drm/uapi: document kernel capabilities

Document all of the DRM_CAP_* defines.

v2 (Pekka):
- Describe what the bit depth is
- Expand on preferred dumb buffer memory access patterns
- Explain what a PRIME buffer is
- Mention DRM_IOCTL_PRIME_FD_TO_HANDLE and DRM_IOCTL_PRIME_HANDLE_TO_FD
- Explicitly reference CLOCK_REALTIME and CLOCK_MONOTONIC
- Make it clear DRM_CAP_CRTC_IN_VBLANK_EVENT applies to both DRM_EVENT_VBLANK
  and DRM_EVENT_FLIP_COMPLETE

v3 (Daniel):
- Specify kernel versions for caps that don't depend on drivers
- Make it clear dumb buffers caps are only about dumb buffers

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210308123421.747836-1-contact@emersion.fr

include/uapi/drm/drm.h

@@ -625,30 +625,147 @@ struct drm_gem_open {
 	__u64 size;
 };
 
+/**
+ * DRM_CAP_DUMB_BUFFER
+ *
+ * If set to 1, the driver supports creating dumb buffers via the
+ * &DRM_IOCTL_MODE_CREATE_DUMB ioctl.
+ */
 #define DRM_CAP_DUMB_BUFFER		0x1
+/**
+ * DRM_CAP_VBLANK_HIGH_CRTC
+ *
+ * If set to 1, the kernel supports specifying a CRTC index in the high bits of
+ * &drm_wait_vblank_request.type.
+ *
+ * Starting kernel version 2.6.39, this capability is always set to 1.
+ */
 #define DRM_CAP_VBLANK_HIGH_CRTC	0x2
+/**
+ * DRM_CAP_DUMB_PREFERRED_DEPTH
+ *
+ * The preferred bit depth for dumb buffers.
+ *
+ * The bit depth is the number of bits used to indicate the color of a single
+ * pixel excluding any padding. This is different from the number of bits per
+ * pixel. For instance, XRGB8888 has a bit depth of 24 but has 32 bits per
+ * pixel.
+ *
+ * Note that this preference only applies to dumb buffers, it's irrelevant for
+ * other types of buffers.
+ */
 #define DRM_CAP_DUMB_PREFERRED_DEPTH	0x3
+/**
+ * DRM_CAP_DUMB_PREFER_SHADOW
+ *
+ * If set to 1, the driver prefers userspace to render to a shadow buffer
+ * instead of directly rendering to a dumb buffer. For best speed, userspace
+ * should do streaming ordered memory copies into the dumb buffer and never
+ * read from it.
+ *
+ * Note that this preference only applies to dumb buffers, it's irrelevant for
+ * other types of buffers.
+ */
 #define DRM_CAP_DUMB_PREFER_SHADOW	0x4
+/**
+ * DRM_CAP_PRIME
+ *
+ * Bitfield of supported PRIME sharing capabilities. See &DRM_PRIME_CAP_IMPORT
+ * and &DRM_PRIME_CAP_EXPORT.
+ *
+ * PRIME buffers are exposed as dma-buf file descriptors. See
+ * Documentation/gpu/drm-mm.rst, section "PRIME Buffer Sharing".
+ */
 #define DRM_CAP_PRIME			0x5
+/**
+ * DRM_PRIME_CAP_IMPORT
+ *
+ * If this bit is set in &DRM_CAP_PRIME, the driver supports importing PRIME
+ * buffers via the &DRM_IOCTL_PRIME_FD_TO_HANDLE ioctl.
+ */
 #define  DRM_PRIME_CAP_IMPORT		0x1
+/**
+ * DRM_PRIME_CAP_EXPORT
+ *
+ * If this bit is set in &DRM_CAP_PRIME, the driver supports exporting PRIME
