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

From: Hans Verkuil
Date: Wed Sep 24 2014 - 08:43:20 EST

On 09/24/2014 02:40 PM, Marek Szyprowski wrote:

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. ",

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.


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