Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations

From: Hans Verkuil
Date: Thu Jun 03 2021 - 07:59:59 EST

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On 27/04/2021 15:13, Sergey Senozhatsky wrote:
> This adds support for new noncontiguous DMA API, which
> requires allocators to have two execution branches: one
> for the current API, and one for the new one.
>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> [hch: untested conversion to the ne API]
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> .../common/videobuf2/videobuf2-dma-contig.c | 140 +++++++++++++++---
> 1 file changed, 116 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-contig.c b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> index 1e218bc440c6..40eaaef1565b 100644
> --- a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> +++ b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> @@ -17,6 +17,7 @@
> #include <linux/sched.h>
> #include <linux/slab.h>
> #include <linux/dma-mapping.h>
> +#include <linux/highmem.h>
>
> #include <media/videobuf2-v4l2.h>
> #include <media/videobuf2-dma-contig.h>
> @@ -42,6 +43,7 @@ struct vb2_dc_buf {
> struct dma_buf_attachment *db_attach;
>
> struct vb2_buffer *vb;
> + bool coherent_mem;
> };
>
> /*********************************************/
> @@ -78,14 +80,22 @@ static void *vb2_dc_cookie(struct vb2_buffer *vb, void *buf_priv)
> static void *vb2_dc_vaddr(struct vb2_buffer *vb, void *buf_priv)
> {
> struct vb2_dc_buf *buf = buf_priv;
> - struct dma_buf_map map;
> - int ret;
>
> - if (!buf->vaddr && buf->db_attach) {
> - ret = dma_buf_vmap(buf->db_attach->dmabuf, &map);
> - buf->vaddr = ret ? NULL : map.vaddr;
> + if (buf->vaddr)
> + return buf->vaddr;
> +
> + if (buf->db_attach) {
> + struct dma_buf_map map;
> +
> + if (!dma_buf_vmap(buf->db_attach->dmabuf, &map))
> + buf->vaddr = map.vaddr;
> +
> + return buf->vaddr;
> }
>
> + /* Non-coherent memory */
> + buf->vaddr = dma_vmap_noncontiguous(buf->dev, buf->size, buf->dma_sgt);
> +

This function can use some comments. What is happening AFAICS is that
buf->vaddr is either set in vb2_dc_alloc_coherent (unless
DMA_ATTR_NO_KERNEL_MAPPING was set), it is obtained through dma_buf_vmap()
if the buffer was attached to a dma_buf, or it is allocated via
dma_vmap_noncontiguous() for non-coherent memory.

But this leaves coherent memory with DMA_ATTR_NO_KERNEL_MAPPING set, what
is vaddr in that case? I think it will call dma_vmap_noncontiguous()
incorrectly in that case, shouldn't there be a check for !buf->coherent_mem
before the call to dma_vmap_noncontiguous()?

It's quite confusing since vaddr can be set in different functions.

> return buf->vaddr;
> }
>
> @@ -101,13 +111,26 @@ static void vb2_dc_prepare(void *buf_priv)
> struct vb2_dc_buf *buf = buf_priv;
> struct sg_table *sgt = buf->dma_sgt;
>
> + /* This takes care of DMABUF and user-enforced cache sync hint */
> if (buf->vb->skip_cache_sync_on_prepare)
> return;
>
> + /*
> + * Coherent MMAP buffers do not need to be synced, unlike USERPTR
> + * and non-coherent MMAP buffers.
> + */
> + if (buf->vb->memory == V4L2_MEMORY_MMAP && buf->coherent_mem)
> + return;
> +
> if (!sgt)
> return;
>
> + /* For both USERPTR and non-coherent MMAP */
> dma_sync_sgtable_for_device(buf->dev, sgt, buf->dma_dir);
> +
> + /* Non-coherent MMAP only */
> + if (!buf->coherent_mem && buf->vaddr)
> + flush_kernel_vmap_range(buf->vaddr, buf->size);
> }
>
> static void vb2_dc_finish(void *buf_priv)
> @@ -115,19 +138,46 @@ static void vb2_dc_finish(void *buf_priv)
> struct vb2_dc_buf *buf = buf_priv;
> struct sg_table *sgt = buf->dma_sgt;
>
> + /* This takes care of DMABUF and user-enforced cache sync hint */
> if (buf->vb->skip_cache_sync_on_finish)
> return;
>
> + /*
> + * Coherent MMAP buffers do not need to be synced, unlike USERPTR
> + * and non-coherent MMAP buffers.
> + */
> + if (buf->vb->memory == V4L2_MEMORY_MMAP && buf->coherent_mem)
> + return;
> +
> if (!sgt)
> return;
>
> + /* For both USERPTR and non-coherent MMAP */
> dma_sync_sgtable_for_cpu(buf->dev, sgt, buf->dma_dir);
> +
> + /* Non-coherent MMAP only */
> + if (!buf->coherent_mem && buf->vaddr)
> + invalidate_kernel_vmap_range(buf->vaddr, buf->size);
> }
>
> /*********************************************/
> /* callbacks for MMAP buffers */
> /*********************************************/
>
> +static void __vb2_dc_put(struct vb2_dc_buf *buf)
> +{
> + if (buf->coherent_mem) {
> + dma_free_attrs(buf->dev, buf->size, buf->cookie,
> + buf->dma_addr, buf->attrs);
> + return;
> + }
> +
> + if (buf->vaddr)
> + dma_vunmap_noncontiguous(buf->dev, buf->vaddr);
> + dma_free_noncontiguous(buf->dev, buf->size,
> + buf->dma_sgt, buf->dma_addr);
> +}

Is there a reason for creating __vb2_dc_put()? I found it more
a hindrance when reviewing than an advantage. I prefer to have
it moved to vb2_dc_put, that way all the clean up happens in that
single function.

