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

Boris Brezillon
2016-03-08T11:15:12Z

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));
+
+ 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)
--
2.1.4