[PATCH v4] drm/vkms: Add documentation

[PATCH v4] drm/vkms: Add documentation

[Louis Chauvet]

Add documentation around vkms_output and its initialization.
Add some documentation on pixel conversion functions.
Update of outdated comments for pixel_write functions.

Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
---
This series does not introduce functionnal changes, only some
documentation and renaming to clarify the code.
---
Changes in v4:
- Applied comments from José
- Link to v3: https://lore.kernel.org/r/20240829-google-clarifications-v3-1-f6604e2f6297@bootlin.com

Changes in v3:
- Merged https://lore.kernel.org/all/20240802-yuv-v9-3-08a706669e16@bootlin.com/
  as it also add documentation
- Apply José's comments, sorry
- Replace =1 by =BIT(0) for possible_crtc value
- Link to v2: https://lore.kernel.org/r/20240826-google-clarifications-v2-1-2574655b0b91@bootlin.com

Changes in v2:
- Dropped already applied patches
- Dropped useless patch as its content is deleted later
- Remove dependency on previous series
- Apply Maíra's comments
- Link to v1: https://lore.kernel.org/r/20240814-google-clarifications-v1-0-3ee76d7d0c28@bootlin.com
---
 drivers/gpu/drm/vkms/vkms_composer.c |   7 +++
 drivers/gpu/drm/vkms/vkms_drv.h      | 101 ++++++++++++++++++++++++++++++-----
 drivers/gpu/drm/vkms/vkms_formats.c  |  62 +++++++++++++++++----
 drivers/gpu/drm/vkms/vkms_output.c   |  14 ++++-
 4 files changed, 161 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
index e7441b227b3c..57a5769fc994 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 5e46ea5b96dc..672fe191e239 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;
@@ -52,9 +63,11 @@ 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 struct vkms_plane_state
+ *	        must ensure that this pointer is valid
  */
 struct vkms_plane_state {
 	struct drm_shadow_plane_state base;
@@ -73,29 +86,56 @@ struct vkms_color_lut {
 };
 
 /**
- * vkms_crtc_state - Driver specific CRTC state
+ * struct vkms_crtc_state - Driver specific CRTC state
+ *
  * @base: base CRTC state
  * @composer_work: work struct to compose and add CRC entries
- * @n_frame_start: start frame number for computed CRC
- * @n_frame_end: end frame number for computed CRC
+ *
+ * @num_active_planes: Number of active planes
+ * @active_planes: List containing all the active planes (counted by
+ *		   @num_active_planes). They should be stored in z-order.
+ * @active_writeback: Current active writeback job
+ * @gamma_lut: Look up table for gamma used in this CRTC
+ * @crc_pending: Protected by @vkms_output.composer_lock, true when the frame CRC is not computed
+ *		 yet. Used by vblank to detect if the composer is too slow.
+ * @wb_pending: Protected by @vkms_output.composer_lock, true when a writeback frame is requested.
+ * @frame_start: Protected by @vkms_output.composer_lock, saves the frame number before the start
+ *		 of the composition process.
+ * @frame_end: Protected by @vkms_output.composer_lock, saves the last requested frame number.
+ *	       This is used to generate enough CRC entries when the composition worker is too slow.
  */
 struct vkms_crtc_state {
 	struct drm_crtc_state base;
 	struct work_struct composer_work;
 
 	int num_active_planes;
-	/* stack of active planes for crc computation, should be in z order */
 	struct vkms_plane_state **active_planes;
 	struct vkms_writeback_job *active_writeback;
 	struct vkms_color_lut gamma_lut;
 
-	/* below four are protected by vkms_output.composer_lock */
 	bool crc_pending;
 	bool wb_pending;
 	u64 frame_start;
 	u64 frame_end;
 };
 
+/**
+ * struct vkms_output - Internal representation of all output components in VKMS
+ *
+ * @crtc: Base CRTC in DRM
+ * @encoder: DRM encoder used for this output
+ * @connector: DRM connector used for this output
+ * @wb_connecter: DRM writeback connector used for this output
+ * @vblank_hrtimer: Timer used to trigger the vblank
+ * @period_ns: vblank period, in nanoseconds, used to configure @vblank_hrtimer and to compute
+ *	       vblank timestamps
+ * @composer_workq: Ordered workqueue for @composer_state.composer_work.
+ * @lock: Lock used to protect concurrent access to the composer
+ * @composer_enabled: Protected by @lock, true when the VKMS composer is active (crc needed or
+ *		      writeback)
+ * @composer_state: Protected by @lock, current state of this VKMS output
+ * @composer_lock: Lock used internally to protect @composer_state members
+ */
 struct vkms_output {
 	struct drm_crtc crtc;
 	struct drm_encoder encoder;
@@ -103,28 +143,38 @@ struct vkms_output {
 	struct drm_writeback_connector wb_connector;
 	struct hrtimer vblank_hrtimer;
 	ktime_t period_ns;
-	/* ordered wq for composer_work */
 	struct workqueue_struct *composer_workq;
-	/* protects concurrent access to composer */
 	spinlock_t lock;
 
