diff mbox series

[v8,03/17] drm/vkms: write/update the documentation for pixel conversion and pixel write functions

Message ID 20240516-yuv-v8-3-cf8d6f86430e@bootlin.com (mailing list archive)
State New, archived
Headers show
Series drm/vkms: Reimplement line-per-line pixel conversion for plane reading | expand

Commit Message

Louis Chauvet May 16, 2024, 1:04 p.m. UTC 
Add some documentation on pixel conversion functions.
Update of outdated comments for pixel_write functions.

Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
---
 drivers/gpu/drm/vkms/vkms_composer.c |  7 ++++
 drivers/gpu/drm/vkms/vkms_drv.h      | 15 ++++++++-
 drivers/gpu/drm/vkms/vkms_formats.c  | 62 ++++++++++++++++++++++++++++++------
 3 files changed, 74 insertions(+), 10 deletions(-)

Comments

Maíra Canal July 13, 2024, 2:35 p.m. UTC | #1 
On 5/16/24 10:04, Louis Chauvet wrote:
> Add some documentation on pixel conversion functions.
> Update of outdated comments for pixel_write functions.
> 
> Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
> Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
> ---
>   drivers/gpu/drm/vkms/vkms_composer.c |  7 ++++
>   drivers/gpu/drm/vkms/vkms_drv.h      | 15 ++++++++-
>   drivers/gpu/drm/vkms/vkms_formats.c  | 62 ++++++++++++++++++++++++++++++------
>   3 files changed, 74 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index c6d9b4a65809..da0651a94c9b 100644
> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> @@ -189,6 +189,13 @@ 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 to avoid heavy memory usage. It is a necessary
> +	 * complexity to avoid poor blending performance.
> +	 *
> +	 * The function vkms_compose_row is used to read a line, pixel-by-pixel, into the staging

Nit: I know it's not kerneldoc, but I'd be glad if you use
vkms_compose_row()

> +	 * buffer.
> +	 */
>   	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..212f4ab6a71f 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

s/structure/Structure

> + *
> + * @fb: backing drm framebuffer
> + * @src: source rectangle of this frame in the source framebuffer, stored in 16.16 fixed-point form
> + * @dst: destination rectangle in the crtc buffer, stored in whole pixel units
> + * @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;
> @@ -49,9 +60,11 @@ struct vkms_writeback_job {
>   };
>   
>   /**
> - * vkms_plane_state - Driver specific plane state
> + * struct 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

s/vkms_plane_state/struct vkms_plane_state

> + * ensure that this pointer is valid

Note: "If the @argument description has multiple lines, the continuation
of the description should start at the same column as the previous
line:" [1]

[1] https://docs.kernel.org/doc-guide/kernel-doc.html#function-parameters

>    */
>   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 d597c48452ac..f157c43da4d6 100644
> --- a/drivers/gpu/drm/vkms/vkms_formats.c
> +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> @@ -9,6 +9,18 @@
>   
>   #include "vkms_formats.h"
>   
> +/**
> + * pixel_offset() - Get the offset of 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 ensure that the framebuffer associated with this request uses a pixel format
> + * where block_h == block_w == 1.
> + * If this requirement is not fulfilled, the resulting offset can point to an other pixel or
> + * outside of the buffer.
> + */
>   static size_t pixel_offset(const struct vkms_frame_info *frame_info, int x, int y)
>   {
>   	struct drm_framebuffer *fb = frame_info->fb;
> @@ -16,18 +28,22 @@ static size_t pixel_offset(const struct vkms_frame_info *frame_info, int x, int
>   	return fb->offsets[0] + (y * fb->pitches[0]) + (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.
>    * This function assumes the channels are packed together, i.e. a color channel
>    * comes immediately after another in the memory. And therefore, this function
>    * doesn't work for YUV with chroma subsampling (e.g. YUV420 and NV21).
> + *
> + * The caller must ensure that the framebuffer associated with this request uses a pixel format
> + * where block_h == block_w == 1, otherwise the returned pointer can be outside the buffer.
>    */
>   static void *packed_pixels_addr(const struct vkms_frame_info *frame_info,
>   				int x, int y)
> @@ -52,6 +68,13 @@ 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.

I believe it is @out_pixel.

The content seems great to me, only those minor nits.

