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SubjectRe: [PATCH v2] drm/bridge: dw-hdmi: Refuse DDC/CI transfers on the internal I2C controller
On Mon, Jul 22, 2019 at 11:19:45AM -0700, Matthias Kaehlcke wrote:
> 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

This is very likely a stupid question, but I didn't see an answer for it, so
I'll just ask :)

If the controller supports xfers of 8 bytes and 1 bytes, could you just split
up any of these transactions into len/8+len%8 transactions?

Sean

> 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
>

--
Sean Paul, Software Engineer, Google / Chromium OS

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