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

[Marek Szyprowski]

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.

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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