Best Regards,
- Maíra

> + *
> + * 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)
>   {
>   	/*
> @@ -144,12 +167,11 @@ 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 &struct pixel_argb_u16 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 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)
>   {
> @@ -215,6 +237,14 @@ static void argb_u16_to_RGB565(u8 *dst_pixels, struct pixel_argb_u16 *in_pixel)
>   	*pixels = cpu_to_le16(r << 11 | g << 5 | b);
>   }
>   
> +/**
> + * vkms_writeback_row() - 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)
>   {
> @@ -228,6 +258,13 @@ void vkms_writeback_row(struct vkms_writeback_job *wb,
>   		wb->pixel_write(dst_pixels, &in_pixels[x]);
>   }
>   
> +/**
> + * get_pixel_conversion_function() - 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: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
> + */
>   void *get_pixel_conversion_function(u32 format)
>   {
>   	switch (format) {
> @@ -246,6 +283,13 @@ void *get_pixel_conversion_function(u32 format)
>   	}
>   }
>   
> +/**
> + * get_pixel_write_function() - 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: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
> + */
>   void *get_pixel_write_function(u32 format)
>   {
>   	switch (format) {
>
Louis Chauvet July 15, 2024, 8:28 a.m. UTC | #2 
Le 13/07/24 - 11:35, Maíra Canal a écrit :
> On 5/16/24 10:04, Louis Chauvet wrote:
> > Add some documentation on pixel conversion functions.
> > Update of outdated comments for pixel_write functions.
> > 
> > Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
> > Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
> > ---
> >   drivers/gpu/drm/vkms/vkms_composer.c |  7 ++++
> >   drivers/gpu/drm/vkms/vkms_drv.h      | 15 ++++++++-
> >   drivers/gpu/drm/vkms/vkms_formats.c  | 62 ++++++++++++++++++++++++++++++------
> >   3 files changed, 74 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> > index c6d9b4a65809..da0651a94c9b 100644
> > --- a/drivers/gpu/drm/vkms/vkms_composer.c
> > +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> > @@ -189,6 +189,13 @@ 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 to avoid heavy memory usage. It is a necessary
> > +	 * complexity to avoid poor blending performance.
> > +	 *
> > +	 * The function vkms_compose_row is used to read a line, pixel-by-pixel, into the staging
> 
> Nit: I know it's not kerneldoc, but I'd be glad if you use
> vkms_compose_row()
> 
> > +	 * buffer.
> > +	 */
> >   	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..212f4ab6a71f 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
> 
> s/structure/Structure
> 
> > + *
> > + * @fb: backing drm framebuffer
> > + * @src: source rectangle of this frame in the source framebuffer, stored in 16.16 fixed-point form
> > + * @dst: destination rectangle in the crtc buffer, stored in whole pixel units
> > + * @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;
> > @@ -49,9 +60,11 @@ struct vkms_writeback_job {
> >   };
> >   
> >   /**
> > - * vkms_plane_state - Driver specific plane state
> > + * struct 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
> 
> s/vkms_plane_state/struct vkms_plane_state
> 
> > + * ensure that this pointer is valid
> 
> Note: "If the @argument description has multiple lines, the continuation
> of the description should start at the same column as the previous
> line:" [1]
> 
> [1] https://docs.kernel.org/doc-guide/kernel-doc.html#function-parameters
> 
> >    */
> >   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 d597c48452ac..f157c43da4d6 100644
> > --- a/drivers/gpu/drm/vkms/vkms_formats.c
> > +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> > @@ -9,6 +9,18 @@
> >   
> >   #include "vkms_formats.h"
> >   
> > +/**
> > + * pixel_offset() - Get the offset of 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 ensure that the framebuffer associated with this request uses a pixel format
> > + * where block_h == block_w == 1.
> > + * If this requirement is not fulfilled, the resulting offset can point to an other pixel or
> > + * outside of the buffer.
> > + */
> >   static size_t pixel_offset(const struct vkms_frame_info *frame_info, int x, int y)
> >   {
> >   	struct drm_framebuffer *fb = frame_info->fb;
> > @@ -16,18 +28,22 @@ static size_t pixel_offset(const struct vkms_frame_info *frame_info, int x, int
> >   	return fb->offsets[0] + (y * fb->pitches[0]) + (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.
> >    * This function assumes the channels are packed together, i.e. a color channel
> >    * comes immediately after another in the memory. And therefore, this function
> >    * doesn't work for YUV with chroma subsampling (e.g. YUV420 and NV21).
> > + *
> > + * The caller must ensure that the framebuffer associated with this request uses a pixel format
> > + * where block_h == block_w == 1, otherwise the returned pointer can be outside the buffer.
> >    */
> >   static void *packed_pixels_addr(const struct vkms_frame_info *frame_info,
> >   				int x, int y)
> > @@ -52,6 +68,13 @@ 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.
> 
> I believe it is @out_pixel.

