[v2,4/7] scatterlist: add sg_alloc_table_from_buf() helper
Commit Message
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 | 161 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 185 insertions(+)
Comments
On Wed, Mar 30, 2016 at 05:39:51PM +0200, 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).
This seems nice. Should we also have a further helper on top of this
which will get constraints from a dmaengine, it seems like it'd be a
common need?
On Wed, 30 Mar 2016 09:51:43 -0700
Mark Brown <broonie@kernel.org> wrote:
> On Wed, Mar 30, 2016 at 05:39:51PM +0200, 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).
>
> This seems nice. Should we also have a further helper on top of this
> which will get constraints from a dmaengine, it seems like it'd be a
> common need?
Yep, we could create a wrapper extracting dma_slave caps info,
converting it to sg_constraints and calling sg_alloc_table_from_buf().
But let's try to get this function accepted first, and I'll send another
patch providing this wrapper.
BTW, do you see other things that should be added in sg_constraints?
On Wed, Mar 30, 2016 at 08:18:31PM +0200, Boris Brezillon wrote:
> BTW, do you see other things that should be added in sg_constraints?
It looked to do everything SPI does which is everything I know about.
Hi,
On 03/30/2016 09:09 PM, Boris BREZILLON wrote:
[...]
> +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);
> + }
> +
If the buf address is in PKMAP_BASE - PAGE_OFFSET-1 region (I have
observed that JFFS2 FS provides buffers in above region to MTD layer),
if CONFIG_DEBUG_SG is set then sg_set_buf() will throw a BUG_ON() as
virt_addr_is_valid() will return false. Is there a sane way to handle
buffers of PKMAP_BASE region with sg_* APIs?
Or, is the function sg_alloc_table_from_buf() not to be used with such
buffers?
Hi Vignesh,
On Thu, 31 Mar 2016 10:26:59 +0530
Vignesh R <vigneshr@ti.com> wrote:
> Hi,
>
> On 03/30/2016 09:09 PM, Boris BREZILLON wrote:
>
> [...]
>
> > +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);
> > + }
> > +
>
> If the buf address is in PKMAP_BASE - PAGE_OFFSET-1 region (I have
> observed that JFFS2 FS provides buffers in above region to MTD layer),
> if CONFIG_DEBUG_SG is set then sg_set_buf() will throw a BUG_ON() as
> virt_addr_is_valid() will return false. Is there a sane way to handle
> buffers of PKMAP_BASE region with sg_* APIs?
> Or, is the function sg_alloc_table_from_buf() not to be used with such
> buffers?
It should be usable with kmapped buffers too: I'll provide a new version
to support that.
That makes me realize I'm not checking the virtual address consistency
in sg_check_constraints().
Thanks,
Boris
On 30/03/16 19:18, Boris Brezillon wrote:
> On Wed, 30 Mar 2016 09:51:43 -0700
> Mark Brown <broonie@kernel.org> wrote:
>
>> On Wed, Mar 30, 2016 at 05:39:51PM +0200, 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).
>>
>> This seems nice. Should we also have a further helper on top of this
>> which will get constraints from a dmaengine, it seems like it'd be a
>> common need?
>
> Yep, we could create a wrapper extracting dma_slave caps info,
> converting it to sg_constraints and calling sg_alloc_table_from_buf().
> But let's try to get this function accepted first, and I'll send another
> patch providing this wrapper.
>
> BTW, do you see other things that should be added in sg_constraints?
>
You could compare with the things Solaris uses to describe the
restrictions on a DMA binding ...
http://docs.oracle.com/cd/E23824_01/html/821-1478/ddi-dma-attr-9s.html#REFMAN9Sddi-dma-attr-9s
.Dave.
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@@ -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);
@@ -433,6 +433,167 @@ 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)) {
+ unsigned long offset_in_page = offset_in_page(buf);
+ size_t contig_len = PAGE_SIZE - offset_in_page;
+ unsigned long phys = vmalloc_to_pfn(buf) - offset_in_page;
+ const void *contig_ptr = buf + contig_len;
+
+ /*
+ * Vmalloced buffer might be composed of several physically
+ * contiguous pages. Avoid extra scattergather entries in
+ * this case.
+ */
+ while (contig_len < chunk_len) {
+ if (phys + PAGE_SIZE != vmalloc_to_pfn(contig_ptr))
+ break;
+
+ contig_len += PAGE_SIZE;
+ contig_ptr += PAGE_SIZE;
+ phys += PAGE_SIZE;
+ }
+
+ chunk_len = min_t(size_t, chunk_len, contig_len);
+ }
+
+ 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 and physically contiguous 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)