-	/* protected by @lock */
 	bool composer_enabled;
 	struct vkms_crtc_state *composer_state;
 
 	spinlock_t composer_lock;
 };
 
-struct vkms_device;
-
+/**
+ * struct vkms_config - General configuration for VKMS driver
+ *
+ * @writeback: If true, a writeback buffer can be attached to the CRTC
+ * @cursor: If true, a cursor plane is created in the VKMS device
+ * @overlay: If true, NUM_OVERLAY_PLANES will be created for the VKMS device
+ * @dev: Used to store the current VKMS device. Only set when the device is instantiated.
+ */
 struct vkms_config {
 	bool writeback;
 	bool cursor;
 	bool overlay;
-	/* only set when instantiated */
 	struct vkms_device *dev;
 };
 
+/**
+ * struct vkms_device - Description of a VKMS device
+ *
+ * @drm - Base device in DRM
+ * @platform - Associated platform device
+ * @output - Configuration and sub-components of the VKMS device
+ * @config: Configuration used in this VKMS device
+ */
 struct vkms_device {
 	struct drm_device drm;
 	struct platform_device *platform;
@@ -132,6 +182,10 @@ struct vkms_device {
 	const struct vkms_config *config;
 };
 
+/*
+ * The following helpers are used to convert a member of a struct into its parent.
+ */
+
 #define drm_crtc_to_vkms_output(target) \
 	container_of(target, struct vkms_output, crtc)
 
@@ -144,12 +198,33 @@ struct vkms_device {
 #define to_vkms_plane_state(target)\
 	container_of(target, struct vkms_plane_state, base.base)
 
-/* CRTC */
+/**
+ * vkms_crtc_init() - Initialize a CRTC for VKMS
+ * @dev: DRM device associated with the VKMS buffer
+ * @crtc: uninitialized CRTC device
+ * @primary: primary plane to attach to the CRTC
+ * @cursor: plane to attach to the CRTC
+ */
 int vkms_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,
 		   struct drm_plane *primary, struct drm_plane *cursor);
 
+/**
+ * vkms_output_init() - Initialize all sub-components needed for a VKMS device.
+ *
+ * @vkmsdev: VKMS device to initialize
+ * @index: CRTC which can be attached to the planes. The caller must ensure that
+ *	   @index is positive and less or equals to 31.
+ */
 int vkms_output_init(struct vkms_device *vkmsdev, int index);
 
+/**
+ * vkms_plane_init() - Initialize a plane
+ *
+ * @vkmsdev: VKMS device containing the plane
+ * @type: type of plane to initialize
+ * @index: CRTC which can be attached to the plane. The caller must ensure that
+ *	   @index is positive and less or equals to 31.
+ */
 struct vkms_plane *vkms_plane_init(struct vkms_device *vkmsdev,
 				   enum drm_plane_type type, int index);
 
diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
index 040b7f113a3b..e8a5cc235ebb 100644
--- a/drivers/gpu/drm/vkms/vkms_formats.c
+++ b/drivers/gpu/drm/vkms/vkms_formats.c
@@ -9,24 +9,40 @@
 
 #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)
 {
 	return frame_info->offset + (y * frame_info->pitch)
 				  + (x * frame_info->cpp);
 }
 
-/*
- * 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)
@@ -51,6 +67,13 @@ 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)
 {
 	/*
@@ -143,12 +166,11 @@ 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)
 {
@@ -214,6 +236,14 @@ 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)
 {
@@ -227,6 +257,13 @@ 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) {
@@ -245,6 +282,13 @@ 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) {
diff --git a/drivers/gpu/drm/vkms/vkms_output.c b/drivers/gpu/drm/vkms/vkms_output.c
index 5ce70dd946aa..0a5a185aa0b0 100644
--- a/drivers/gpu/drm/vkms/vkms_output.c
+++ b/drivers/gpu/drm/vkms/vkms_output.c
@@ -21,6 +21,7 @@ static int vkms_conn_get_modes(struct drm_connector *connector)
 {
 	int count;
 
