drm: crc: Introduce verify_crc_source callback

This patch adds a new callback function "verify_crc_source" which will
be used during setting the crc source in control node. This will help
in avoiding setting of wrong string for source.

Changes since V1:
 - do not yet verify_crc_source during open.
Changes since V1:
 - improve callback description

Signed-off-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: dri-devel@lists.freedesktop.org
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180713135942.25061-2-mahesh1.kumar@intel.com

drivers/gpu/drm/drm_debugfs_crc.c

@@ -87,6 +87,8 @@ static ssize_t crc_control_write(struct file *file, const char __user *ubuf,
 	struct drm_crtc *crtc = m->private;
 	struct drm_crtc_crc *crc = &crtc->crc;
 	char *source;
+	size_t values_cnt;
+	int ret;
 
 	if (len == 0)
 		return 0;
@@ -104,6 +106,12 @@ static ssize_t crc_control_write(struct file *file, const char __user *ubuf,
 	if (source[len] == '\n')
 		source[len] = '\0';
 
+	if (crtc->funcs->verify_crc_source) {
+		ret = crtc->funcs->verify_crc_source(crtc, source, &values_cnt);
+		if (ret)
+			return ret;
+	}
+
 	spin_lock_irq(&crc->lock);
 
 	if (crc->opened) {

include/drm/drm_crtc.h

@@ -746,6 +746,22 @@ struct drm_crtc_funcs {
 	 */
 	int (*set_crc_source)(struct drm_crtc *crtc, const char *source,
 			      size_t *values_cnt);
+	/**
+	 * @verify_crc_source:
+	 *
+	 * verifies the source of CRC checksums of frames before setting the
+	 * source for CRC and during crc open. Source parameter can be NULL
+	 * while disabling crc source.
+	 *
+	 * This callback is optional if the driver does not support any CRC
+	 * generation functionality.
+	 *
+	 * RETURNS:
+	 *
+	 * 0 on success or a negative error code on failure.
+	 */
+	int (*verify_crc_source)(struct drm_crtc *crtc, const char *source,
+				 size_t *values_cnt);
 
 	/**
 	 * @atomic_print_state: