Re: [PATCH v10 30/30] videobuf2: use sgtable-based scatterlist wrappers

On Mon, Sep 7, 2020 at 4:02 PM Marek Szyprowski
[off-list ref] wrote:

Hi Tomasz,

On 07.09.2020 15:07, Tomasz Figa wrote:

quoted

On Fri, Sep 4, 2020 at 3:35 PM Marek Szyprowski
[off-list ref] wrote:

quoted

Use recently introduced common wrappers operating directly on the struct
sg_table objects and scatterlist page iterators to make the code a bit
more compact, robust, easier to follow and copy/paste safe.

No functional change, because the code already properly did all the
scatterlist related calls.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
---
  .../common/videobuf2/videobuf2-dma-contig.c   | 34 ++++++++-----------
  .../media/common/videobuf2/videobuf2-dma-sg.c | 32 +++++++----------
  .../common/videobuf2/videobuf2-vmalloc.c      | 12 +++----
  3 files changed, 31 insertions(+), 47 deletions(-)

Thanks for the patch! Please see my comments inline.

quoted

diff --git a/drivers/media/common/videobuf2/videobuf2-dma-contig.c b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
index ec3446cc45b8..1b242d844dde 100644
--- a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
+++ b/drivers/media/common/videobuf2/videobuf2-dma-contig.c

@@ -58,10 +58,10 @@ static unsigned long vb2_dc_get_contiguous_size(struct sg_table *sgt)
         unsigned int i;
         unsigned long size = 0;

-       for_each_sg(sgt->sgl, s, sgt->nents, i) {
+       for_each_sgtable_dma_sg(sgt, s, i) {
                 if (sg_dma_address(s) != expected)
                         break;
-               expected = sg_dma_address(s) + sg_dma_len(s);
+               expected += sg_dma_len(s);
                 size += sg_dma_len(s);
         }
         return size;

@@ -103,8 +103,7 @@ static void vb2_dc_prepare(void *buf_priv)
         if (!sgt)
                 return;

-       dma_sync_sg_for_device(buf->dev, sgt->sgl, sgt->orig_nents,
-                              buf->dma_dir);
+       dma_sync_sgtable_for_device(buf->dev, sgt, buf->dma_dir);
  }

  static void vb2_dc_finish(void *buf_priv)

@@ -115,7 +114,7 @@ static void vb2_dc_finish(void *buf_priv)
         if (!sgt)
                 return;

-       dma_sync_sg_for_cpu(buf->dev, sgt->sgl, sgt->orig_nents, buf->dma_dir);
+       dma_sync_sgtable_for_cpu(buf->dev, sgt, buf->dma_dir);
  }

  /*********************************************/

@@ -275,8 +274,8 @@ static void vb2_dc_dmabuf_ops_detach(struct dma_buf *dbuf,
                  * memory locations do not require any explicit cache
                  * maintenance prior or after being used by the device.
                  */
-               dma_unmap_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
-                                  attach->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
+               dma_unmap_sgtable(db_attach->dev, sgt, attach->dma_dir,
+                                 DMA_ATTR_SKIP_CPU_SYNC);
         sg_free_table(sgt);
         kfree(attach);
         db_attach->priv = NULL;

@@ -301,8 +300,8 @@ static struct sg_table *vb2_dc_dmabuf_ops_map(

         /* release any previous cache */
         if (attach->dma_dir != DMA_NONE) {
-               dma_unmap_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
-                                  attach->dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
+               dma_unmap_sgtable(db_attach->dev, sgt, attach->dma_dir,
+                                 DMA_ATTR_SKIP_CPU_SYNC);
                 attach->dma_dir = DMA_NONE;
         }

@@ -310,9 +309,8 @@ static struct sg_table *vb2_dc_dmabuf_ops_map(
          * mapping to the client with new direction, no cache sync
          * required see comment in vb2_dc_dmabuf_ops_detach()
          */
-       sgt->nents = dma_map_sg_attrs(db_attach->dev, sgt->sgl, sgt->orig_nents,
-                                     dma_dir, DMA_ATTR_SKIP_CPU_SYNC);
-       if (!sgt->nents) {
+       if (dma_map_sgtable(db_attach->dev, sgt, dma_dir,
+                           DMA_ATTR_SKIP_CPU_SYNC)) {
                 pr_err("failed to map scatterlist\n");
                 mutex_unlock(lock);
                 return ERR_PTR(-EIO);

As opposed to dma_map_sg_attrs(), dma_map_sgtable() now returns an
error code on its own. Is it expected to ignore it and return -EIO?

Those errors are more or less propagated to userspace and -EIO has been
already widely documented in V4L2 documentation as the error code for
the most of the V4L2 ioctls. I don't want to change it. A possible
-EINVAL returned from dma_map_sgtable() was just one of the 'generic'
error codes, not very descriptive in that case. Probably the main
problem here is that dma_map_sg() and friend doesn't return any error
codes...

True for the alloc/get_*() callbacks, but the dmabuf_ops_map() ones
are used for the in-kernel DMA-buf exporter ops, called by DMA-buf
importers.

As a side note, returning user-facing error codes from deep internals
of vb2 and having the userspace rely on particular values sounds quite
fragile. For example, I see vb2_dc_attach_dmabuf() returning a return
value coming from dma_buf_attach() directly and __prepare_dmabuf()
propagating it back to __buf_prepare(), which can just return that
back to the userspace. I guess we might have to do some follow-up work
to clean it up.

Best regards,
Tomasz

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