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

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Date:   Mon, 7 Sep 2020 17:51:26 +0200
From:   Tomasz Figa <tfiga@...omium.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>, linaro-mm-sig@...ts.linaro.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [PATCH v10 30/30] videobuf2: use sgtable-based scatterlist wrappers

On Mon, Sep 7, 2020 at 4:02 PM Marek Szyprowski
<m.szyprowski@...sung.com> wrote:
>
> Hi Tomasz,
>
> On 07.09.2020 15:07, Tomasz Figa wrote:
> > On Fri, Sep 4, 2020 at 3:35 PM Marek Szyprowski
> > <m.szyprowski@...sung.com> wrote:
> >> 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@...sung.com>
> >> Reviewed-by: Robin Murphy <robin.murphy@....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.
> >
> >> 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