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

Boris Brezillon
2016-03-21T15:46:31Z

On Tue, 8 Mar 2016 12:15:12 +0100
Boris Brezillon <boris.brezillon@free-electrons.com> 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;
> +};
> +

Hm, we seem to have another case in NAND controller drivers. Some
drivers do not support scattergather accesses and have to provide a
single physically contiguous memory region to transfer the whole
NAND page.

Could we add a ->max_entries field to the sg_contraints struct to allow
those drivers to use the generic mtd_map_buf() function, and fallback
to a bounce buffer approach (or PIO access approach) when
sg_alloc_table_from_buf() fails to fulfill this constraint.

We could also define another 'non-sg' function to do the 'physically
contiguous' check, but in any case, I'd like to avoid letting each
driver code its own checking function.

What do you think?

> /*
> * 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));
> +
> + 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.
> + */
> +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)
> +{
> + struct sg_constraints cons = { };
> + size_t remaining, chunk_len;
> + const void *sg_buf;
> + int i, ret;
> +
> + if (constraints)
> + cons = *constraints;
> +
> + ret = sg_check_constraints(&cons, buf, len);
> + if (ret)
> + return ret;
> +
> + sg_buf = buf;
> + remaining = len;
> + i = 0;
> + sg_for_each_chunk_in_buf(sg_buf, remaining, chunk_len, &cons)
> + i++;
> +
> + ret = sg_alloc_table(sgt, i, gfp_mask);
> + if (ret)
> + return ret;
> +
> + sg_buf = buf;
> + remaining = len;
> + i = 0;
> + sg_for_each_chunk_in_buf(sg_buf, remaining, chunk_len, &cons) {
> + if (is_vmalloc_addr(sg_buf)) {
> + struct page *vm_page;
> +
> + vm_page = vmalloc_to_page(sg_buf);
> + if (!vm_page) {
> + ret = -ENOMEM;
> + goto err_free_table;
> + }
> +
> + sg_set_page(&sgt->sgl[i], vm_page, chunk_len,
> + offset_in_page(sg_buf));
> + } else {
> + sg_set_buf(&sgt->sgl[i], sg_buf, chunk_len);
> + }
> +
> + i++;
> + }
> +
> + return 0;
> +
> +err_free_table:
> + sg_free_table(sgt);
> +
> + return ret;
> +}
> +EXPORT_SYMBOL(sg_alloc_table_from_buf);
> +
> void __sg_page_iter_start(struct sg_page_iter *piter,
> struct scatterlist *sglist, unsigned int nents,
> unsigned long pgoffset)

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