+	/* Use the default modes list from DRM */
 	count = drm_add_modes_noedid(connector, XRES_MAX, YRES_MAX);
 	drm_set_preferred_mode(connector, XRES_DEF, YRES_DEF);
 
@@ -58,6 +59,12 @@ int vkms_output_init(struct vkms_device *vkmsdev, int index)
 	int writeback;
 	unsigned int n;
 
+	/*
+	 * Initialize used plane. One primary plane is required to perform the composition.
+	 *
+	 * The overlay and cursor planes are not mandatory, but can be used to perform complex
+	 * composition.
+	 */
 	primary = vkms_plane_init(vkmsdev, DRM_PLANE_TYPE_PRIMARY, index);
 	if (IS_ERR(primary))
 		return PTR_ERR(primary);
@@ -76,6 +83,7 @@ int vkms_output_init(struct vkms_device *vkmsdev, int index)
 			return PTR_ERR(cursor);
 	}
 
+	/* [1]: Allocation of a CRTC, its index will be BIT(0) = 1 */
 	ret = vkms_crtc_init(dev, crtc, &primary->base, &cursor->base);
 	if (ret)
 		return ret;
@@ -95,7 +103,11 @@ int vkms_output_init(struct vkms_device *vkmsdev, int index)
 		DRM_ERROR("Failed to init encoder\n");
 		goto err_encoder;
 	}
-	encoder->possible_crtcs = 1;
+	/*
+	 * This is a hardcoded value to select crtc for the encoder.
+	 * BIT(0) here designate the first registered CRTC, the one allocated in [1]
+	 */
+	encoder->possible_crtcs = BIT(0);
 
 	ret = drm_connector_attach_encoder(connector, encoder);
 	if (ret) {

---
base-commit: 84addde447fd9d713e101437db0d4924855eff4f
change-id: 20240520-google-clarifications-dede8dcbe38a

Best regards,
-- 
Louis Chauvet <louis.chauvet@bootlin.com>

Re: [PATCH v4] drm/vkms: Add documentation

[Maíra Canal]

Hi Louis,

On 9/5/24 10:27, Louis Chauvet wrote:
> Add documentation around vkms_output and its initialization.
> Add some documentation on pixel conversion functions.
> Update of outdated comments for pixel_write functions.
> 
> Reviewed-by: José Expósito <jose.exposito89@gmail.com>
> Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com
Applied to misc/kernel.git (drm-misc-next).

