Re: [PATCH] [media] videobuf-dma-contig: replace vm_iomap_memory() with remap_pfn_range().

[Hans Verkuil]

On 09/24/2014 02:40 PM, Marek Szyprowski wrote:
> Hello,
>
> On 2014-09-10 09:00, Hans Verkuil wrote:
> > On 09/10/14 07:28, Fancy Fang wrote:
> > > When user requests V4L2_MEMORY_MMAP type buffers, the videobuf-core
> > > will assign the corresponding offset to the 'boff' field of the
> > > videobuf_buffer for each requested buffer sequentially. Later, user
> > > may call mmap() to map one or all of the buffers with the 'offset'
> > > parameter which is equal to its 'boff' value. Obviously, the 'offset'
> > > value is only used to find the matched buffer instead of to be the
> > > real offset from the buffer's physical start address as used by
> > > vm_iomap_memory(). So, in some case that if the offset is not zero,
> > > vm_iomap_memory() will fail.
> > Is this just a fix for something that can fail theoretically, or do you
> > actually have a case where this happens? I am very reluctant to make
> > any changes to videobuf. Drivers should all migrate to vb2.
> >
> > I have CC-ed Marek as well since he knows a lot more about this stuff
> > than I do.
>
> I'm sorry for a delay, I was really busy with other things.
>
> > > Signed-off-by: Fancy Fang <chen.fang@xxxxxxxxxxxxx>
> > > ---
> > >   drivers/media/v4l2-core/videobuf-dma-contig.c | 4 +++-
> > >   1 file changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/media/v4l2-core/videobuf-dma-contig.c b/drivers/media/v4l2-core/videobuf-dma-contig.c
> > > index bf80f0f..8bd9889 100644
> > > --- a/drivers/media/v4l2-core/videobuf-dma-contig.c
> > > +++ b/drivers/media/v4l2-core/videobuf-dma-contig.c
> > > @@ -305,7 +305,9 @@ static int __videobuf_mmap_mapper(struct videobuf_queue *q,
> > >       /* Try to remap memory */
> > >       size = vma->vm_end - vma->vm_start;
> > >       vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> > > -    retval = vm_iomap_memory(vma, mem->dma_handle, size);
> > > +    retval = remap_pfn_range(vma, vma->vm_start,
> > > +                 mem->dma_handle >> PAGE_SHIFT,
> > > +                 size, vma->vm_page_prot);
> > >       if (retval) {
> > >           dev_err(q->dev, "mmap: remap failed with error %d. ",
> > >               retval);
>
> I think we don't need to revert the code to use remap_pfn_range() again (like
> it was in pre v3.10 times). The simplest way will be to correctly fix
> vma->vm_pgoff and set it to zero before calling vm_iomap_memory(). It is
> done the same way in vb2_dma_contig.c:vb2_dc_mmap().
>
> To sum up - please change your patch: keep vm_iomap_memory() call and add
> "vma->vm_pgoff = 0;" line before it with suitable comment.

Much better, I agree completely.

Regards,

	Hans
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