+ * buffers via the &DRM_IOCTL_PRIME_HANDLE_TO_FD ioctl.
+ */
 #define  DRM_PRIME_CAP_EXPORT		0x2
+/**
+ * DRM_CAP_TIMESTAMP_MONOTONIC
+ *
+ * If set to 0, the kernel will report timestamps with ``CLOCK_REALTIME`` in
+ * struct drm_event_vblank. If set to 1, the kernel will report timestamps with
+ * ``CLOCK_MONOTONIC``. See ``clock_gettime(2)`` for the definition of these
+ * clocks.
+ *
+ * Starting from kernel version 2.6.39, the default value for this capability
+ * is 1. Starting kernel version 4.15, this capability is always set to 1.
+ */
 #define DRM_CAP_TIMESTAMP_MONOTONIC	0x6
+/**
+ * DRM_CAP_ASYNC_PAGE_FLIP
+ *
+ * If set to 1, the driver supports &DRM_MODE_PAGE_FLIP_ASYNC.
+ */
 #define DRM_CAP_ASYNC_PAGE_FLIP		0x7
-/*
+/**
- * The CURSOR_WIDTH and CURSOR_HEIGHT capabilities return a valid widthxheight
+ * DRM_CAP_CURSOR_WIDTH
- * combination for the hardware cursor. The intention is that a hardware
+ *
- * agnostic userspace can query a cursor plane size to use.
+ * The ``CURSOR_WIDTH`` and ``CURSOR_HEIGHT`` capabilities return a valid
+ * width x height combination for the hardware cursor. The intention is that a
+ * hardware agnostic userspace can query a cursor plane size to use.
  *
  * Note that the cross-driver contract is to merely return a valid size;
  * drivers are free to attach another meaning on top, eg. i915 returns the
  * maximum plane size.
  */
 #define DRM_CAP_CURSOR_WIDTH		0x8
+/**
+ * DRM_CAP_CURSOR_HEIGHT
+ *
+ * See &DRM_CAP_CURSOR_WIDTH.
+ */
 #define DRM_CAP_CURSOR_HEIGHT		0x9
+/**
+ * DRM_CAP_ADDFB2_MODIFIERS
+ *
+ * If set to 1, the driver supports supplying modifiers in the
+ * &DRM_IOCTL_MODE_ADDFB2 ioctl.
+ */
 #define DRM_CAP_ADDFB2_MODIFIERS	0x10
+/**
+ * DRM_CAP_PAGE_FLIP_TARGET
+ *
+ * If set to 1, the driver supports the &DRM_MODE_PAGE_FLIP_TARGET_ABSOLUTE and
+ * &DRM_MODE_PAGE_FLIP_TARGET_RELATIVE flags in
+ * &drm_mode_crtc_page_flip_target.flags for the &DRM_IOCTL_MODE_PAGE_FLIP
+ * ioctl.
+ */
 #define DRM_CAP_PAGE_FLIP_TARGET	0x11
+/**
+ * DRM_CAP_CRTC_IN_VBLANK_EVENT
+ *
+ * If set to 1, the kernel supports reporting the CRTC ID in
+ * &drm_event_vblank.crtc_id for the &DRM_EVENT_VBLANK and
+ * &DRM_EVENT_FLIP_COMPLETE events.
+ *
+ * Starting kernel version 4.12, this capability is always set to 1.
+ */
 #define DRM_CAP_CRTC_IN_VBLANK_EVENT	0x12
+/**
+ * DRM_CAP_SYNCOBJ
+ *
+ * If set to 1, the driver supports sync objects. See
+ * Documentation/gpu/drm-mm.rst, section "DRM Sync Objects".
+ */
 #define DRM_CAP_SYNCOBJ		0x13
+/**
+ * DRM_CAP_SYNCOBJ_TIMELINE
+ *
+ * If set to 1, the driver supports timeline operations on sync objects. See
+ * Documentation/gpu/drm-mm.rst, section "DRM Sync Objects".
+ */
 #define DRM_CAP_SYNCOBJ_TIMELINE	0x14
 
 /* DRM_IOCTL_GET_CAP ioctl argument type */