Re: [PATCH v2 1/9] media: v4l2-subdev: Add a pad variant of .query_dv_timings()

From: Hans Verkuil <hverkuil-cisco@xs4all.nl>
To: "Paweł Anikiel" <panikiel@google.com>
Cc: airlied@gmail.com, akpm@linux-foundation.org,
	conor+dt@kernel.org, daniel@ffwll.ch, dinguyen@kernel.org,
	krzysztof.kozlowski+dt@linaro.org,
	maarten.lankhorst@linux.intel.com, mchehab@kernel.org,
	mripard@kernel.org, robh+dt@kernel.org, tzimmermann@suse.de,
	devicetree@vger.kernel.org, dri-devel@lists.freedesktop.org,
	linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
	chromeos-krk-upstreaming@google.com, ribalda@chromium.org
Subject: Re: [PATCH v2 1/9] media: v4l2-subdev: Add a pad variant of .query_dv_timings()
Date: Thu, 29 Feb 2024 13:05:33 +0100	[thread overview]
Message-ID: <e6c16850-50cf-442c-a854-15134e416954@xs4all.nl> (raw)
In-Reply-To: <CAM5zL5r5JtA2HojaYZkSfUvoMTSNWALQM8HVuuXq-Znu7+TvGw@mail.gmail.com>

On 2/29/24 12:33, Paweł Anikiel wrote:
> On Thu, Feb 29, 2024 at 9:02 AM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
>>
>> On 28/02/2024 16:34, Paweł Anikiel wrote:
>>> On Wed, Feb 28, 2024 at 12:25 PM Hans Verkuil <hverkuil-cisco@xs4all.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