[PATCH 04/12] v4l: vb2-dma-contig: add setup of sglist for MMAP buffers

[Subash Patel]

Hello Tomasz,

On 06/07/2012 06:06 AM, Laurent Pinchart wrote:
> Hi Tomasz,
>
> On Wednesday 06 June 2012 13:56:42 Tomasz Stanislawski wrote:
>> On 06/06/2012 10:06 AM, Laurent Pinchart wrote:
>>> On Wednesday 23 May 2012 15:07:27 Tomasz Stanislawski wrote:
>>>> This patch adds the setup of sglist list for MMAP buffers.
>>>> It is needed for buffer exporting via DMABUF mechanism.
>>>>
>>>> Signed-off-by: Tomasz Stanislawski<t.stanislaws at samsung.com>
>>>> Signed-off-by: Kyungmin Park<kyungmin.park at samsung.com>
>>>> ---
>>>>
>>>>   drivers/media/video/videobuf2-dma-contig.c |   70 +++++++++++++++++++++-
>>>>   1 file changed, 68 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/media/video/videobuf2-dma-contig.c
>>>> b/drivers/media/video/videobuf2-dma-contig.c index 52b4f59..ae656be
>>>> 100644
>>>> --- a/drivers/media/video/videobuf2-dma-contig.c
>>>> +++ b/drivers/media/video/videobuf2-dma-contig.c
>
> [snip]
>
>>>> +static int vb2_dc_kaddr_to_pages(unsigned long kaddr,
>>>> +	struct page **pages, unsigned int n_pages)
>>>> +{
>>>> +	unsigned int i;
>>>> +	unsigned long pfn;
>>>> +	struct vm_area_struct vma = {
>>>> +		.vm_flags = VM_IO | VM_PFNMAP,
>>>> +		.vm_mm = current->mm,
>>>> +	};
>>>> +
>>>> +	for (i = 0; i<  n_pages; ++i, kaddr += PAGE_SIZE) {
>>>
>>> The follow_pfn() kerneldoc mentions that it looks up a PFN for a user
>>> address. The only users I've found in the kernel sources pass a user
>>> address. Is it legal to use it for kernel addresses ?
>>
>> It is not completely legal :). As I understand the mm code, the follow_pfn
>> works only for IO/PFN mappings. This is the typical case (every case?) of
>> mappings created by dma_alloc_coherent.
>>
>> In order to make this function work for a kernel pointer, one has to create
>> an artificial VMA that has IO/PFN bits on.
>>
>> This solution is a hack-around for dma_get_pages (aka dma_get_sgtable). This
>> way the dependency on dma_get_pages was broken giving a small hope of
>> merging vb2 exporting.
>>
>> Marek prepared a patchset 'ARM: DMA-mapping: new extensions for buffer
>> sharing' that adds dma buffers with no kernel mappings and dma_get_sgtable
>> function.
>>
>> However this patchset is still in a RFC state.
>
> That's totally understood :-) I'm fine with keeping the hack for now until the
> dma_get_sgtable() gets in a usable/mergeable state, please just mention it in
> the code with something like
>
> /* HACK: This is a temporary workaround until the dma_get_sgtable() function
> becomes available. */
>
>> I have prepared a patch that removes vb2_dc_kaddr_to_pages and substitutes
>> it with dma_get_pages. It will become a part of vb2-exporter patches just
>> after dma_get_sgtable is merged (or at least acked by major maintainers).
>
The above function call (because of follow_pfn) doesn't succeed for all 
the allocated pages. Hence I created a patch(attached) which is based on 
[1] series. One can apply it for using your present patch-set in the 
meantime.

Regards,
Subash
[1] http://www.spinics.net/lists/kernel/msg1343092.html