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

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Date:   Mon, 7 Oct 2019 13:54:05 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Matthias Kaehlcke <mka@...omium.org>,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        David Airlie <airlied@...ux.ie>
Cc:     Douglas Anderson <dianders@...omium.org>,
        Sean Paul <sean@...rly.run>, Yakir Yang <kuankuan.y@...il.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Tzung-Bi Shih <tzungbi@...omium.org>,
        Jerome Brunet <jbrunet@...libre.com>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Cheng-Yi Chiang <cychiang@...omium.org>
Subject: Re: [PATCH v2] drm/bridge: dw-hdmi: Refuse DDC/CI transfers on the
 internal I2C controller

Hi Matthias,

On 02/10/2019 21:44, 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
> 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@...omium.org>
> Reviewed-by: Douglas Anderson <dianders@...omium.org>
> Reviewed-by: Sean Paul <sean@...rly.run>
> Acked-by: Neil Armstrong <narmstrong@...libre.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++) {
> 

Applying to drm-misc-next

Thanks !
Neil