Re: [GIT PULL FOR v3.19] R-Car DU changes

[Lars-Peter Clausen <lars@metafoo.de>]

On 11/24/2014 09:01 PM, Dave Airlie wrote:
> On 25 November 2014 at 00:18, Lars-Peter Clausen <lars@metafoo.de> wrote:
>> On 11/24/2014 03:00 PM, Laurent Pinchart wrote:
>>>
>>> Hi Daniel,
>>>
>>> (CC'ing Rob Clark and Lars-Peter. As a reminder we're discussing the "drm:
>>> Decouple EDID parsing from I2C adapter" patch available at
>>> git://linuxtv.org/pinchartl/fbdev.git drm/next/du)
>>>
>>> On Monday 24 November 2014 14:09:39 Daniel Vetter wrote:
>>>>
>>>> On Mon, Nov 24, 2014 at 11:46:18AM +0200, Laurent Pinchart wrote:
>>>>>>
>>>>>> - the interface looks rather backwards: Either this still does i2c
>>>>>>     reads, and then you'd just need a i2c-over-whatever adapter to make
>>>>>> it
>>>>>>     work. Or you have other magic means to optain an edid block, in
>>>>>> which
>>>>>>     case just do that and then feed the edid drm_add_edid_modes.
>>>>>
>>>>>
>>>>> I have a magic way to get EDID over I2C :-) Basically the ADV7511
>>>>> controls
>>>>> the DDC bus, and exposes EDID data over I2C using vendor commands. To
>>>>> read an EDID block I have to write an ADV7511 register over I2C with the
>>>>> block number, wait for an interrupt, read a status register to check
>>>>> whether EDID data is available or whether an error occurred, and then
>>>>> read EDID data from the ADV7511 over I2C in 64-bytes chunks. This needs
>>>>> to be repeated for every block. I thus can't use drm_get_edid()
>>>>> directly.
>>>>
>>>>
>>>> Sounds familiar. See the special ddc-over-sdvo i2c bus we register in
>>>> intel_sdvo.c, specifically look at intel_sdvo_init_ddc_proxy. It is a bit
>>>> of boilerplate, but in the end just amounts to 3 small functions and one
>>>> tiny vtable to wire it all up cleanly.
>>>
>>>
>>> That's what I would have done as well if I had a device-specific I2C
>>> adapter
>>> connected to the DDC bus, but in this case the interface exposed by the
>>> ADV7511 to the SoC over I2C consists of higher level device-specific I2C
>>> commands to read EDID data. There is no low-level I2C read/write
>>> primitives
>>> available. I would thus need to expose a fake adapter that would receive
>>> I2C
>>> commands, parse them to detect an EDID block read, retrieve the EDID data
>>> and
>>> return them from the fake read. That doesn't make much sense to me.
>>
>>
>> The intel sdvo looks just like a simple I2C mux which will just pass-through
>> messages from the master to the EDID EEPROM. The ADV7511 is unfortunately a
>> bit different. You tell it to fetch the EDID information, then it will do
>> some magic and then you can read the EDID back. Abstracting this as a this
>> as a I2C controller will, while possible, result in a fair amount of boiler
>> plate code that will not look particularly pretty.
>
> It sounds also a bit like DP auxch also, or even how on UDL we get the edid
> over USB.
>
> I'd rather see not pretty code that only one person had to look at though :-)
> with lots of comments on the hw design that demands ugly.

I'd rather not see that if I'm the person who has to look at it ;)

But looking around it appears as if this is a more common thing among 
external HDMI encoders though. E.g. the tda998x has the exact same issue and 
the current driver just copy&pastes drm_do_get_edid() and plugs in its own 
EDID block retrieval method.

On the other hand the patch that makes it possible to use a different 
backend then a raw I2C adapter to fetch the EDID is extremely trivial and 
doesn't make the existing code more complex in any way.
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