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@xxxxxxxxxxx>
> >> Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
> >> ---
> >>
> >> 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).
--
Regards,
Laurent Pinchart
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