linux-kernel - Re: [PATCH] media: v4l2-subdev.h: drop the guard CONFIG_VIDEO_V4L2_SUBDEV_API for v4l2_subdev_get_try

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Date:	Tue, 02 Dec 2014 08:45:36 +0100
From:	Hans Verkuil <hverkuil@...all.nl>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Prabhakar Lad <prabhakar.csengg@...il.com>
CC:	linux-media <linux-media@...r.kernel.org>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Sakari Ailus <sakari.ailus@....fi>
Subject: Re: [PATCH] media: v4l2-subdev.h: drop the guard CONFIG_VIDEO_V4L2_SUBDEV_API
 for v4l2_subdev_get_try_*()

On 12/02/2014 12:26 AM, Laurent Pinchart wrote:
> Hi Prabhakar,
> 
> On Sunday 30 November 2014 21:30:35 Prabhakar Lad wrote:
>> On Sun, Nov 30, 2014 at 9:16 PM, Laurent Pinchart wrote:
>>> On Sunday 30 November 2014 21:05:50 Prabhakar Lad wrote:
>>>> On Sat, Nov 29, 2014 at 7:12 PM, Laurent Pinchart wrote:
>>>>> Hi Prabhakar,
>>>>
>>>> [Snip]
>>>>
>>>>>>> Sure. That's a better choice than removing the config option
>>>>>>> dependency of the fields struct v4l2_subdev.
>>>>>
>>>>> Decoupling CONFIG_VIDEO_V4L2_SUBDEV_API from the availability of the
>>>>> in-kernel pad format and selection rectangles helpers is definitely a
>>>>> good idea. I was thinking about decoupling the try format and
>>>>> rectangles from v4l2_subdev_fh by creating a kind of configuration store
>>>>> structure to store them, and embedding that structure in v4l2_subdev_fh.
>>>>> The pad-level operations would then take a pointer to the configuration
>>>>> store instead of the v4l2_subdev_fh. Bridge drivers that want to
>>>>> implement TRY_FMT based on pad-level operations would create a
>>>>> configuration store, use the pad-level operations, and destroy the
>>>>> configuration store. The userspace subdev API would use the
>>>>> configuration store from the file handle.
>>>>
>>>> are planning to work/post any time soon ? Or are you OK with suggestion
>>>> from Hans ?
>>>
>>> I have no plan to work on that myself now, I was hoping you could
>>> implement it ;-)
>>
>> OK will implement it.
>>
>> Can you please elaborate a more on this "The userspace subdev API would use
>> the configuration store from the file handle."
> 
> Basically,
> 
> 1. Create a subdev pad configuration store structure to store the formats and 
> selection rectangles for each pad.

I wouldn't call it a 'store'. Just call it fmt_config or pad_config something like
that.

> 
> 2. Embed an instance of that structure in v4l2_subdev_fh.
> 
> 3. Modify the subdev pad ops to take a configuration store pointer instead of 
> a file handle pointer.
> 
> The userspace API implementation (v4l2-subdev.c) would then pass &fh->store to 
> the pad operations instead of fh.
> 
> Bridge drivers that need to implement TRY_FMT on top of pad ops would create a 
> temporary store (or temporary stores when multiple subsdevs are involved), 
> call the pad ops with a pointer to the temporary store to propagate TRY 
> formats, destroy the store(s) and return the resulting format.

That will work. I think this is a good approach and it shouldn't be too difficult.

Regards,

	Hans
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