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

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Message-ID: <ea3219fd-b8eb-279b-aa08-c77705a31249@baylibre.com>
Date:   Tue, 30 Jul 2019 15:38:23 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Sean Paul <sean@...rly.run>, 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>,
        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

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.

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