Re: [PATCH] [media] vb2: map dmabuf for planes on driver queue instead of vidioc_qbuf

From: Javier Martinez Canillas
Date: Wed Jul 20 2016 - 10:19:16 EST

Hello Hans,

On 07/20/2016 10:12 AM, Hans Verkuil wrote:
> On 07/20/2016 04:06 PM, Javier Martinez Canillas wrote:
>> Hello Sakari,
>> On 07/20/2016 09:20 AM, Sakari Ailus wrote:
>>> Hi Javier,
>>> On Fri, Jul 15, 2016 at 12:26:06PM -0400, Javier Martinez Canillas wrote:
>>>> The buffer planes' dma-buf are currently mapped when buffers are queued
>>>> from userspace but it's more appropriate to do the mapping when buffers
>>>> are queued in the driver since that's when the actual DMA operation are
>>>> going to happen.
>>>> Suggested-by: Nicolas Dufresne <nicolas.dufresne@xxxxxxxxxxxxx>
>>>> Signed-off-by: Javier Martinez Canillas <javier@xxxxxxxxxxxxxxx>
>>>> ---
>>>> Hello,
>>>> A side effect of this change is that if the dmabuf map fails for some
>>>> reasons (i.e: a driver using the DMA contig memory allocator but CMA
>>>> not being enabled), the fail will no longer happen on VIDIOC_QBUF but
>>>> later (i.e: in VIDIOC_STREAMON).
>>>> I don't know if that's an issue though but I think is worth mentioning.
>>> I have the same question has Hans --- why?
>> Yes, sorry for missing this information. Nicolas already explained a little
>> bit but the context is that I want to add dma-buf fence support to videobuf2,
>> and currently the dma-buf is unmapped in VIDIOC_DQBUF.
>> But with dma-buf fence, the idea is to be able to dequeue a buffer even when
>> the driver has not yet finished processing the buffer. So the dma-buf needs to
>> be mapped until vb2_buffer_done() when the driver is done processing the vb2,
>> and is able to signal the pending fence.
>> Since the unmapping was going to be delayed to vb2_buffer_done(), I thought
>> it would make sense to also move the mapping closer to when is really going
>> to be used and that's why I moved it to __enqueue_in_driver() in this patch.
>> But I didn't know that user-space was using the dma-buf map as a way to know
>> if the dma-buf will be compatible and fallback to a different streaming I/O
>> method if that's not the case. So $SUBJECT is wrong if it prevents user-space
>> to recover gracefully from a dma-buf mapping failure.
>> In any case, only delaying the unmapping is needed to support fence and doing
>> the map early in VIDIOC_QBUF is not an issue.
> OK. I've rejected this patch. I understand the DQBUF part and I happily accept

Ok, thanks.

> a patch for that. But the other side should be left as-is. The TODO comment
> should probably be dropped, now that I think about it.

I can post such a patch, do you want me to also add a comment about why is done
in QBUF instead of when the buffer is queued in the driver (e.g: that user-space
is able to recover in QBUF but no in STREAMON) or just removing it and mention
that in the commit message is enough?

> Regards,
> Hans

Best regards,
Javier Martinez Canillas
Open Source Group
Samsung Research America