg.global_alpha = pipe_ctx->plane_state->global_alpha_value;
>   	else