[PATCH v2 2/3] drm/edid: Implement SCDC support detection

[Thierry Reding <thierry.reding@xxxxxxxxx>]

From: Thierry Reding <treding@xxxxxxxxxx>

Sinks compliant with the HDMI 2.0 specification may support SCDC, a
mechanism for the source and the sink to communicate. Sinks advertise
support for this feature in the HDMI Forum Vendor Specific Data Block
as defined in the HDMI 2.0 specification, section 10.4 "Status and
Control Data Channel". Implement a small helper that find the HF-VSDB
and parses it to check if the sink supports SCDC.

Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
---
Changes in v2:
- rename internal function to cea_db_is_hdmi_forum_vsdb() for more
  clarity (Ville Syrjälä)

 drivers/gpu/drm/drm_edid.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++
 include/drm/drm_edid.h     |  1 +
 include/linux/hdmi.h       |  1 +
 3 files changed, 51 insertions(+)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 336be31ff3de..369961597ee5 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -3238,6 +3238,21 @@ static bool cea_db_is_hdmi_vsdb(const u8 *db)
 	return hdmi_id == HDMI_IEEE_OUI;
 }
 
+static bool cea_db_is_hdmi_forum_vsdb(const u8 *db)
+{
+	unsigned int oui;
+
+	if (cea_db_tag(db) != VENDOR_BLOCK)
+		return false;
+
+	if (cea_db_payload_len(db) < 7)
+		return false;
+
+	oui = db[3] << 16 | db[2] << 8 | db[1];
+
+	return oui == HDMI_FORUM_IEEE_OUI;
+}
+
 #define for_each_cea_db(cea, i, start, end) \
 	for ((i) = (start); (i) < (end) && (i) + cea_db_payload_len(&(cea)[(i)]) < (end); (i) += cea_db_payload_len(&(cea)[(i)]) + 1)
 
@@ -3687,6 +3702,40 @@ bool drm_detect_hdmi_monitor(struct edid *edid)
 EXPORT_SYMBOL(drm_detect_hdmi_monitor);
 
 /**
+ * drm_detect_hdmi_scdc - detect whether an HDMI sink supports SCDC
+ * @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 SCDC_Present bit (bit 7 of byte 6) is set.
+ *
+ * Returns:
+ * True if the sink supports SCDC, false otherwise.
+ */
+bool drm_detect_hdmi_scdc(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] & 0x80)
+				return true;
+		}
+	}
+
+	return false;
+}
+EXPORT_SYMBOL(drm_detect_hdmi_scdc);
+
+/**
  * 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 38eabf65f19d..7ea7e90846d8 100644
--- a/include/drm/drm_edid.h
+++ b/include/drm/drm_edid.h
@@ -440,6 +440,7 @@ int drm_add_edid_modes(struct drm_connector *connector, struct edid *edid);
 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_monitor_audio(struct edid *edid);
 bool drm_rgb_quant_range_selectable(struct edid *edid);
 int drm_add_modes_noedid(struct drm_connector *connector,
diff --git a/include/linux/hdmi.h b/include/linux/hdmi.h
index edbb4fc674ed..d271ff23984f 100644
--- a/include/linux/hdmi.h
+++ b/include/linux/hdmi.h
@@ -35,6 +35,7 @@ enum hdmi_infoframe_type {
 };
 
 #define HDMI_IEEE_OUI 0x000c03
+#define HDMI_FORUM_IEEE_OUI 0xc45dd8
 #define HDMI_INFOFRAME_HEADER_SIZE  4
 #define HDMI_AVI_INFOFRAME_SIZE    13
 #define HDMI_SPD_INFOFRAME_SIZE    25
-- 
2.10.2

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