linux-kernel - Re: [PATCH v3] media: usb: uvc: use vb2_ops_wait

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Message-ID: <CA+V-a8vVepBkjMdQyckqXjciqPKGJJLHnW0Y3144BUx4q42=FA@mail.gmail.com>
Date:	Sat, 29 Nov 2014 18:25:42 +0000
From:	Prabhakar Lad <prabhakar.csengg@...il.com>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	LMML <linux-media@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [PATCH v3] media: usb: uvc: use vb2_ops_wait_prepare/finish helper

Hi Laurent,

On Sat, Nov 29, 2014 at 6:11 PM, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
> Hi Prabhakar,
>
> On Friday 28 November 2014 17:06:52 Prabhakar Lad wrote:
>> On Thu, Nov 27, 2014 at 9:32 PM, Laurent Pinchart wrote:
>> > Hi Prabhakar,
>>
>> [Snip]
>>
>> >> +     queue->queue.lock = &queue->mutex;
>> >
>> > I'm a bit concerned that this would introduce future breakages. Setting
>> > the queue.lock pointer enables locking in all vb2_fop_* and vb2_ops_wait_*
>> > functions. The uvcvideo driver isn't ready for that, but doesn't use the
>> > vb2_fop_* functions yet, so that's not an issue. However, in the future,
>> > videobuf2 might use the lock in more places, including functions used by
>> > the uvcvideo driver. This could then cause breakages.
>>
>> Even if in future if videobuf2 uses this lock it would be in helpers mostly,
>> so any way it doesn’t harm :)
>
> My concern is about vb2 starting using the lock in existing helpers used by
> the uvcvideo driver. I suppose we can deal with it later.
>
>> > It would be better to completely convert the uvcvideo driver to the
>> > vb2_fop_* functions if we want to use vb2_ops_*. I'm not sure how complex
>> > that would be though, and whether it would be possible while still
>> > keeping the fine-grained locking implemented by the uvcvideo driver. Do
>> > you think it should be attempted ?
>>
>> mmap & poll should be fairly simple, looks like open & release cannot be
>> dropped as it does some usb_autopm_get/put_interface() calls which I am not
>> aware of.
>
> I've looked at that, and there's a race condition in vb2_fop_poll() (for which
> I've already sent a patch) and possible in vb2_mmap() (raised the issue on
> #v4l today) as well that need to be fixed first.
>
> Anyway, for this patch
>
> Acked-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
>
> Have you tested it by the way ?
>
I have just compile tested it.

> Should I take it in my tree or will you send a pull request for the whole
> series ?
>
Probably you can pick this up via your tree.

Thanks,
--Prabhakar Lad

>> >>       ret = vb2_queue_init(&queue->queue);
>> >>       if (ret)
>> >>               return ret;
>
> --
> Regards,
>
> Laurent Pinchart
>
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