[PATCH v2 3/9] drm/vkms: write/update the documentation for pixel conversion and pixel write functions

[Louis Chauvet <louis.chauvet@bootlin.com>]

Add some documentation on pixel conversion functions.
Update of outdated comments for pixel_write functions.

Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
---
 drivers/gpu/drm/vkms/vkms_composer.c |  4 +++
 drivers/gpu/drm/vkms/vkms_drv.h      | 13 ++++++++
 drivers/gpu/drm/vkms/vkms_formats.c  | 58 ++++++++++++++++++++++++++++++------
 3 files changed, 66 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index c6d9b4a65809..5b341222d239 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -189,6 +189,10 @@ static void blend(struct vkms_writeback_job *wb,
 
 	size_t crtc_y_limit = crtc_state->base.crtc->mode.vdisplay;
 
+	/*
+	 * The planes are composed line-by-line. It is a necessary complexity to avoid poor
+	 * blending performance.
+	 */
 	for (size_t y = 0; y < crtc_y_limit; y++) {
 		fill_background(&background_color, output_buffer);
 
diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
index b4b357447292..18086423a3a7 100644
--- a/drivers/gpu/drm/vkms/vkms_drv.h
+++ b/drivers/gpu/drm/vkms/vkms_drv.h
@@ -25,6 +25,17 @@
 
 #define VKMS_LUT_SIZE 256
 
+/**
+ * struct vkms_frame_info - structure to store the state of a frame
+ *
+ * @fb: backing drm framebuffer
+ * @src: source rectangle of this frame in the source framebuffer
+ * @dst: destination rectangle in the crtc buffer
+ * @map: see drm_shadow_plane_state@data
+ * @rotation: rotation applied to the source.
+ *
+ * @src and @dst should have the same size modulo the rotation.
+ */
 struct vkms_frame_info {
 	struct drm_framebuffer *fb;
 	struct drm_rect src, dst;
@@ -52,6 +63,8 @@ struct vkms_writeback_job {
  * vkms_plane_state - Driver specific plane state
  * @base: base plane state
  * @frame_info: data required for composing computation
+ * @pixel_read: function to read a pixel in this plane. The creator of a vkms_plane_state must
+ * ensure that this pointer is valid
  */
 struct vkms_plane_state {
 	struct drm_shadow_plane_state base;
diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
index 172830a3936a..cb7a49b7c8e7 100644
--- a/drivers/gpu/drm/vkms/vkms_formats.c
+++ b/drivers/gpu/drm/vkms/vkms_formats.c
@@ -9,6 +9,17 @@
 
 #include "vkms_formats.h"
 
+/**
+ * packed_pixels_offset() - Get the offset of the block containing the pixel at coordinates x/y
+ * in the first plane
+ *
+ * @frame_info: Buffer metadata
+ * @x: The x coordinate of the wanted pixel in the buffer
+ * @y: The y coordinate of the wanted pixel in the buffer
+ *
+ * The caller must be aware that this offset is not always a pointer to a pixel. If individual
+ * pixel values are needed, they have to be extracted from the resulting block.
+ */
 static size_t pixel_offset(const struct vkms_frame_info *frame_info, int x, int y)
 {
 	struct drm_framebuffer *fb = frame_info->fb;
@@ -17,12 +28,13 @@ static size_t pixel_offset(const struct vkms_frame_info *frame_info, int x, int
 			      + (x * fb->format->cpp[0]);
 }
 
-/*
- * packed_pixels_addr - Get the pointer to pixel of a given pair of coordinates
+/**
+ * packed_pixels_addr() - Get the pointer to the block containing the pixel at the given
+ * coordinates
  *
  * @frame_info: Buffer metadata
- * @x: The x(width) coordinate of the 2D buffer
- * @y: The y(Heigth) coordinate of the 2D buffer
+ * @x: The x(width) coordinate inside the plane
+ * @y: The y(height) coordinate inside the plane
  *
  * Takes the information stored in the frame_info, a pair of coordinates, and
  * returns the address of the first color channel.
@@ -53,6 +65,13 @@ static int get_x_position(const struct vkms_frame_info *frame_info, int limit, i
 	return x;
 }
 
+/*
+ * The following  functions take pixel data from the buffer and convert them to the format
+ * ARGB16161616 in out_pixel.
+ *
+ * They are used in the `vkms_compose_row` function to handle multiple formats.
+ */
+
 static void ARGB8888_to_argb_u16(u8 *src_pixels, struct pixel_argb_u16 *out_pixel)
 {
 	/*
@@ -145,12 +164,11 @@ void vkms_compose_row(struct line_buffer *stage_buffer, struct vkms_plane_state
 }
 
 /*
- * The following  functions take an line of argb_u16 pixels from the
- * src_buffer, convert them to a specific format, and store them in the
- * destination.
+ * The following functions take one argb_u16 pixel and convert it to a specific format. The
+ * result is stored in @dst_pixels.
  *
- * They are used in the `compose_active_planes` to convert and store a line
- * from the src_buffer to the writeback buffer.
+ * They are used in the `vkms_writeback_row` to convert and store a pixel from the src_buffer to
+ * the writeback buffer.
  */
 static void argb_u16_to_ARGB8888(u8 *dst_pixels, struct pixel_argb_u16 *in_pixel)
 {
@@ -216,6 +234,14 @@ static void argb_u16_to_RGB565(u8 *dst_pixels, struct pixel_argb_u16 *in_pixel)
 	*pixels = cpu_to_le16(r << 11 | g << 5 | b);
 }
 
+/**
+ * Generic loop for all supported writeback format. It is executed just after the blending to
+ * write a line in the writeback buffer.
+ *
+ * @wb: Job where to insert the final image
+ * @src_buffer: Line to write
+ * @y: Row to write in the writeback buffer
+ */
 void vkms_writeback_row(struct vkms_writeback_job *wb,
 			const struct line_buffer *src_buffer, int y)
 {
@@ -229,6 +255,13 @@ void vkms_writeback_row(struct vkms_writeback_job *wb,
 		wb->pixel_write(dst_pixels, &in_pixels[x]);
 }
 
+/**
+ * Retrieve the correct read_pixel function for a specific format.
+ * The returned pointer is NULL for unsupported pixel formats. The caller must ensure that the
+ * pointer is valid before using it in a vkms_plane_state.
+ *
+ * @format: 4cc of the format
+ */
 void *get_pixel_conversion_function(u32 format)
 {
 	switch (format) {
@@ -247,6 +280,13 @@ void *get_pixel_conversion_function(u32 format)
 	}
 }
 
+/**
+ * Retrieve the correct write_pixel function for a specific format.
+ * The returned pointer is NULL for unsupported pixel formats. The caller must ensure that the
+ * pointer is valid before using it in a vkms_writeback_job.
+ *
+ * @format: 4cc of the format
+ */
 void *get_pixel_write_function(u32 format)
 {
 	switch (format) {

-- 
2.43.0