linux-kernel - Re: [PATCH v2 1/9] media: v4l2-subdev: Add a pad variant of .query_dv

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Date: Thu, 29 Feb 2024 13:05:33 +0100
From: Hans Verkuil <hverkuil-cisco@...all.nl>
To: Paweł Anikiel <panikiel@...gle.com>
Cc: airlied@...il.com, akpm@...ux-foundation.org, conor+dt@...nel.org,
 daniel@...ll.ch, dinguyen@...nel.org, krzysztof.kozlowski+dt@...aro.org,
 maarten.lankhorst@...ux.intel.com, mchehab@...nel.org, mripard@...nel.org,
 robh+dt@...nel.org, tzimmermann@...e.de, devicetree@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 linux-media@...r.kernel.org, chromeos-krk-upstreaming@...gle.com,
 ribalda@...omium.org
Subject: Re: [PATCH v2 1/9] media: v4l2-subdev: Add a pad variant of
 .query_dv_timings()

On 2/29/24 12:33, Paweł Anikiel wrote:
> On Thu, Feb 29, 2024 at 9:02 AM Hans Verkuil <hverkuil-cisco@...all.nl> wrote:
>>
>> On 28/02/2024 16:34, Paweł Anikiel wrote:
>>> On Wed, Feb 28, 2024 at 12:25 PM Hans Verkuil <hverkuil-cisco@...all.nl> wrote:
>>>>
>>>> Hi Paweł,
>>>>
>>>> On 21/02/2024 17:02, Paweł Anikiel wrote:
>>>>> Currently, .query_dv_timings() is defined as a video callback without
>>>>> a pad argument. This is a problem if the subdevice can have different
>>>>> dv timings for each pad (e.g. a DisplayPort receiver with multiple
>>>>> virtual channels).
>>>>>
>>>>> To solve this, add a pad variant of this callback which includes
>>>>> the pad number as an argument.
>>>>
>>>> So now we have two query_dv_timings ops: one for video ops, and one
>>>> for pad ops. That's not very maintainable. I would suggest switching
>>>> all current users of the video op over to the pad op.
>>>
>>> I agree it would be better if there was only one. However, I have some concerns:
>>> 1. Isn't there a problem with backwards compatibility? For example, an
>>> out-of-tree driver is likely to use this callback, which would break.
>>> I'm asking because I'm not familiar with how such API changes are
>>> handled.
>>
>> It's out of tree, so they will just have to adapt. That's how life is if
>> you are not part of the mainline kernel.
>>
>>> 2. If I do switch all current users to the pad op, I can't test those
>>> changes. Isn't that a problem?
>>
>> I can test one or two drivers, but in general I don't expect this to be
>> a problem.
>>
>>> 3. Would I need to get ACK from all the driver maintainers?
>>
>> CC the patches to the maintainers. Generally you will get back Acks from
>> some but not all maintainers, but that's OK. For changes affecting multiple
>> drivers you never reach 100% on that. I can review the remainder. The DV
>> Timings API is my expert area, so that shouldn't be a problem.
>>
>> A quick grep gives me these subdev drivers that implement it:
>>
>> adv748x, adv7604, adv7842, tc358743, tda1997x, tvp7002, gs1662.
>>
>> And these bridge drivers that call the subdevs:
>>
>> cobalt, rcar-vin, vpif_capture.
>>
>> The bridge drivers can use the following pad when calling query_dv_timings:
>>
>> cobalt: ADV76XX_PAD_HDMI_PORT_A
>> rcar_vin: vin->parallel.sink_pad
>> vpif_capture: 0
>>
>> The converted subdev drivers should check if the pad is an input pad.
>> Ideally it should check if the pad is equal to the current input pad
>> since most devices can only query the timings for the currently selected
>> input pad. But some older drivers predate the pad concept and they
>> ignore the pad value.
> 
> Thank you for the helpful info. I will switch all these drivers to the
> pad op, then. Would you like me to prepare a separate patchset, or
> should I include the changes in this one?

I think I prefer a separate patchset for this.

Regards,

	Hans