drm/vkms: Add documentation

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Update of outdated comments for pixel_write functions.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>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240905-google-clarifications-v4-1-e43c1f2e3d87@bootlin.com

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) {