[PATCH v8 06/39] drm/display: scdc-helper: Add helper to set SCDC version information

[Cristian Ciocaltea]

The HDMI 2.x specs mandate that compliant Sink devices report their SCDC
version in the Sink Version register, which reads as 1 on any
SCDC-capable sink.

There is also a dedicated Source Version register.  Writing it is not
compulsory, but the spec advises that compliant Source devices do so, in
which case the value must be 1.

Add drm_scdc_set_source_version() to follow this recommendation.  The
Sink Version register is read first, both to log the advertised SCDC
version and to guard against non-conformant devices: a sink reporting
version 0 is either not SCDC-version-aware or broken, so writing the
source version gains nothing and risks upsetting such hardware.  In that
case the write is skipped.

The source version is passed as a parameter rather than hardcoded, as
future spec revisions may define additional rules for the allowable
version values.  The written value is additionally clamped to the sink's
reported version so the source never advertises a version the sink does
not understand.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea at collabora.com>
---
 drivers/gpu/drm/display/drm_scdc_helper.c | 46 ++++++++++++++++++++++++++++++-
 include/drm/display/drm_scdc_helper.h     |  2 ++
 2 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/display/drm_scdc_helper.c b/drivers/gpu/drm/display/drm_scdc_helper.c
index cb6632346aad..6d804ee19420 100644
--- a/drivers/gpu/drm/display/drm_scdc_helper.c
+++ b/drivers/gpu/drm/display/drm_scdc_helper.c
@@ -21,10 +21,11 @@
  * DEALINGS IN THE SOFTWARE.
  */
 
+#include <linux/delay.h>
 #include <linux/export.h>
 #include <linux/i2c.h>
+#include <linux/minmax.h>
 #include <linux/slab.h>
-#include <linux/delay.h>
 
 #include <drm/display/drm_scdc_helper.h>
 #include <drm/drm_connector.h>
@@ -270,3 +271,46 @@ bool drm_scdc_set_high_tmds_clock_ratio(struct drm_connector *connector,
 	return true;
 }
 EXPORT_SYMBOL(drm_scdc_set_high_tmds_clock_ratio);
+
+/**
+ * drm_scdc_set_source_version - set SCDC source version on the sink
+ * @connector: connector
+ * @ver: source version to advertise (per spec, 1)
+ *
+ * Reads the sink version for diagnostics and as a guard, then writes
+ * the source version unless the sink reports version 0.
+ *
+ * Returns:
+ * 0 on success or when skipped; a negative error code when either
+ * the read or the write failed.
+ */
+int drm_scdc_set_source_version(struct drm_connector *connector, u8 ver)
+{
+	u8 sink_ver;
+	int ret;
+
+	ret = drm_scdc_readb(connector->ddc, SCDC_SINK_VERSION, &sink_ver);
+	if (ret) {
+		drm_scdc_dbg(connector, "Failed to read SCDC_SINK_VERSION: %d\n", ret);
+		return ret;
+	}
+
+	drm_scdc_dbg(connector, "Sink reported SCDC ver. %u\n", sink_ver);
+
+	/*
+	 * Only advertise our source version to sinks that report a
+	 * non-zero sink version.  A sink reporting version 0 is either
+	 * not SCDC-version-aware or non-conformant; writing the source
+	 * version gains nothing and may upset broken hardware.
+	 */
+	if (sink_ver) {
+		ret = drm_scdc_writeb(connector->ddc, SCDC_SOURCE_VERSION,
+				      min_t(u8, sink_ver, ver));
+		if (ret)
+			drm_scdc_dbg(connector,
+				     "Failed to write SCDC_SOURCE_VERSION: %d\n", ret);
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(drm_scdc_set_source_version);
diff --git a/include/drm/display/drm_scdc_helper.h b/include/drm/display/drm_scdc_helper.h
index 34600476a1b9..90b0828364c2 100644
--- a/include/drm/display/drm_scdc_helper.h
+++ b/include/drm/display/drm_scdc_helper.h
@@ -77,4 +77,6 @@ bool drm_scdc_get_scrambling_status(struct drm_connector *connector);
 bool drm_scdc_set_scrambling(struct drm_connector *connector, bool enable);
 bool drm_scdc_set_high_tmds_clock_ratio(struct drm_connector *connector, bool set);
 
+int drm_scdc_set_source_version(struct drm_connector *connector, u8 ver);
+
 #endif

-- 
2.54.0