Thanks for the review!

Yes it @out_pixel, I will fix those nits for the next iteration.

Thanks,
Louis Chauvet
 
> The content seems great to me, only those minor nits.
> 
> Best Regards,
> - Maíra
> 

[...]
diff mbox series

Patch

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index c6d9b4a65809..da0651a94c9b 100644
--- a/drivers/gpu/drm/vkms/vkms_composer.c
+++ b/drivers/gpu/drm/vkms/vkms_composer.c
@@ -189,6 +189,13 @@  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 to avoid heavy memory usage. It is a necessary
+	 * complexity to avoid poor blending performance.
+	 *
+	 * The function vkms_compose_row is used to read a line, pixel-by-pixel, into the staging
+	 * buffer.
+	 */
 	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..212f4ab6a71f 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, stored in 16.16 fixed-point form
+ * @dst: destination rectangle in the crtc buffer, stored in whole pixel units
+ * @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;
@@ -49,9 +60,11 @@  struct vkms_writeback_job {
 };
 
 /**
- * vkms_plane_state - Driver specific plane state
+ * struct 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 d597c48452ac..f157c43da4d6 100644
--- a/drivers/gpu/drm/vkms/vkms_formats.c
+++ b/drivers/gpu/drm/vkms/vkms_formats.c
@@ -9,6 +9,18 @@ 
 
 #include "vkms_formats.h"
 
+/**
+ * pixel_offset() - Get the offset of 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 ensure that the framebuffer associated with this request uses a pixel format
+ * where block_h == block_w == 1.
+ * If this requirement is not fulfilled, the resulting offset can point to an other pixel or
+ * outside of the buffer.
+ */
 static size_t pixel_offset(const struct vkms_frame_info *frame_info, int x, int y)
 {
 	struct drm_framebuffer *fb = frame_info->fb;
@@ -16,18 +28,22 @@  static size_t pixel_offset(const struct vkms_frame_info *frame_info, int x, int
 	return fb->offsets[0] + (y * fb->pitches[0]) + (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.
  * This function assumes the channels are packed together, i.e. a color channel
  * comes immediately after another in the memory. And therefore, this function
  * doesn't work for YUV with chroma subsampling (e.g. YUV420 and NV21).
+ *
+ * The caller must ensure that the framebuffer associated with this request uses a pixel format
+ * where block_h == block_w == 1, otherwise the returned pointer can be outside the buffer.
  */
 static void *packed_pixels_addr(const struct vkms_frame_info *frame_info,
 				int x, int y)
@@ -52,6 +68,13 @@  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)
 {
 	/*
@@ -144,12 +167,11 @@  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 &struct pixel_argb_u16 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 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)
 {
@@ -215,6 +237,14 @@  static void argb_u16_to_RGB565(u8 *dst_pixels, struct pixel_argb_u16 *in_pixel)
 	*pixels = cpu_to_le16(r << 11 | g << 5 | b);
 }
 
+/**
+ * vkms_writeback_row() - 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)
 {
@@ -228,6 +258,13 @@  void vkms_writeback_row(struct vkms_writeback_job *wb,
 		wb->pixel_write(dst_pixels, &in_pixels[x]);
 }
 
+/**
+ * get_pixel_conversion_function() - 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: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
+ */
 void *get_pixel_conversion_function(u32 format)
 {
 	switch (format) {
@@ -246,6 +283,13 @@  void *get_pixel_conversion_function(u32 format)
 	}
 }
 
+/**
+ * get_pixel_write_function() - 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: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
+ */
 void *get_pixel_write_function(u32 format)
 {
 	switch (format) {