Re: [PATCH 4/5] media: i2c: cat24c208: driver for the cat24c208 EDID EEPROM

[Hans Verkuil <hverkuil-cisco@xs4all.nl>]

On 29/07/2022 16:47, Andy Shevchenko wrote:
> On Fri, Jul 29, 2022 at 2:11 PM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
>> On 29/07/2022 14:00, Andy Shevchenko wrote:
>>> On Fri, Jul 29, 2022 at 9:21 AM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
>>>> On 28/07/2022 22:56, Andy Shevchenko wrote:
>>>>> On Thu, Jul 28, 2022 at 3:23 PM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
>>>>>> On 28/07/2022 14:02, Andy Shevchenko wrote:
>>>>>>> On Thursday, July 28, 2022, Erling Ljunggren <hljunggr@cisco.com <mailto:hljunggr@cisco.com>> wrote:
>>>>>
>>>>>>>     Support reading and writing the EDID EEPROM through the
>>>>>>>     v4l2 API.
>>>>>>>
>>>>>>> Why the normal way of representing as a memory (we have framework and drivers) can’t work?
>>>>>>
>>>>>> Because support for EDID for video sinks is already part of the media subsystem (V4L2).
>>>>>> Normally it is integrated into an HDMI receiver, but in this case it is just the EDID
>>>>>> support without the video receiver. It belongs in drivers/media in any case since EDIDs
>>>>>> are closely tied to media.
>>>>>
>>>>> It's fine. From the Linux perspective we do not reduplicate the
>>>>> drivers that are done by other frameworks, right?
>>>>>
>>>>>>> Moreover, this driver seems limited in support of variety of the eeprom chips.
>>>>>>
>>>>>> Not quite sure what you mean. The cat24c208 is what this was developed for and
>>>>>> the only one we have.
>>>>>>
>>>>>> Note that an EDID EEPROM != a regular EEPROM: it has to support the VESA E-DDC
>>>>>> standard, which a normal EEPROM doesn't. So these devices are specifically made
>>>>>> for this use-case.
>>>>>
>>>>> What is the difference from a programming interface?
>>>>> Can the nvmem driver(s) be reused (at24?)?
>>>>
>>>> No. EDID EEPROM devices are specific to storing EDIDs: they have two i2c
>>>> ports, one connected to (typically) the HDMI bus (DDC lines) allowing a
>>>> video source to read the EDID, the other is connected to the SoC to write to
>>>> and configure the device. The HDMI bus side has two i2c addresses (reading the
>>>> EEPROM and to write to the segment address for EDIDs > 256 bytes), the SoC
>>>> side has three i2c addresses: to configure the behavior, the segment address,
>>>> and to write the EDID from the SoC.
>>>>
>>>> So it is a much more complex device than a regular eeprom, and it really
>>>> is dedicated to EDIDs only.
>>>
>>> Thanks for the explanation, but it's still unclear what the
>>> differences are in the programming interface there. Perhaps you may
>>> simply register a platform device in this driver and reuse the rest
>>> from at24?
>>
>> No, it's really different from a regular eeprom.
>>
>>>> Also note that the V4L2 API is already used to get/set EDIDs, everything is
>>>> in place for supporting that, including support for parsing EDIDs for the
>>>> physical address, which is something that is needed if this is combined with
>>>> HDMI CEC hardware. It's not implemented in this driver since it is not
>>>> needed in our use-case, but that might change in the future.
>>>>
>>>> And by using the V4L2 API you can use v4l2-ctl --get-edid and --set-edid
>>>> out of the box, using the standard API for EDIDs.
>>>
>>> Bonus question: we have cat24c04/cat24c05 are recognized by at24
>>> already, are they different to cat24c08?
>>>
>>
>> Yes, they are different.
> 
> Thanks for your patience and elaboration, I got it.
> 
> Would this driver be used only by v4l2? Or potentially some other
> hardware may need it (DRM?)?

Only V4L2: an EDID describes the capabilities of a video sink (e.g. a display),
so it is specific to video receivers, and that's the domain of V4L2.

Regards,

	Hans