> +
> static void vb2_dc_put(void *buf_priv)
> {
> struct vb2_dc_buf *buf = buf_priv;
> @@ -139,17 +189,52 @@ static void vb2_dc_put(void *buf_priv)
> sg_free_table(buf->sgt_base);
> kfree(buf->sgt_base);
> }
> - dma_free_attrs(buf->dev, buf->size, buf->cookie, buf->dma_addr,
> - buf->attrs);
> + __vb2_dc_put(buf);
> put_device(buf->dev);
> kfree(buf);
> }
>
> +static int vb2_dc_alloc_coherent(struct vb2_dc_buf *buf)
> +{
> + struct vb2_queue *q = buf->vb->vb2_queue;
> +
> + buf->cookie = dma_alloc_attrs(buf->dev,
> + buf->size,
> + &buf->dma_addr,
> + GFP_KERNEL | q->gfp_flags,
> + buf->attrs);
> + if (!buf->cookie)
> + return -ENOMEM;
> + if ((q->dma_attrs & DMA_ATTR_NO_KERNEL_MAPPING) == 0)
> + buf->vaddr = buf->cookie;
> + return 0;
> +}
> +
> +static int vb2_dc_alloc_non_coherent(struct vb2_dc_buf *buf)
> +{
> + struct vb2_queue *q = buf->vb->vb2_queue;
> +
> + buf->dma_sgt = dma_alloc_noncontiguous(buf->dev,
> + buf->size,
> + buf->dma_dir,
> + GFP_KERNEL | q->gfp_flags,
> + buf->attrs);
> + if (!buf->dma_sgt)
> + return -ENOMEM;
> + /*
> + * For requests that need kernel mapping (DMA_ATTR_NO_KERNEL_MAPPING

I'm confused about this comment: DMA_ATTR_NO_KERNEL_MAPPING is only checked
in vb2_dc_alloc_coherent(), so is this comment valid?

> + * bit is cleared) we perform dma_vmap_noncontiguous() later, in
> + * vb2_dc_vadd().

Typo: vb2_dc_vadd -> vb2_dc_vaddr

> + */

So this leaves buf->vaddr to NULL.

Is it possible that after allocating the buffer, the buffer is exported as a
dma_buf and another device calls dma_buf_ops vb2_dc_dmabuf_ops_vmap, which
in turn calls dma_buf_map_set_vaddr(map, buf->vaddr); with a NULL buf->vaddr?

I may be barking up the wrong tree here, to really understand it I would have
to spend more time.

> + return 0;
> +}
> +
> static void *vb2_dc_alloc(struct vb2_buffer *vb,
> struct device *dev,
> unsigned long size)
> {
> struct vb2_dc_buf *buf;
> + int ret;
>
> if (WARN_ON(!dev))
> return ERR_PTR(-EINVAL);
> @@ -159,27 +244,28 @@ static void *vb2_dc_alloc(struct vb2_buffer *vb,
> return ERR_PTR(-ENOMEM);
>
> buf->attrs = vb->vb2_queue->dma_attrs;
> - buf->cookie = dma_alloc_attrs(dev, size, &buf->dma_addr,
> - GFP_KERNEL | vb->vb2_queue->gfp_flags,
> - buf->attrs);
> - if (!buf->cookie) {
> - dev_err(dev, "dma_alloc_coherent of size %ld failed\n", size);
> - kfree(buf);
> - return ERR_PTR(-ENOMEM);
> - }
> -
> - if ((buf->attrs & DMA_ATTR_NO_KERNEL_MAPPING) == 0)
> - buf->vaddr = buf->cookie;
> + buf->dma_dir = vb->vb2_queue->dma_dir;
> + buf->vb = vb;
> + buf->coherent_mem = vb->vb2_queue->coherent_mem;
>
> + buf->size = size;
> /* Prevent the device from being released while the buffer is used */
> buf->dev = get_device(dev);
> - buf->size = size;
> - buf->dma_dir = vb->vb2_queue->dma_dir;
> +
> + if (buf->coherent_mem)
> + ret = vb2_dc_alloc_coherent(buf);
> + else
> + ret = vb2_dc_alloc_non_coherent(buf);
> +
> + if (ret) {
> + dev_err(dev, "dma alloc of size %ld failed\n", size);
> + kfree(buf);
> + return ERR_PTR(-ENOMEM);
> + }
>
> buf->handler.refcount = &buf->refcount;
> buf->handler.put = vb2_dc_put;
> buf->handler.arg = buf;
> - buf->vb = vb;
>
> refcount_set(&buf->refcount, 1);
>
> @@ -196,9 +282,12 @@ static int vb2_dc_mmap(void *buf_priv, struct vm_area_struct *vma)
> return -EINVAL;
> }
>
> - ret = dma_mmap_attrs(buf->dev, vma, buf->cookie,
> - buf->dma_addr, buf->size, buf->attrs);
> -
> + if (buf->coherent_mem)
> + ret = dma_mmap_attrs(buf->dev, vma, buf->cookie, buf->dma_addr,
> + buf->size, buf->attrs);
> + else
> + ret = dma_mmap_noncontiguous(buf->dev, vma, buf->size,
> + buf->dma_sgt);
> if (ret) {
> pr_err("Remapping memory failed, error: %d\n", ret);
> return ret;
> @@ -390,6 +479,9 @@ static struct sg_table *vb2_dc_get_base_sgt(struct vb2_dc_buf *buf)
> int ret;
> struct sg_table *sgt;
>
> + if (!buf->coherent_mem)
> + return buf->dma_sgt;
> +
> sgt = kmalloc(sizeof(*sgt), GFP_KERNEL);
> if (!sgt) {
> dev_err(buf->dev, "failed to alloc sg table\n");
>

Regards,

Hans

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