[PATCH v2] drm/bridge: dw-hdmi: Refuse DDC/CI transfers on the internal I2C controller

[Matthias Kaehlcke <mka@chromium.org>]

The DDC/CI protocol involves sending a multi-byte request to the
display via I2C, which is typically followed by a multi-byte
response. The internal I2C controller only allows single byte
reads/writes or reads of 8 sequential bytes, hence DDC/CI is not
supported when the internal I2C controller is used. The I2C
transfers complete without errors, however the data in the response
is garbage. Abort transfers to/from slave address 0x37 (DDC) with
-EOPNOTSUPP, to make it evident that the communication is failing.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Sean Paul <sean@poorly.run>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
---
Sorry for the delay with sending v2, I completely forgot about this patch ...

Changes in v2:
- updated comment with 'TOFIX' entry as requested by Neil
- added Neil's 'Acked-by' tag

 drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
index 52d220a70362..ac24bceaf415 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
@@ -41,6 +41,7 @@
 
 #include <media/cec-notifier.h>
 
+#define DDC_CI_ADDR		0x37
 #define DDC_SEGMENT_ADDR	0x30
 
 #define HDMI_EDID_LEN		512
@@ -439,6 +440,15 @@ static int dw_hdmi_i2c_xfer(struct i2c_adapter *adap,
 	u8 addr = msgs[0].addr;
 	int i, ret = 0;
 
+	if (addr == DDC_CI_ADDR)
+		/*
+		 * The internal I2C controller does not support the multi-byte
+		 * read and write operations needed for DDC/CI.
+		 * TOFIX: Blacklist the DDC/CI address until we filter out
+		 * unsupported I2C operations.
+		 */
+		return -EOPNOTSUPP;
+
 	dev_dbg(hdmi->dev, "xfer: num: %d, addr: %#x\n", num, addr);
 
 	for (i = 0; i < num; i++) {
-- 
2.23.0.444.g18eeb5a265-goog