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

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Date:   Mon, 22 Jul 2019 13:12:40 -0700
From:   Doug Anderson <dianders@...omium.org>
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 <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Adam Jackson <ajax@...hat.com>
Subject: Re: [PATCH v2] drm/bridge: dw-hdmi: Refuse DDC/CI transfers on the
 internal I2C controller

Hi,

On Mon, Jul 22, 2019 at 11:19 AM Matthias Kaehlcke <mka@...omium.org> 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>
> ---
> 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;
> +

In theory we could also solve this by detecting (in other parts of the
function) the bad multi-byte read/write operations.  That would maybe
be more generic (AKA it would more properly handle random userspace
tools fiddling with i2c-dev) but add complexity to the code.

...possibly a better long-term solution is to just totally stop
emulating a regular i2c adapter when the hardware just doesn't support
that.  In theory we could remove the "drm_get_edid()" call in
dw_hdmi_connector_get_modes() and replace it with a direct call to
drm_do_get_edid() if we're using the built-in adapter.  Possibly
"tda998x_drv.c" would be a good reference.  If we did that, we could
remove all the weird / hacky i2c code in this driver.


Since the bigger cleanup seems like a bit much to ask, I'm OK with
this as a stopgap to at least prevent userspace tools from getting
confused.  Thus:

Reviewed-by: Douglas Anderson <dianders@...omium.org>