Hi Laurent,

On Tue, 08 Mar 2016 18:51:49 +0200
Laurent Pinchart [off-list ref] wrote:

Hi Boris,

Thank you for the patch.

On Tuesday 08 March 2016 12:15:12 Boris Brezillon wrote:

quoted

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-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
---
 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.

quoted

+	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
http://free-electrons.com

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