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

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Date:	Fri, 28 Nov 2014 17:06:52 +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,

Thanks for the review.

On Thu, Nov 27, 2014 at 9:32 PM, Laurent Pinchart
<laurent.pinchart@...asonboard.com> 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 :)

> 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.

Thanks,
--Prabhakar Lad

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