[PATCH v2 3/3] drm/edid: Implement SCDC Read Request capability detection

[Thierry Reding <thierry.reding@gmail.com>]

From: Thierry Reding <treding@nvidia.com>

Sinks that support SCDC can optionally have the capability to initiate
read requests, which are a mechanism by which a sink can notify its
source that it should read the Update Flags. If either the sink or the
source are not Read Request capable, polling of the Update Flags shall
be employed.

Signed-off-by: Thierry Reding <treding@nvidia.com>
---
Changes in v2:
- new patch

 drivers/gpu/drm/drm_edid.c | 36 ++++++++++++++++++++++++++++++++++++
 include/drm/drm_edid.h     |  1 +
 2 files changed, 37 insertions(+)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 369961597ee5..8211cce3e09e 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -3736,6 +3736,42 @@ bool drm_detect_hdmi_scdc(struct edid *edid)
 EXPORT_SYMBOL(drm_detect_hdmi_scdc);
 
 /**
+ * drm_detect_hdmi_scdc_rr_capable - detect whether an HDMI sink is capable of
+ *    initiating an SCDC Read Request
+ * @edid: sink EDID information
+ *
+ * Parse the CEA extension according to CEA-861-B to find an HF-VSDB as
+ * defined in HDMI 2.0, section 10.3.2 "HDMI Forum Vendor Specific Data
+ * Block" and checks if the RR_Capable bit (bit 6 of byte 6) is set.
+ *
+ * Returns:
+ * True if the sink is capable of initiating an SCDC Read Request, false
+ * otherwise.
+ */
+bool drm_detect_hdmi_scdc_rr_capable(struct edid *edid)
+{
+	unsigned int start, end, i;
+	const u8 *cea;
+
+	cea = drm_find_cea_extension(edid);
+	if (!cea)
+		return false;
+
+	if (cea_db_offsets(cea, &start, &end))
+		return false;
+
+	for_each_cea_db(cea, i, start, end) {
+		if (cea_db_is_hdmi_forum_vsdb(&cea[i])) {
+			if (cea[i + 6] & 0x40)
+				return true;
+		}
+	}
+
+	return false;
+}
+EXPORT_SYMBOL(drm_detect_hdmi_scdc_rr_capable);
+
+/**
  * drm_detect_monitor_audio - check monitor audio capability
  * @edid: EDID block to scan
  *
diff --git a/include/drm/drm_edid.h b/include/drm/drm_edid.h
index 7ea7e90846d8..d1c29586035e 100644
--- a/include/drm/drm_edid.h
+++ b/include/drm/drm_edid.h
@@ -441,6 +441,7 @@ u8 drm_match_cea_mode(const struct drm_display_mode *to_match);
 enum hdmi_picture_aspect drm_get_cea_aspect_ratio(const u8 video_code);
 bool drm_detect_hdmi_monitor(struct edid *edid);
 bool drm_detect_hdmi_scdc(struct edid *edid);
+bool drm_detect_hdmi_scdc_rr_capable(struct edid *edid);
 bool drm_detect_monitor_audio(struct edid *edid);
 bool drm_rgb_quant_range_selectable(struct edid *edid);
 int drm_add_modes_noedid(struct drm_connector *connector,
-- 
2.10.2

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