On Mon, Jan 16, 2012 at 11:03 AM, Semwal, Sumit <sumit.semwal@xxxxxx> wrote:
>
> On Sun, Jan 15, 2012 at 2:08 AM, Sakari Ailus <sakari.ailus@xxxxxx> wrote:
>>
>> Hi Sumit,
>>
>> Thanks for the patch!
>
Hi Sakari,
Thanks for reviewing this :)
>>
>>
>> <snip>
>> Shouldn't the buffer mapping only be done at the first call to
>> __qbuf_dmabuf()? On latter calls, the cache would need to be handled
>> according to presence of V4L2_BUF_FLAG_NO_CACHE_CLEAN /
>> V4L2_BUF_FLAG_NO_CACHE_INVALIDATE in v4l2_buffer.
>
Well, the 'map / unmap' implementation is by design exporter-specific;
so the exporter of the buffer may choose to, depending on the use
case, 'map-and-keep' on first call to map_dmabuf, and do actual unmap
only at 'release' time. This will mean that the {map,unmap}_dmabuf
calls will be used mostly for 'access-bracketing' between multiple
users, and not for actual map/unmap each time.
Again, the framework is flexible enough to allow exporters to actually
map/unmap as required (think cases where backing-storage migration
might be needed while buffers are in use - in that case, when all
current users have called unmap_XXX() on a buffer, it can be migrated,
and the next map_XXX() calls could give different mappings back to the
users).
> The kernel 'users' of dma-buf [in case of this RFC, v4l2] should not ideally need to worry about the actual mapping/unmapping that is done; the buffer exporter in a particular use-case should be able to handle it.
> <snip>
>>
>> Same here, except reverse: this only should be done when the buffer is
>> destroyed --- either when the user explicitly calls reqbufs(0) or when
>> the file handle owning this buffer is being closed.
>>
>> Mapping buffers at every prepare_buf and unmapping them in dqbuf is
>> prohibitively expensive. Same goes for many other APIs than V4L2, I think.
>>
>> I wonder if the means to do this exists already.
>>
>> I have to admit I haven't followed the dma_buf discussion closely so I
>> might be missing something relevant here.
>
Hope the above explanation helps. Please do not hesitate to contact if
you need more details.
>>
>>
>> Kind regards,
>>
>> --
>> Sakari Ailus
>> sakari.ailus@xxxxxx
Best regards,
~Sumit.
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