Hi Fang,
On Thu, Sep 14, 2023 at 4:41 PM Fang Hui <hui.fang@xxxxxxx> wrote:
>
> On system with "CONFIG_ZONE_DMA32=y", if the allocated physical address is
First of all, thanks a lot for the patch! Please check my review comments below.
Is CONFIG_ZONE_DMA32 really the factor that triggers the problem? My
understanding was that the problem was that the hardware has 32-bit
DMA, but the system has physical memory at addresses beyond the first
4G.
> greater than 4G, swiotlb will be used. It will lead below defects.
> 1) Impact performance due to an extra memcpy.
> 2) May meet below error due to swiotlb_max_mapping_size()
> is 256K (IO_TLB_SIZE * IO_TLB_SEGSIZE).
> "swiotlb buffer is full (sz: 393216 bytes), total 65536 (slots),
> used 2358 (slots)"
>
> To avoid those defects, use dma_alloc_pages() instead of alloc_pages()
> in vb2_dma_sg_alloc_compacted().
>
> Suggested-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> Signed-off-by: Fang Hui <hui.fang@xxxxxxx>
> ---
> drivers/media/common/videobuf2/videobuf2-dma-sg.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
Please remove MA-21654 from the subject and prefix it with the right
tags for the path (`git log drivers/media/common/videobuf2` should be
helpful to find the right one).
> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-sg.c b/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> index 28f3fdfe23a2..b938582c68f4 100644
> --- a/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> +++ b/drivers/media/common/videobuf2/videobuf2-dma-sg.c
> @@ -58,7 +58,7 @@ struct vb2_dma_sg_buf {
> static void vb2_dma_sg_put(void *buf_priv);
>
> static int vb2_dma_sg_alloc_compacted(struct vb2_dma_sg_buf *buf,
> - gfp_t gfp_flags)
> + gfp_t gfp_flags, struct device *dev)
FWIW buf->dev already points to the right device - although we would
need to move the assignment in vb2_dma_sg_alloc() to a place higher in
that function before calling this function.
> {
> unsigned int last_page = 0;
> unsigned long size = buf->size;
> @@ -67,6 +67,7 @@ static int vb2_dma_sg_alloc_compacted(struct vb2_dma_sg_buf *buf,
> struct page *pages;
> int order;
> int i;
> + dma_addr_t dma_handle;
>
> order = get_order(size);
> /* Don't over allocate*/
> @@ -75,8 +76,9 @@ static int vb2_dma_sg_alloc_compacted(struct vb2_dma_sg_buf *buf,
>
> pages = NULL;
> while (!pages) {
> - pages = alloc_pages(GFP_KERNEL | __GFP_ZERO |
> - __GFP_NOWARN | gfp_flags, order);
> + pages = dma_alloc_pages(dev, PAGE_SIZE << order, &dma_handle,
Hmm, when I was proposing dma_alloc_pages(), I missed that it returns
a DMA handle. That on its own can be handled by saving the returned
handles somewhere in struct vb2_dma_sg_buf, but there is a bigger
problem - the function would actually create a mapping if the DMA
device requires some mapping management (e.g. is behind an IOMMU),
which is undesirable, because we create the mapping ourselves below
anyway...
@Christoph Hellwig @Robin Murphy I need your thoughts on this as
well. Would it make sense to have a variant of dma_alloc_pages() that
only allocates the pages, but doesn't perform the mapping? (Or a flag
that tells the implementation to skip creating a mapping.)
> + DMA_BIDIRECTIONAL,
The right value should be already available in buf->dma_dir.
> + GFP_KERNEL | __GFP_ZERO | __GFP_NOWARN | gfp_flags);
> if (pages)
> break;
>
> @@ -96,6 +98,7 @@ static int vb2_dma_sg_alloc_compacted(struct vb2_dma_sg_buf *buf,
> }
>
> return 0;
> +
Unnecessary blank line.
> }
>
> static void *vb2_dma_sg_alloc(struct vb2_buffer *vb, struct device *dev,
> @@ -130,7 +133,7 @@ static void *vb2_dma_sg_alloc(struct vb2_buffer *vb, struct device *dev,
> if (!buf->pages)
> goto fail_pages_array_alloc;
>
> - ret = vb2_dma_sg_alloc_compacted(buf, vb->vb2_queue->gfp_flags);
> + ret = vb2_dma_sg_alloc_compacted(buf, vb->vb2_queue->gfp_flags, dev);
> if (ret)
> goto fail_pages_alloc;
>
> --
> 2.17.1
>
We also need to use dma_free_pages() to free the memory.
Best regards,
Tomasz
