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

[Neil Armstrong <narmstrong@baylibre.com>]

On 30/07/2019 18:14, Sean Paul wrote:
> 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.

Acked with an updated comment, example :
+		/*
+		 * 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.
+		 */

Neil

> 
> 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@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@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++) {
>>>>>
>>>
>>
>