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.

Acked-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
---
 drivers/gpu/drm/vkms/vkms_composer.c | 44 ++++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/vkms/vkms_drv.h      | 11 +++++++++
 drivers/gpu/drm/vkms/vkms_formats.c  | 32 ++++++++++++++++++++++++++
 3 files changed, 87 insertions(+)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index ecac0bc858a0..601e33431b45 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -159,6 +159,50 @@ 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;
+
+	/*
+	 * 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.
+	 * The reading direction is computed by rotating the vector AB (top-left to top-right) in a
+	 * 1x1 square.
+	 */
+
+	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 && y == 0)
+		return READ_LEFT_TO_RIGHT;
+	else if (x == -1 && y == 0)
+		return READ_RIGHT_TO_LEFT;
+	else if (y == 1 && x == 0)
+		return READ_TOP_TO_BOTTOM;
+	else if (y == -1 && x == 0)
+		return READ_BOTTOM_TO_TOP;
+
+	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 3f45290a0c5d..777b7bd91f27 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 7f932d42394d..d0e7dfc1f0d3 100644
--- a/drivers/gpu/drm/vkms/vkms_formats.c
+++ b/drivers/gpu/drm/vkms/vkms_formats.c
@@ -79,6 +79,38 @@ static void packed_pixels_addr(const struct vkms_frame_info *frame_info,
 	*addr = (u8 *)frame_info->map[0].vaddr + offset;
 }
 
+/**
+ * get_block_step_bytes() - Common helper to compute the correct step value between each pixel block
+ * 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 count is the number of bytes between two consecutive blocks in a direction,
+ * the caller may have to read multiple pixels before using the next one (for example, to read from
+ * left to right in a DRM_FORMAT_R1 plane, each block contains 8 pixels, so the step must be used
+ * only every 8 pixels).
+ */
+static int get_block_step_bytes(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] * drm_format_info_block_width(fb->format,
+										   plane_index);
+	case READ_BOTTOM_TO_TOP:
+		return -(int)fb->pitches[plane_index] * drm_format_info_block_width(fb->format,
+										    plane_index);
+	}
+
+	return 0;
+}
+
 /**
  * packed_pixels_addr_1x1() - Get the pointer to the block containing the pixel at the given
  * coordinates

-- 
2.46.2

> 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.
> 
> Acked-by: Maíra Canal <mairacanal@riseup.net>
> Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
> ---
>  drivers/gpu/drm/vkms/vkms_composer.c | 44 ++++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/vkms/vkms_drv.h      | 11 +++++++++
>  drivers/gpu/drm/vkms/vkms_formats.c  | 32 ++++++++++++++++++++++++++
>  3 files changed, 87 insertions(+)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index ecac0bc858a0..601e33431b45 100644
> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> @@ -159,6 +159,50 @@ 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;
> +
> +	/*
> +	 * 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.
> +	 * The reading direction is computed by rotating the vector AB (top-left to top-right) in a
> +	 * 1x1 square.
> +	 */
> +
> +	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 && y == 0)
> +		return READ_LEFT_TO_RIGHT;
> +	else if (x == -1 && y == 0)
> +		return READ_RIGHT_TO_LEFT;
> +	else if (y == 1 && x == 0)
> +		return READ_TOP_TO_BOTTOM;
> +	else if (y == -1 && x == 0)
> +		return READ_BOTTOM_TO_TOP;
> +
> +	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 3f45290a0c5d..777b7bd91f27 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

Minor typo:

s/internaly/internally

Everything else looks great, thanks!

> + * 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 7f932d42394d..d0e7dfc1f0d3 100644
> --- a/drivers/gpu/drm/vkms/vkms_formats.c
> +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> @@ -79,6 +79,38 @@ static void packed_pixels_addr(const struct vkms_frame_info *frame_info,
>  	*addr = (u8 *)frame_info->map[0].vaddr + offset;
>  }
>  
> +/**
> + * get_block_step_bytes() - Common helper to compute the correct step value between each pixel block
> + * 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 count is the number of bytes between two consecutive blocks in a direction,
> + * the caller may have to read multiple pixels before using the next one (for example, to read from
> + * left to right in a DRM_FORMAT_R1 plane, each block contains 8 pixels, so the step must be used
> + * only every 8 pixels).
> + */
> +static int get_block_step_bytes(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] * drm_format_info_block_width(fb->format,
> +										   plane_index);
> +	case READ_BOTTOM_TO_TOP:
> +		return -(int)fb->pitches[plane_index] * drm_format_info_block_width(fb->format,
> +										    plane_index);
> +	}
> +
> +	return 0;
> +}
> +
>  /**
>   * packed_pixels_addr_1x1() - Get the pointer to the block containing the pixel at the given
>   * coordinates
>