Best Regards,
- Maíra

> ---
> This series does not introduce functionnal changes, only some
> documentation and renaming to clarify the code.
> ---
> Changes in v4:
> - Applied comments from José
> - Link to v3: https://lore.kernel.org/r/20240829-google-clarifications-v3-1-f6604e2f6297@bootlin.com
> 
> Changes in v3:
> - Merged https://lore.kernel.org/all/20240802-yuv-v9-3-08a706669e16@bootlin.com/
>    as it also add documentation
> - Apply José's comments, sorry
> - Replace =1 by =BIT(0) for possible_crtc value
> - Link to v2: https://lore.kernel.org/r/20240826-google-clarifications-v2-1-2574655b0b91@bootlin.com
> 
> Changes in v2:
> - Dropped already applied patches
> - Dropped useless patch as its content is deleted later
> - Remove dependency on previous series
> - Apply Maíra's comments
> - Link to v1: https://lore.kernel.org/r/20240814-google-clarifications-v1-0-3ee76d7d0c28@bootlin.com
> ---
>   drivers/gpu/drm/vkms/vkms_composer.c |   7 +++
>   drivers/gpu/drm/vkms/vkms_drv.h      | 101 ++++++++++++++++++++++++++++++-----
>   drivers/gpu/drm/vkms/vkms_formats.c  |  62 +++++++++++++++++----
>   drivers/gpu/drm/vkms/vkms_output.c   |  14 ++++-
>   4 files changed, 161 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index e7441b227b3c..57a5769fc994 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 5e46ea5b96dc..672fe191e239 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;
> @@ -52,9 +63,11 @@ 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 struct vkms_plane_state
> + *	        must ensure that this pointer is valid
>    */
>   struct vkms_plane_state {
>   	struct drm_shadow_plane_state base;
> @@ -73,29 +86,56 @@ struct vkms_color_lut {
>   };
>   
>   /**
> - * vkms_crtc_state - Driver specific CRTC state
> + * struct vkms_crtc_state - Driver specific CRTC state
> + *
>    * @base: base CRTC state
>    * @composer_work: work struct to compose and add CRC entries
> - * @n_frame_start: start frame number for computed CRC
> - * @n_frame_end: end frame number for computed CRC
> + *
> + * @num_active_planes: Number of active planes
> + * @active_planes: List containing all the active planes (counted by
> + *		   @num_active_planes). They should be stored in z-order.
> + * @active_writeback: Current active writeback job
> + * @gamma_lut: Look up table for gamma used in this CRTC
> + * @crc_pending: Protected by @vkms_output.composer_lock, true when the frame CRC is not computed
> + *		 yet. Used by vblank to detect if the composer is too slow.
> + * @wb_pending: Protected by @vkms_output.composer_lock, true when a writeback frame is requested.
> + * @frame_start: Protected by @vkms_output.composer_lock, saves the frame number before the start
> + *		 of the composition process.
> + * @frame_end: Protected by @vkms_output.composer_lock, saves the last requested frame number.
> + *	       This is used to generate enough CRC entries when the composition worker is too slow.
>    */
>   struct vkms_crtc_state {
>   	struct drm_crtc_state base;
>   	struct work_struct composer_work;
>   
>   	int num_active_planes;
> -	/* stack of active planes for crc computation, should be in z order */
>   	struct vkms_plane_state **active_planes;
>   	struct vkms_writeback_job *active_writeback;
>   	struct vkms_color_lut gamma_lut;
>   
> -	/* below four are protected by vkms_output.composer_lock */
>   	bool crc_pending;
>   	bool wb_pending;
>   	u64 frame_start;
>   	u64 frame_end;
>   };
>   
> +/**
> + * struct vkms_output - Internal representation of all output components in VKMS
> + *
> + * @crtc: Base CRTC in DRM
> + * @encoder: DRM encoder used for this output
> + * @connector: DRM connector used for this output
> + * @wb_connecter: DRM writeback connector used for this output
> + * @vblank_hrtimer: Timer used to trigger the vblank
> + * @period_ns: vblank period, in nanoseconds, used to configure @vblank_hrtimer and to compute
> + *	       vblank timestamps
> + * @composer_workq: Ordered workqueue for @composer_state.composer_work.
> + * @lock: Lock used to protect concurrent access to the composer
> + * @composer_enabled: Protected by @lock, true when the VKMS composer is active (crc needed or
> + *		      writeback)
> + * @composer_state: Protected by @lock, current state of this VKMS output
> + * @composer_lock: Lock used internally to protect @composer_state members
> + */
>   struct vkms_output {
>   	struct drm_crtc crtc;
>   	struct drm_encoder encoder;
> @@ -103,28 +143,38 @@ struct vkms_output {
>   	struct drm_writeback_connector wb_connector;
>   	struct hrtimer vblank_hrtimer;
>   	ktime_t period_ns;
> -	/* ordered wq for composer_work */
>   	struct workqueue_struct *composer_workq;
> -	/* protects concurrent access to composer */
>   	spinlock_t lock;
>   
> -	/* protected by @lock */
>   	bool composer_enabled;
>   	struct vkms_crtc_state *composer_state;
>   
>   	spinlock_t composer_lock;
>   };
>   
> -struct vkms_device;
> -
> +/**
> + * struct vkms_config - General configuration for VKMS driver
> + *
> + * @writeback: If true, a writeback buffer can be attached to the CRTC
