linux-kernel - Re: [PATCH] [media] videobuf-dma-contig: replace vm_iomap_memory() with remap_pfn

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Date:	Wed, 24 Sep 2014 14:42:49 +0200
From:	Hans Verkuil <hverkuil@...all.nl>
To:	Marek Szyprowski <m.szyprowski@...sung.com>,
	Fancy Fang <chen.fang@...escale.com>, m.chehab@...sung.com,
	viro@...IV.linux.org.uk
CC:	shawn.guo@...escale.com, linux-media@...r.kernel.org,
	linux-kernel@...r.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@...escale.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 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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