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Date: Tue, 26 Mar 2024 16:57:00 +0100
From: Louis Chauvet <louis.chauvet@bootlin.com>
To: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>,
	Melissa Wen <melissa.srw@gmail.com>,
	=?iso-8859-1?Q?Ma=EDra?= Canal <mairacanal@riseup.net>,
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	Daniel Vetter <daniel@ffwll.ch>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
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Subject: Re: [PATCH v5 08/16] drm/vkms: Avoid computing blending limits
 inside pre_mul_alpha_blend
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 <20240325144101.6d9fcf7e.pekka.paalanen@collabora.com>
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Le 25/03/24 - 14:41, Pekka Paalanen a ?crit :
> On Wed, 13 Mar 2024 18:45:02 +0100
> Louis Chauvet <louis.chauvet@bootlin.com> 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@bootlin.com>
> > ---
> >  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.

I will change it to:

 * Using @x_start and @count information, only few pixel can be blended instead of the whole line
 * each time. @x_start is only used for the output buffer. The staging buffer is always read from
 * the start (0..@count in stage_buffer is blended at @x_start..@x_start+@count in output_buffer).

> > + * @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.

Oops, fixed in v6.
 
> >   *
> > - * 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?

n_pixel is size_t, drm_rect_width is int. I will change everything to int. 
Is there a way to ask the compiler "please don't do implicit conversion 
and report them as warn/errors"?

> >  			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.

It is effectively a pixel count. I just took those naming from the 
original pre_mul_alpha_blend. I will change it to pixel_count.

Thanks,
Louis Chauvet

> >  		}
> >  
> >  		apply_lut(crtc_state, output_buffer);
> > 
> 
> The details aside, this is a good move.
> 
> 
> Thanks,
> pq


-- 
Louis Chauvet, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com