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

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

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.

- Lars



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