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Subject[PATCH v2] drm/bridge: dw-hdmi: Refuse DDC/CI transfers on the internal I2C controller
Date
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>
---
Changes in v2:
- changed DDC_I2C_ADDR to DDC_CI_ADDR
---
drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
index 045b1b13fd0e..28933629f3c7 100644
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
@@ -35,6 +35,7 @@

#include <media/cec-notifier.h>

+#define DDC_CI_ADDR 0x37
#define DDC_SEGMENT_ADDR 0x30

#define HDMI_EDID_LEN 512
@@ -322,6 +323,13 @@ 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.
+ */
+ return -EOPNOTSUPP;
+
dev_dbg(hdmi->dev, "xfer: num: %d, addr: %#x\n", num, addr);

for (i = 0; i < num; i++) {
--
2.22.0.657.g960e92d24f-goog
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