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

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Message-ID: <20190722202426.GL104440@art_vandelay>
Date:   Mon, 22 Jul 2019 16:24:26 -0400
From:   Sean Paul <sean@...rly.run>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Douglas Anderson <dianders@...omium.org>,
        Adam Jackson <ajax@...hat.com>
Subject: Re: [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@...omium.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