4 files changed, 215 insertions, 0 deletions

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 698e906ca730..556ec1ea2574 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -247,6 +247,11 @@ struct scatterlist *sg_next(struct scatterlist *);
 struct scatterlist *sg_last(struct scatterlist *s, unsigned int);
 void sg_init_table(struct scatterlist *, unsigned int);
 void sg_init_one(struct scatterlist *, const void *, unsigned int);
+int sg_split(struct scatterlist *in, const int in_mapped_nents,
+             const off_t skip, const int nb_splits,
+             const size_t *split_sizes,
+             struct scatterlist **out, int *out_mapped_nents,
+             gfp_t gfp_mask);
 
 typedef struct scatterlist *(sg_alloc_fn)(unsigned int, gfp_t);
 typedef void (sg_free_fn)(struct scatterlist *, unsigned int);
diff --git a/lib/Kconfig b/lib/Kconfig
index 3a2ef67db6c7..dc516164415a 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -521,6 +521,13 @@ config UCS2_STRING
 
 source "lib/fonts/Kconfig"
 
+config SG_SPLIT
+        def_bool n
+        help
+         Provides a heler to split scatterlists into chunks, each chunk being a
+         scatterlist. This should be selected by a driver or an API which
+         whishes to split a scatterlist amongst multiple DMA channel.
+
 #
 # sg chaining option
 #
diff --git a/lib/Makefile b/lib/Makefile
index 6897b527581a..2ee6ea2e9b08 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -160,6 +160,7 @@ obj-$(CONFIG_GENERIC_STRNLEN_USER) += strnlen_user.o
 
 obj-$(CONFIG_GENERIC_NET_UTILS) += net_utils.o
 
+obj-$(CONFIG_SG_SPLIT) += sg_split.o
 obj-$(CONFIG_STMP_DEVICE) += stmp_device.o
 
