[PATCHv8 21/26] v4l: vb2-dma-contig: add reference counting for a device from allocator context

[Tomasz Stanislawski]

Hi Laurent,

On 08/15/2012 10:04 PM, Laurent Pinchart wrote:
> Hi Tomasz,
> 
> Thanks for the patch.
> 
> On Tuesday 14 August 2012 17:34:51 Tomasz Stanislawski wrote:
>> This patch adds taking reference to the device for MMAP buffers.
>>
>> Such buffers, may be exported using DMABUF mechanism. If the driver that
>> created a queue is unloaded then the queue is released, the device might be
>> released too.  However, buffers cannot be released if they are referenced by
>> DMABUF descriptor(s). The device pointer kept in a buffer must be valid for
>> the whole buffer's lifetime. Therefore MMAP buffers should take a reference
>> to the device to avoid risk of dangling pointers.
> 
> That patch looks good, but that approach won't scale if we ever need a more 
> complex context that can't be copied. We can ignore that for now, but if we 
> decide to support "orphan" vb2 buffers, that's an issue we need to be aware of 
> as it might come back to bite us in the future.
> 

Good news is that vb2-dma-contig knows the structure of the context.
Therefore it will always be able to implement a proper 'copy constructor'.
One could always add reference counting for contexts but I consider it
little over-engineering.

> I assume that this patch requires your dmabuf owner patch to fix the orphan 
> buffer case properly.
> 

The 'dmabuf owner' patch is not required for this patchset.
It will only protect 'videobuf2-dma-contig' module from being
unloaded while its dmabuf is in use.

Regards,
Tomasz Stanislawski