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

[Sean Paul]

On Tue, Jul 30, 2019 at 03:38:23PM +0200, Neil Armstrong wrote:
> Hi,
> 
> On 25/07/2019 19:49, Sean Paul wrote:
> > 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,
> 
> I'm not qualified as a dw-hdmi expert but until the internal i2c controller
> is exposed as a "standard" i2c adapter (which is a valuable feature),
> blacklisting a fixed address is wrong, and we should detect invalid/malformed
> transactions instead that doesn't fit in the HW model OR really stop emulating
> an i2c adapter.

I think we all agree on this (and Doug mentioned it upthread). That said, the
driver is currently returning successful status and garbage data. I think that's
objectively worse than returning an error, and this patch really doesn't
prevent us from doing it right in the future.

If the code wasn't already upstream, I agree we should pivot to the correct
solution. But unless someone volunteers to fix this the right way, I don't have
a problem with this patch for now.

Sean

> 
> Moving to drm_do_get_edid() would need to entirely rewrite or refactor communication
> code to handle the SCDC transactions, since they use an i2c adapter...
> 
> Neil
> 
> > 
> > Reviewed-by: Sean Paul <sean at poorly.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 at 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++) {
> >>>
> > 
> 

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