Re: [PATCH v6 09/17] drm/vkms: Introduce pixel_read_direction enum

[Pekka Paalanen]

On Tue, 09 Apr 2024 15:25:27 +0200
Louis Chauvet <louis.chauvet@xxxxxxxxxxx> wrote:

> The pixel_read_direction enum is useful to describe the reading direction
> in a plane. It avoids using the rotation property of DRM, which not
> practical to know the direction of reading.
> This patch also introduce two helpers, one to compute the
> pixel_read_direction from the DRM rotation property, and one to compute
> the step, in byte, between two successive pixel in a specific direction.
> 
> Signed-off-by: Louis Chauvet <louis.chauvet@xxxxxxxxxxx>
> ---
>  drivers/gpu/drm/vkms/vkms_composer.c | 42 ++++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/vkms/vkms_drv.h      | 11 ++++++++++
>  drivers/gpu/drm/vkms/vkms_formats.c  | 30 ++++++++++++++++++++++++++
>  3 files changed, 83 insertions(+)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index 45b111c74884..7c2e328c9510 100644
> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> @@ -159,6 +159,48 @@ static void apply_lut(const struct vkms_crtc_state *crtc_state, struct line_buff
>  	}
>  }
>  
> +/**
> + * direction_for_rotation() - Get the correct reading direction for a given rotation
> + *
> + * @rotation: Rotation to analyze. It correspond the field @frame_info.rotation.
> + *
> + * This function will use the @rotation setting of a source plane to compute the reading
> + * direction in this plane which correspond to a "left to right writing" in the CRTC.
> + * For example, if the buffer is reflected on X axis, the pixel must be read from right to left
> + * to be written from left to right on the CRTC.
> + */
> +static enum pixel_read_direction direction_for_rotation(unsigned int rotation)
> +{
> +	struct drm_rect tmp_a, tmp_b;
> +	int x, y;
> +
> +	/*
> +	 * The direction is computed by rotating the vector AB (top-left to top-right) in a
> +	 * 1x1 square.

Points A and B are depicted as zero-size rectangles on the CRTC.
The CRTC writing direction is from A to B. The plane reading direction
is discovered by inverse-transforming A and B.

(If you want, you can add that to the comment.)

> +	 */
> +
> +	tmp_a = DRM_RECT_INIT(0, 0, 0, 0);
> +	tmp_b = DRM_RECT_INIT(1, 0, 0, 0);
> +	drm_rect_rotate_inv(&tmp_a, 1, 1, rotation);
> +	drm_rect_rotate_inv(&tmp_b, 1, 1, rotation);
> +
> +	x = tmp_b.x1 - tmp_a.x1;
> +	y = tmp_b.y1 - tmp_a.y1;
> +
> +	if (x == 1)
> +		return READ_LEFT_TO_RIGHT;
> +	else if (x == -1)
> +		return READ_RIGHT_TO_LEFT;
> +	else if (y == 1)
> +		return READ_TOP_TO_BOTTOM;
> +	else if (y == -1)
> +		return READ_BOTTOM_TO_TOP;

I find this code practically obvious. Excellent!

If you want to be more strict, each condition could also require the
other component to be zero.

> +
> +
> +	WARN_ONCE(true, "The inverse of the rotation gives an incorrect direction.");
> +	return READ_LEFT_TO_RIGHT;
> +}
> +
>  /**
>   * blend - blend the pixels from all planes and compute crc
>   * @wb: The writeback frame buffer metadata
> diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> index 2e1a1b824a3c..16317b063c20 100644
> --- a/drivers/gpu/drm/vkms/vkms_drv.h
> +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> @@ -69,6 +69,17 @@ struct vkms_writeback_job {
>  	pixel_write_t pixel_write;
>  };
>  
> +/**
> + * enum pixel_read_direction - Enum used internaly by VKMS to represent a reading direction in a
> + * plane.
> + */
> +enum pixel_read_direction {
> +	READ_BOTTOM_TO_TOP,
> +	READ_TOP_TO_BOTTOM,
> +	READ_RIGHT_TO_LEFT,
> +	READ_LEFT_TO_RIGHT
> +};
> +
>  /**
>   * typedef pixel_read_t - These functions are used to read a pixel in the source frame,
>   * convert it to `struct pixel_argb_u16` and write it to @out_pixel.
> diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
> index 9a1400ad4db6..f76944874fe7 100644
> --- a/drivers/gpu/drm/vkms/vkms_formats.c
> +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> @@ -75,6 +75,36 @@ static void packed_pixels_addr(const struct vkms_frame_info *frame_info,
>  	*addr = (u8 *)frame_info->map[0].vaddr + offset;
>  }
>  
> +/**
> + * get_block_step_byte() - Common helper to compute the correct step value between each pixel block

This should be called get_block_step_bytes(). "Byte" sounds like it
returns a single byte.

> + * to read in a certain direction.
> + *
> + * @fb: Framebuffer to iter on
> + * @direction: Direction of the reading
> + * @plane_index: Plane to get the step from
> + *
> + * As the returned offset is the number of bytes between two consecutive blocks in a direction,

I'd call it "returned count" rather than "returned offset".

> + * the caller may have to read multiple pixel before using the next one (for example, to read from

..multiple pixels before using the next block

> + * left to right in a DRM_FORMAT_R1 plane, each block contains 8 pixels, so the step must be used
> + * only every 8 pixels.

Close parenthesis.

> + */
> +static int get_block_step_byte(struct drm_framebuffer *fb, enum pixel_read_direction direction,
> +			       int plane_index)
> +{
> +	switch (direction) {
> +	case READ_LEFT_TO_RIGHT:
> +		return fb->format->char_per_block[plane_index];
> +	case READ_RIGHT_TO_LEFT:
> +		return -fb->format->char_per_block[plane_index];
> +	case READ_TOP_TO_BOTTOM:
> +		return (int)fb->pitches[plane_index];
> +	case READ_BOTTOM_TO_TOP:
> +		return -(int)fb->pitches[plane_index];

I'm not sure if this is correct for formats with block_h > 1.

If a pitch is the theoretical count of bytes per line, then this should
return block_h * pitch. But I'm not exactly sure what is correct here.

Aside from this problem, looks good.


Thanks,
pq

> +	}
> +
> +	return 0;
> +}
> +
>  /**
>   * packed_pixels_addr_1x1() - Get the pointer to the block containing the pixel at the given
>   * coordinates
>

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