Re: [PATCH v5 08/16] drm/vkms: Avoid computing blending limits inside pre_mul_alpha_blend

[Pekka Paalanen]

On Wed, 13 Mar 2024 18:45:02 +0100
Louis Chauvet <louis.chauvet@xxxxxxxxxxx> wrote:

> The pre_mul_alpha_blend is dedicated to blending, so to avoid mixing
> different concepts (coordinate calculation and color management), extract
> the x_limit and x_dst computation outside of this helper.
> It also increases the maintainability by grouping the computation related
> to coordinates in the same place: the loop in `blend`.
> 
> Signed-off-by: Louis Chauvet <louis.chauvet@xxxxxxxxxxx>
> ---
>  drivers/gpu/drm/vkms/vkms_composer.c | 40 +++++++++++++++++-------------------
>  1 file changed, 19 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index da0651a94c9b..9254086f23ff 100644
> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> @@ -24,34 +24,30 @@ static u16 pre_mul_blend_channel(u16 src, u16 dst, u16 alpha)
>  
>  /**
>   * pre_mul_alpha_blend - alpha blending equation
> - * @frame_info: Source framebuffer's metadata
>   * @stage_buffer: The line with the pixels from src_plane
>   * @output_buffer: A line buffer that receives all the blends output
> + * @x_start: The start offset to avoid useless copy

I'd say just:

+ * @x_start: The start offset

It describes the parameter, and the paragraph below explains the why.

It would be explaining, that x_start applies to output_buffer, but
input_buffer is always read starting from 0.

> + * @count: The number of byte to copy

You named it pixel_count, and it counts pixels, not bytes. It's not a
copy but a blend into output_buffer.

>   *
> - * Using the information from the `frame_info`, this blends only the
> - * necessary pixels from the `stage_buffer` to the `output_buffer`
> - * using premultiplied blend formula.
> + * Using @x_start and @count information, only few pixel can be blended instead of the whole line
> + * each time.
>   *
>   * The current DRM assumption is that pixel color values have been already
>   * pre-multiplied with the alpha channel values. See more
>   * drm_plane_create_blend_mode_property(). Also, this formula assumes a
>   * completely opaque background.
>   */
> -static void pre_mul_alpha_blend(struct vkms_frame_info *frame_info,
> -				struct line_buffer *stage_buffer,
> -				struct line_buffer *output_buffer)
> +static void pre_mul_alpha_blend(const struct line_buffer *stage_buffer,
> +				struct line_buffer *output_buffer, int x_start, int pixel_count)
>  {
> -	int x_dst = frame_info->dst.x1;
> -	struct pixel_argb_u16 *out = output_buffer->pixels + x_dst;
> -	struct pixel_argb_u16 *in = stage_buffer->pixels;
> -	int x_limit = min_t(size_t, drm_rect_width(&frame_info->dst),
> -			    stage_buffer->n_pixels);
> -
> -	for (int x = 0; x < x_limit; x++) {
> -		out[x].a = (u16)0xffff;
> -		out[x].r = pre_mul_blend_channel(in[x].r, out[x].r, in[x].a);
> -		out[x].g = pre_mul_blend_channel(in[x].g, out[x].g, in[x].a);
> -		out[x].b = pre_mul_blend_channel(in[x].b, out[x].b, in[x].a);
> +	struct pixel_argb_u16 *out = &output_buffer->pixels[x_start];
> +	const struct pixel_argb_u16 *in = stage_buffer->pixels;
> +
> +	for (int i = 0; i < pixel_count; i++) {
> +		out[i].a = (u16)0xffff;
> +		out[i].r = pre_mul_blend_channel(in[i].r, out[i].r, in[i].a);
> +		out[i].g = pre_mul_blend_channel(in[i].g, out[i].g, in[i].a);
> +		out[i].b = pre_mul_blend_channel(in[i].b, out[i].b, in[i].a);
>  	}
>  }
>  
> @@ -183,7 +179,7 @@ static void blend(struct vkms_writeback_job *wb,
>  {
>  	struct vkms_plane_state **plane = crtc_state->active_planes;
>  	u32 n_active_planes = crtc_state->num_active_planes;
> -	int y_pos;
> +	int y_pos, x_dst, x_limit;
>  
>  	const struct pixel_argb_u16 background_color = { .a = 0xffff };
>  
> @@ -201,14 +197,16 @@ static void blend(struct vkms_writeback_job *wb,
>  
>  		/* The active planes are composed associatively in z-order. */
>  		for (size_t i = 0; i < n_active_planes; i++) {
> +			x_dst = plane[i]->frame_info->dst.x1;
> +			x_limit = min_t(size_t, drm_rect_width(&plane[i]->frame_info->dst),
> +					stage_buffer->n_pixels);

Are those input values to min_t() really of type size_t? Or why is
size_t here?

>  			y_pos = get_y_pos(plane[i]->frame_info, y);
>  
>  			if (!check_limit(plane[i]->frame_info, y_pos))
>  				continue;
>  
>  			vkms_compose_row(stage_buffer, plane[i], y_pos);
> -			pre_mul_alpha_blend(plane[i]->frame_info, stage_buffer,
> -					    output_buffer);
> +			pre_mul_alpha_blend(stage_buffer, output_buffer, x_dst, x_limit);

I thought it was a count, not a limit?

"Limit" sounds to me like "end", and end - start = count.

>  		}
>  
>  		apply_lut(crtc_state, output_buffer);
> 

The details aside, this is a good move.


Thanks,
pq

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