Re: [PATCH 4/7] scatterlist: add sg_alloc_table_from_buf() helper

Boris Brezillon
2016-03-08T16:57:28Z

Hi Laurent,

On Tue, 08 Mar 2016 18:51:49 +0200
Laurent Pinchart <laurent.pinchart@ideasonboard.com> wrote:

> Hi Boris,
>
> Thank you for the patch.
>
> On Tuesday 08 March 2016 12:15:12 Boris Brezillon wrote:
> > sg_alloc_table_from_buf() provides an easy solution to create an sg_table
> > from a virtual address pointer. This function takes care of dealing with
> > vmallocated buffers, buffer alignment, or DMA engine limitations (maximum
> > DMA transfer size).
> >
> > Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
> > ---
> > include/linux/scatterlist.h | 24 ++++++++
> > lib/scatterlist.c | 142 +++++++++++++++++++++++++++++++++++++++++
> > 2 files changed, 166 insertions(+)
> >
> > diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> > index 556ec1e..4a75362 100644
> > --- a/include/linux/scatterlist.h
> > +++ b/include/linux/scatterlist.h
> > @@ -41,6 +41,27 @@ struct sg_table {
> > unsigned int orig_nents; /* original size of list */
> > };
> >
> > +/**
> > + * struct sg_constraints - SG constraints structure
> > + *
> > + * @max_chunk_len: maximum chunk buffer length. Each SG entry has to be
> > smaller
> > + * than this value. Zero means no constraint.
> > + * @required_alignment: minimum alignment. Is used for both size and
> > pointer
> > + * alignment. If this constraint is not met, the function
> > + * should return -EINVAL.
> > + * @preferred_alignment: preferred alignment. Mainly used to optimize
> > + * throughput when the DMA engine performs better when
> > + * doing aligned accesses.
> > + *
> > + * This structure is here to help sg_alloc_table_from_buf() create the
> > optimal
> > + * SG list based on DMA engine constraints.
> > + */
> > +struct sg_constraints {
> > + size_t max_chunk_len;
> > + size_t required_alignment;
> > + size_t preferred_alignment;
> > +};
> > +
> > /*
> > * Notes on SG table design.
> > *
> > @@ -265,6 +286,9 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
> > struct page **pages, unsigned int n_pages,
> > unsigned long offset, unsigned long size,
> > gfp_t gfp_mask);
> > +int sg_alloc_table_from_buf(struct sg_table *sgt, const void *buf, size_t
> > len,
> > + const struct sg_constraints *constraints,
> > + gfp_t gfp_mask);
> >
> > size_t sg_copy_buffer(struct scatterlist *sgl, unsigned int nents, void
> > *buf, size_t buflen, off_t skip, bool to_buffer);
> > diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> > index bafa993..706b583 100644
> > --- a/lib/scatterlist.c
> > +++ b/lib/scatterlist.c
> > @@ -433,6 +433,148 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
> > }
> > EXPORT_SYMBOL(sg_alloc_table_from_pages);
> >
> > +static size_t sg_buf_chunk_len(const void *buf, size_t len,
> > + const struct sg_constraints *cons)
> > +{
> > + size_t chunk_len = len;
> > +
> > + if (cons->max_chunk_len)
> > + chunk_len = min_t(size_t, chunk_len, cons->max_chunk_len);
> > +
> > + if (is_vmalloc_addr(buf))
> > + chunk_len = min_t(size_t, chunk_len,
> > + PAGE_SIZE - offset_in_page(buf));
>
> This will lead to page-sized scatter-gather entries even for pages of the
> vmalloc memory that happen to be physically contiguous. That works, but I
> wonder whether we'd want to be more efficient.

Hm, I thought dma_map_sg() was taking care of merging pĥysically
contiguous memory regions, but maybe I'm wrong. Anyway, that's
definitely something I can add at this level.

>
> > + if (!IS_ALIGNED((unsigned long)buf, cons->preferred_alignment)) {
> > + const void *aligned_buf = PTR_ALIGN(buf,
> > + cons->preferred_alignment);
> > + size_t unaligned_len = (unsigned long)(aligned_buf - buf);
> > +
> > + chunk_len = min_t(size_t, chunk_len, unaligned_len);
> > + } else if (chunk_len > cons->preferred_alignment) {
> > + chunk_len &= ~(cons->preferred_alignment - 1);
> > + }
> > +
> > + return chunk_len;
> > +}
> > +
> > +#define sg_for_each_chunk_in_buf(buf, len, chunk_len, constraints) \
> > + for (chunk_len = sg_buf_chunk_len(buf, len, constraints); \
> > + len; \
> > + len -= chunk_len, buf += chunk_len, \
> > + chunk_len = sg_buf_chunk_len(buf, len, constraints))
> > +
> > +static int sg_check_constraints(struct sg_constraints *cons,
> > + const void *buf, size_t len)
> > +{
> > + if (!cons->required_alignment)
> > + cons->required_alignment = 1;
> > +
> > + if (!cons->preferred_alignment)
> > + cons->preferred_alignment = cons->required_alignment;
> > +
> > + /* Test if buf and len are properly aligned. */
> > + if (!IS_ALIGNED((unsigned long)buf, cons->required_alignment) ||
> > + !IS_ALIGNED(len, cons->required_alignment))
> > + return -EINVAL;
> > +
> > + /*
> > + * if the buffer has been vmallocated and required_alignment is
> > + * more than PAGE_SIZE we cannot guarantee it.
> > + */
> > + if (is_vmalloc_addr(buf) && cons->required_alignment > PAGE_SIZE)
> > + return -EINVAL;
> > +
> > + /*
> > + * max_chunk_len has to be aligned to required_alignment to
> > + * guarantee that all buffer chunks are aligned correctly.
> > + */
> > + if (!IS_ALIGNED(cons->max_chunk_len, cons->required_alignment))
> > + return -EINVAL;
> > +
> > + /*
> > + * preferred_alignment has to be aligned to required_alignment
> > + * to avoid misalignment of buffer chunks.
> > + */
> > + if (!IS_ALIGNED(cons->preferred_alignment, cons->required_alignment))
> > + return -EINVAL;
> > +
> > + return 0;
> > +}
> > +
> > +/**
> > + * sg_alloc_table_from_buf - create an SG table from a buffer
> > + *
> > + * @sgt: SG table
> > + * @buf: buffer you want to create this SG table from
> > + * @len: length of buf
> > + * @constraints: optional constraints to take into account when creating
> > + * the SG table. Can be NULL if no specific constraints are
> > + * required.
> > + * @gfp_mask: type of allocation to use when creating the table
> > + *
> > + * This function creates an SG table from a buffer, its length and some
> > + * SG constraints.
> > + *
> > + * Note: This function supports vmallocated buffers.
>
> What other types of memory does it support ? kmalloc() quite obviously, are
> there others ? I think you should explicitly list the memory types that the
> function intends to support.

Sure, I can explicitly list all memory types. Do you see any other kind
of memory other than the vmalloced and physically-contiguous ones?

Thanks for your review.

Boris
--
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering