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

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>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191002124354.v2.1.I709dfec496f5f0b44a7b61dcd4937924da8d8382@changeid

diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
index d77fb8a..a15fbf7 100644 (file)
--- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
+++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
@@ -37,6 +37,7 @@
 #include "dw-hdmi-cec.h"
 #include "dw-hdmi.h"
 
+#define DDC_CI_ADDR            0x37
 #define DDC_SEGMENT_ADDR       0x30
 
 #define HDMI_EDID_LEN          512
@@ -399,6 +400,15 @@ 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++) {