 libfdt_files = fdt.o fdt_ro.o fdt_wip.o fdt_rw.o fdt_sw.o fdt_strerror.o \
diff --git a/lib/sg_split.c b/lib/sg_split.c
new file mode 100644
index 000000000000..b063410c3593
--- /dev/null
+++ b/lib/sg_split.c
@@ -0,0 +1,202 @@
+/*
+ * Copyright (C) 2015 Robert Jarzmik <robert.jarzmik@free.fr>
+ *
+ * Scatterlist splitting helpers.
+ *
+ * This source code is licensed under the GNU General Public License,
+ * Version 2. See the file COPYING for more details.
+ */
+
+#include <linux/scatterlist.h>
+#include <linux/slab.h>
+
+struct sg_splitter {
+        struct scatterlist *in_sg0;
+        int nents;
+        off_t skip_sg0;
+        unsigned int length_last_sg;
+
+        struct scatterlist *out_sg;
+};
+
+static int sg_calculate_split(struct scatterlist *in, int nents, int nb_splits,
+                              off_t skip, const size_t *sizes,
+                              struct sg_splitter *splitters, bool mapped)
+{
+        int i;
+        unsigned int sglen;
+        size_t size = sizes[0], len;
+        struct sg_splitter *curr = splitters;
+        struct scatterlist *sg;
+
+        for (i = 0; i < nb_splits; i++) {
+                splitters[i].in_sg0 = NULL;
+                splitters[i].nents = 0;
+        }
+
+        for_each_sg(in, sg, nents, i) {
+                sglen = mapped ? sg_dma_len(sg) : sg->length;
+                if (skip > sglen) {
+                        skip -= sglen;
+                        continue;
+                }
+
+                len = min_t(size_t, size, sglen - skip);
+                if (!curr->in_sg0) {
+                        curr->in_sg0 = sg;
+                        curr->skip_sg0 = skip;
+                }
+                size -= len;
+                curr->nents++;
+                curr->length_last_sg = len;
+
+                while (!size && (skip + len < sglen) && (--nb_splits > 0)) {
+                        curr++;
+                        size = *(++sizes);
+                        skip += len;
+                        len = min_t(size_t, size, sglen - skip);
+
+                        curr->in_sg0 = sg;
+                        curr->skip_sg0 = skip;
+                        curr->nents = 1;
+                        curr->length_last_sg = len;
+                        size -= len;
+                }
+                skip = 0;
+
+                if (!size && --nb_splits > 0) {
+                        curr++;
+                        size = *(++sizes);
+                }
+
+                if (!nb_splits)
+                        break;
+        }
+
+        return (size || !splitters[0].in_sg0) ? -EINVAL : 0;
+}
+
+static void sg_split_phys(struct sg_splitter *splitters, const int nb_splits)
+{
+        int i, j;
+        struct scatterlist *in_sg, *out_sg;
+        struct sg_splitter *split;
+
+        for (i = 0, split = splitters; i < nb_splits; i++, split++) {
+                in_sg = split->in_sg0;
+                out_sg = split->out_sg;
+                for (j = 0; j < split->nents; j++, out_sg++) {
+                        *out_sg = *in_sg;
+                        if (!j) {
+                                out_sg->offset += split->skip_sg0;
+                                out_sg->length -= split->skip_sg0;
+                        } else {
+                                out_sg->offset = 0;
+                        }
+                        sg_dma_address(out_sg) = 0;
+                        sg_dma_len(out_sg) = 0;
+                        in_sg = sg_next(in_sg);
+                }
+                out_sg[-1].length = split->length_last_sg;
+                sg_mark_end(out_sg - 1);
+        }
+}
+
+static void sg_split_mapped(struct sg_splitter *splitters, const int nb_splits)
+{
+        int i, j;
+        struct scatterlist *in_sg, *out_sg;
+        struct sg_splitter *split;
+
+        for (i = 0, split = splitters; i < nb_splits; i++, split++) {
+                in_sg = split->in_sg0;
+                out_sg = split->out_sg;
+                for (j = 0; j < split->nents; j++, out_sg++) {
+                        sg_dma_address(out_sg) = sg_dma_address(in_sg);
+                        sg_dma_len(out_sg) = sg_dma_len(in_sg);
+                        if (!j) {
+                                sg_dma_address(out_sg) += split->skip_sg0;
+                                sg_dma_len(out_sg) -= split->skip_sg0;
+                        }
+                        in_sg = sg_next(in_sg);
+                }
+                sg_dma_len(--out_sg) = split->length_last_sg;
+        }
+}
+
+/**
+ * sg_split - split a scatterlist into several scatterlists
+ * @in: the input sg list
+ * @in_mapped_nents: the result of a dma_map_sg(in, ...), or 0 if not mapped.
+ * @skip: the number of bytes to skip in the input sg list
+ * @nb_splits: the number of desired sg outputs
+ * @split_sizes: the respective size of each output sg list in bytes
+ * @out: an array where to store the allocated output sg lists
+ * @out_mapped_nents: the resulting sg lists mapped number of sg entries. Might
+ *                    be NULL if sglist not already mapped (in_mapped_nents = 0)
+ * @gfp_mask: the allocation flag
+ *
+ * This function splits the input sg list into nb_splits sg lists, which are
+ * allocated and stored into out.
+ * The @in is split into :
+ *  - @out[0], which covers bytes [@skip .. @skip + @split_sizes[0] - 1] of @in
+ *  - @out[1], which covers bytes [@skip + split_sizes[0] ..
+ *                                 @skip + @split_sizes[0] + @split_sizes[1] -1]
+ * etc ...
+ * It will be the caller's duty to kfree() out array members.
+ *
+ * Returns 0 upon success, or error code
+ */
+int sg_split(struct scatterlist *in, const int in_mapped_nents,
+             const off_t skip, const int nb_splits,
+             const size_t *split_sizes,
+             struct scatterlist **out, int *out_mapped_nents,
+             gfp_t gfp_mask)
+{
+        int i, ret;
+        struct sg_splitter *splitters;
+
+        splitters = kcalloc(nb_splits, sizeof(*splitters), gfp_mask);
+        if (!splitters)
+                return -ENOMEM;
+
+        ret = sg_calculate_split(in, sg_nents(in), nb_splits, skip, split_sizes,
+                           splitters, false);
+        if (ret < 0)
+                goto err;
+
+        ret = -ENOMEM;
+        for (i = 0; i < nb_splits; i++) {
+                splitters[i].out_sg = kmalloc_array(splitters[i].nents,
+                                                    sizeof(struct scatterlist),
+                                                    gfp_mask);
+                if (!splitters[i].out_sg)
+                        goto err;
+        }
+
+        /*
+         * The order of these 3 calls is important and should be kept.
+         */
+        sg_split_phys(splitters, nb_splits);
+        ret = sg_calculate_split(in, in_mapped_nents, nb_splits, skip,
+                                 split_sizes, splitters, true);
+        if (ret < 0)
+                goto err;
+        sg_split_mapped(splitters, nb_splits);
+
+        for (i = 0; i < nb_splits; i++) {
+                out[i] = splitters[i].out_sg;
+                if (out_mapped_nents)
+                        out_mapped_nents[i] = splitters[i].nents;
+        }
+
+        kfree(splitters);
+        return 0;
+
+err:
+        for (i = 0; i < nb_splits; i++)
+                kfree(splitters[i].out_sg);
+        kfree(splitters);
+        return ret;
+}
+EXPORT_SYMBOL(sg_split);