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

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Message-ID: <20190725174927.GN104440@art_vandelay>
Date:   Thu, 25 Jul 2019 13:49:27 -0400
From:   Sean Paul <sean@...rly.run>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Sean Paul <sean@...rly.run>, 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 02:02:07PM -0700, Matthias Kaehlcke wrote:
> On Mon, Jul 22, 2019 at 04:24:26PM -0400, Sean Paul wrote:
> > 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?
> 
> The controller interprets all transfers to be register accesses. It is
> not possible to just send the sequence '0x0a 0x0b 0x0c' as three byte
> transfers, the controller expects an address for each byte and
> (supposedly) sends it over the wire, which typically isn't what you
> want.
> 
> Also the 8-byte reads only seem to be supported in certain
> configurations ("when the DWC_HDMI_TX_20 parameter is enabled").

Thanks for the detailed answers (both you and Doug)!

This change looks good to me, but I'll leave it to a dw-hdmi expert to apply. So
fwiw,

Reviewed-by: Sean Paul <sean@...rly.run>


> 
> > > 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++) {
> > 

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