> + * @cursor: If true, a cursor plane is created in the VKMS device
> + * @overlay: If true, NUM_OVERLAY_PLANES will be created for the VKMS device
> + * @dev: Used to store the current VKMS device. Only set when the device is instantiated.
> + */
>   struct vkms_config {
>   	bool writeback;
>   	bool cursor;
>   	bool overlay;
> -	/* only set when instantiated */
>   	struct vkms_device *dev;
>   };
>   
> +/**
> + * struct vkms_device - Description of a VKMS device
> + *
> + * @drm - Base device in DRM
> + * @platform - Associated platform device
> + * @output - Configuration and sub-components of the VKMS device
> + * @config: Configuration used in this VKMS device
> + */
>   struct vkms_device {
>   	struct drm_device drm;
>   	struct platform_device *platform;
> @@ -132,6 +182,10 @@ struct vkms_device {
>   	const struct vkms_config *config;
>   };
>   
> +/*
> + * The following helpers are used to convert a member of a struct into its parent.
> + */
> +
>   #define drm_crtc_to_vkms_output(target) \
>   	container_of(target, struct vkms_output, crtc)
>   
> @@ -144,12 +198,33 @@ struct vkms_device {
>   #define to_vkms_plane_state(target)\
>   	container_of(target, struct vkms_plane_state, base.base)
>   
> -/* CRTC */
> +/**
> + * vkms_crtc_init() - Initialize a CRTC for VKMS
> + * @dev: DRM device associated with the VKMS buffer
> + * @crtc: uninitialized CRTC device
> + * @primary: primary plane to attach to the CRTC
> + * @cursor: plane to attach to the CRTC
> + */
>   int vkms_crtc_init(struct drm_device *dev, struct drm_crtc *crtc,
>   		   struct drm_plane *primary, struct drm_plane *cursor);
>   
> +/**
> + * vkms_output_init() - Initialize all sub-components needed for a VKMS device.
> + *
> + * @vkmsdev: VKMS device to initialize
> + * @index: CRTC which can be attached to the planes. The caller must ensure that
> + *	   @index is positive and less or equals to 31.
> + */
>   int vkms_output_init(struct vkms_device *vkmsdev, int index);
>   
> +/**
> + * vkms_plane_init() - Initialize a plane
> + *
> + * @vkmsdev: VKMS device containing the plane
> + * @type: type of plane to initialize
> + * @index: CRTC which can be attached to the plane. The caller must ensure that
> + *	   @index is positive and less or equals to 31.
> + */
>   struct vkms_plane *vkms_plane_init(struct vkms_device *vkmsdev,
>   				   enum drm_plane_type type, int index);
>   
> diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
> index 040b7f113a3b..e8a5cc235ebb 100644
> --- a/drivers/gpu/drm/vkms/vkms_formats.c
> +++ b/drivers/gpu/drm/vkms/vkms_formats.c
> @@ -9,24 +9,40 @@
>   
>   #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)
>   {
>   	return frame_info->offset + (y * frame_info->pitch)
>   				  + (x * frame_info->cpp);
>   }
>   
> -/*
> - * 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)
> @@ -51,6 +67,13 @@ 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)
>   {
>   	/*
> @@ -143,12 +166,11 @@ 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)
>   {
> @@ -214,6 +236,14 @@ 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)
>   {
> @@ -227,6 +257,13 @@ 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) {
> @@ -245,6 +282,13 @@ 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) {
> diff --git a/drivers/gpu/drm/vkms/vkms_output.c b/drivers/gpu/drm/vkms/vkms_output.c
> index 5ce70dd946aa..0a5a185aa0b0 100644
> --- a/drivers/gpu/drm/vkms/vkms_output.c
> +++ b/drivers/gpu/drm/vkms/vkms_output.c
> @@ -21,6 +21,7 @@ static int vkms_conn_get_modes(struct drm_connector *connector)
>   {
>   	int count;
>   
> +	/* Use the default modes list from DRM */
>   	count = drm_add_modes_noedid(connector, XRES_MAX, YRES_MAX);
>   	drm_set_preferred_mode(connector, XRES_DEF, YRES_DEF);
>   
> @@ -58,6 +59,12 @@ int vkms_output_init(struct vkms_device *vkmsdev, int index)
>   	int writeback;
>   	unsigned int n;
>   
> +	/*
> +	 * Initialize used plane. One primary plane is required to perform the composition.
> +	 *
> +	 * The overlay and cursor planes are not mandatory, but can be used to perform complex
> +	 * composition.
> +	 */
>   	primary = vkms_plane_init(vkmsdev, DRM_PLANE_TYPE_PRIMARY, index);
>   	if (IS_ERR(primary))
>   		return PTR_ERR(primary);
> @@ -76,6 +83,7 @@ int vkms_output_init(struct vkms_device *vkmsdev, int index)
>   			return PTR_ERR(cursor);
>   	}
>   
> +	/* [1]: Allocation of a CRTC, its index will be BIT(0) = 1 */
>   	ret = vkms_crtc_init(dev, crtc, &primary->base, &cursor->base);
>   	if (ret)
>   		return ret;
> @@ -95,7 +103,11 @@ int vkms_output_init(struct vkms_device *vkmsdev, int index)
>   		DRM_ERROR("Failed to init encoder\n");
>   		goto err_encoder;
>   	}
> -	encoder->possible_crtcs = 1;
> +	/*
> +	 * This is a hardcoded value to select crtc for the encoder.
> +	 * BIT(0) here designate the first registered CRTC, the one allocated in [1]
> +	 */
> +	encoder->possible_crtcs = BIT(0);
>   
>   	ret = drm_connector_attach_encoder(connector, encoder);
>   	if (ret) {
> 
> ---
> base-commit: 84addde447fd9d713e101437db0d4924855eff4f
> change-id: 20240520-google-clarifications-dede8dcbe38a
> 
> Best regards,