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

chen.fang
2014-09-25T02:17:48Z

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Thanks for your idea, Marek and Hans. This change makes sense. And I've already
verified it. I'll resubmit my change soon.

Best regards,
Fancy Fang

-----Original Message-----
From: Hans Verkuil [mailto:hverkuil@xs4all.nl]
Sent: Wednesday, September 24, 2014 8:43 PM
To: Marek Szyprowski; Fang Chen-B47543; m.chehab@samsung.com; viro@ZenIV.linux.org.uk
Cc: Guo Shawn-R65073; linux-media@vger.kernel.org; linux-kernel@vger.kernel.org
Subject: Re: [PATCH] [media] videobuf-dma-contig: replace vm_iomap_memory() with remap_pfn_range().

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@freescale.com>
>>> ---
>>> 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
> vma->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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