drm/crc-debugfs: Add notes about CRC<->commit interactions

CRC generation can be impacted by commits coming from userspace, and
enabling CRC generation may itself trigger a commit. Add notes about
this to the kerneldoc.

Changes since v1:
 - Clarified that anything that would disable CRCs counts as a full
   modeset, and so userspace needs to reconfigure after full modesets

Changes since v2:
 - Add these notes
 - Rebase onto drm-misc-next (trivial conflict in comment)
Signed-off-by: NBrian Starkey <brian.starkey@arm.com>
Signed-off-by: NAyan Kumar Halder <ayan.halder@arm.com>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

Link:- https://patchwork.freedesktop.org/patch/321974/

drivers/gpu/drm/drm_debugfs_crc.c

@@ -69,6 +69,15 @@
  * implement &drm_crtc_funcs.set_crc_source and &drm_crtc_funcs.verify_crc_source.
  * The debugfs files are automatically set up if those vfuncs are set. CRC samples
  * need to be captured in the driver by calling drm_crtc_add_crc_entry().
+ * Depending on the driver and HW requirements, &drm_crtc_funcs.set_crc_source
+ * may result in a commit (even a full modeset).
+ *
+ * CRC results must be reliable across non-full-modeset atomic commits, so if a
+ * commit via DRM_IOCTL_MODE_ATOMIC would disable or otherwise interfere with
+ * CRC generation, then the driver must mark that commit as a full modeset
+ * (drm_atomic_crtc_needs_modeset() should return true). As a result, to ensure
+ * consistent results, generic userspace must re-setup CRC generation after a
+ * legacy SETCRTC or an atomic commit with DRM_MODE_ATOMIC_ALLOW_MODESET.
  */
 
 static int crc_control_show(struct seq_file *m, void *data)

include/drm/drm_crtc.h

@@ -756,6 +756,9 @@ struct drm_crtc_funcs {
 	 * provided from the configured source. Drivers must accept an "auto"
 	 * source name that will select a default source for this CRTC.
 	 *
+	 * This may trigger an atomic modeset commit if necessary, to enable CRC
+	 * generation.
+	 *
 	 * Note that "auto" can depend upon the current modeset configuration,
 	 * e.g. it could pick an encoder or output specific CRC sampling point.
 	 *
@@ -767,6 +770,7 @@ struct drm_crtc_funcs {
 	 * 0 on success or a negative error code on failure.
 	 */
 	int (*set_crc_source)(struct drm_crtc *crtc, const char *source);
+
 	/**
 	 * @verify_crc_source:
 	 *