Hi Laurent,
Thank your for your comments.
On 06/06/2012 10:06 AM, Laurent Pinchart wrote:
> Hi Tomasz,
>
> Thanks for the patch.
>
> On Wednesday 23 May 2012 15:07:27 Tomasz Stanislawski wrote:
>> This patch adds the setup of sglist list for MMAP buffers.
>> It is needed for buffer exporting via DMABUF mechanism.
>>
>> Signed-off-by: Tomasz Stanislawski <t.stanislaws@xxxxxxxxxxx>
>> Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
>> ---
>> drivers/media/video/videobuf2-dma-contig.c | 70 ++++++++++++++++++++++++-
>> 1 file changed, 68 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/media/video/videobuf2-dma-contig.c
>> b/drivers/media/video/videobuf2-dma-contig.c index 52b4f59..ae656be 100644
>> --- a/drivers/media/video/videobuf2-dma-contig.c
>> +++ b/drivers/media/video/videobuf2-dma-contig.c
>> @@ -32,6 +32,7 @@ struct vb2_dc_buf {
>> /* MMAP related */
>> struct vb2_vmarea_handler handler;
>> atomic_t refcount;
>> + struct sg_table *sgt_base;
>>
>> /* USERPTR related */
>> struct vm_area_struct *vma;
>> @@ -189,14 +190,37 @@ static void vb2_dc_put(void *buf_priv)
>> if (!atomic_dec_and_test(&buf->refcount))
>> return;
>>
>> + vb2_dc_release_sgtable(buf->sgt_base);
>> dma_free_coherent(buf->dev, buf->size, buf->vaddr, buf->dma_addr);
>> kfree(buf);
>> }
>>
>> +static int vb2_dc_kaddr_to_pages(unsigned long kaddr,
>> + struct page **pages, unsigned int n_pages)
>> +{
>> + unsigned int i;
>> + unsigned long pfn;
>> + struct vm_area_struct vma = {
>> + .vm_flags = VM_IO | VM_PFNMAP,
>> + .vm_mm = current->mm,
>> + };
>> +
>> + for (i = 0; i < n_pages; ++i, kaddr += PAGE_SIZE) {
>
> The follow_pfn() kerneldoc mentions that it looks up a PFN for a user address.
> The only users I've found in the kernel sources pass a user address. Is it
> legal to use it for kernel addresses ?
>
It is not completely legal :). As I understand the mm code,
the follow_pfn works only for IO/PFN mappings.
This is the typical case (every case?) of mappings
created by dma_alloc_coherent.
In order to make this function work for a kernel pointer,
one has to create an artificial VMA that has IO/PFN bits on.
This solution is a hack-around for dma_get_pages (aka dma_get_sgtable).
This way the dependency on dma_get_pages was broken giving a small
hope of merging vb2 exporting.
Marek prepared a patchset 'ARM: DMA-mapping: new extensions for
buffer sharing' that adds dma buffers with no kernel mappings
and dma_get_sgtable function.
However this patchset is still in a RFC state.
I have prepared a patch that removes vb2_dc_kaddr_to_pages
and substitutes it with dma_get_pages. It will become
a part of vb2-exporter patches just after dma_get_sgtable
is merged (or at least acked by major maintainers).
>> + if (follow_pfn(&vma, kaddr, &pfn))
>> + break;
>> + pages[i] = pfn_to_page(pfn);
>> + }
>> +
>> + return i;
>> +}
>> +
>> static void *vb2_dc_alloc(void *alloc_ctx, unsigned long size)
>> {
>> struct device *dev = alloc_ctx;
>> struct vb2_dc_buf *buf;
>> + int ret = -ENOMEM;
>> + int n_pages;
>> + struct page **pages = NULL;
>>
>> buf = kzalloc(sizeof *buf, GFP_KERNEL);
>> if (!buf)
>> @@ -205,10 +229,41 @@ static void *vb2_dc_alloc(void *alloc_ctx, unsigned
>> long size) buf->vaddr = dma_alloc_coherent(dev, size, &buf->dma_addr,
>> GFP_KERNEL); if (!buf->vaddr) {
>> dev_err(dev, "dma_alloc_coherent of size %ld failed\n", size);
>> - kfree(buf);
>> - return ERR_PTR(-ENOMEM);
>> + goto fail_buf;
>> + }
>> +
>> + WARN_ON((unsigned long)buf->vaddr & ~PAGE_MASK);
>> + WARN_ON(buf->dma_addr & ~PAGE_MASK);
>> +
>> + n_pages = PAGE_ALIGN(size) >> PAGE_SHIFT;
>> +
>> + pages = kmalloc(n_pages * sizeof pages[0], GFP_KERNEL);
>> + if (!pages) {
>> + dev_err(dev, "failed to alloc page table\n");
>> + goto fail_dma;
>> + }
>> +
>> + ret = vb2_dc_kaddr_to_pages((unsigned long)buf->vaddr, pages, n_pages);
>> + if (ret < 0) {
>> + dev_err(dev, "failed to get buffer pages from DMA API\n");
>> + goto fail_pages;
>> + }
>> + if (ret != n_pages) {
>> + ret = -EFAULT;
>> + dev_err(dev, "failed to get all pages from DMA API\n");
>> + goto fail_pages;
>> + }
>> +
>> + buf->sgt_base = vb2_dc_pages_to_sgt(pages, n_pages, 0, size);
>> + if (IS_ERR(buf->sgt_base)) {
>> + ret = PTR_ERR(buf->sgt_base);
>> + dev_err(dev, "failed to prepare sg table\n");
>> + goto fail_pages;
>> }
>>
>> + /* pages are no longer needed */
>> + kfree(pages);
>> +
>> buf->dev = dev;
>> buf->size = size;
>>
>> @@ -219,6 +274,17 @@ static void *vb2_dc_alloc(void *alloc_ctx, unsigned
>> long size) atomic_inc(&buf->refcount);
>>
>> return buf;
>> +
>> +fail_pages:
>> + kfree(pages);
>> +
>> +fail_dma:
>
> You can merge the fail_pages and fail_dma labels, as kfree(NULL) is valid.
>
Yes, I can. But there are two reasons for not doing that:
- first: calling a dummy kfree introduces a negligible but non-zero overhead
- second: the fail-path becomes more difficult to understand
Regards,
Tomasz Stanislawski
>> + dma_free_coherent(dev, size, buf->vaddr, buf->dma_addr);
>> +
>> +fail_buf:
>> + kfree(buf);
>> +
>> + return ERR_PTR(ret);
>> }
>>
>> static int vb2_dc_mmap(void *buf_priv, struct vm_area_struct *vma)
>
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