lib/scatterlist: Provide a DMA page iterator

Commit 2db76d7c3c6d ("lib/scatterlist: sg_page_iter: support sg lists w/o
backing pages") introduced the sg_page_iter_dma_address() function without
providing a way to use it in the general case. If the sg_dma_len() is not
equal to the sg length callers cannot safely use the
for_each_sg_page/sg_page_iter_dma_address combination.

Resolve this API mistake by providing a DMA specific iterator,
for_each_sg_dma_page(), that uses the right length so
sg_page_iter_dma_address() works as expected with all sglists.

A new iterator type is introduced to provide compile-time safety against
wrongly mixing accessors and iterators.

Acked-by: Christoph Hellwig <hch@lst.de> (for scatterlist)
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com> (ipu3-cio2)
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

5 files changed, 76 insertions, 12 deletions

diff --git a/.clang-format b/.clang-format
index bc2ffb2a0b53..335ce29ab813 100644
--- a/.clang-format
+++ b/.clang-format
@@ -240,6 +240,7 @@ ForEachMacros:
   - 'for_each_set_bit'
   - 'for_each_set_bit_from'
   - 'for_each_sg'
+  - 'for_each_sg_dma_page'
   - 'for_each_sg_page'
   - 'for_each_sibling_event'
   - '__for_each_thread'
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
index 31786b200afc..a3357ff7540d 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_ttm_buffer.c
@@ -311,7 +311,13 @@ static dma_addr_t __vmw_piter_dma_addr(struct vmw_piter *viter)
 
 static dma_addr_t __vmw_piter_sg_addr(struct vmw_piter *viter)
 {
-        return sg_page_iter_dma_address(&viter->iter);
+        /*
+         * FIXME: This driver wrongly mixes DMA and CPU SG list iteration and
+         * needs revision. See
+         * https://lore.kernel.org/lkml/20190104223531.GA1705@ziepe.ca/
+         */
+        return sg_page_iter_dma_address(
+                container_of(&viter->iter, struct sg_dma_page_iter, base));
 }
 
 
diff --git a/drivers/media/pci/intel/ipu3/ipu3-cio2.c b/drivers/media/pci/intel/ipu3/ipu3-cio2.c
index cdb79ae2d8dc..9fbfbda74171 100644
--- a/drivers/media/pci/intel/ipu3/ipu3-cio2.c
+++ b/drivers/media/pci/intel/ipu3/ipu3-cio2.c
@@ -846,7 +846,7 @@ static int cio2_vb2_buf_init(struct vb2_buffer *vb)
         unsigned int pages = DIV_ROUND_UP(vb->planes[0].length, CIO2_PAGE_SIZE);
         unsigned int lops = DIV_ROUND_UP(pages + 1, entries_per_page);
         struct sg_table *sg;
-        struct sg_page_iter sg_iter;
+        struct sg_dma_page_iter sg_iter;
         int i, j;
 
         if (lops <= 0 || lops > CIO2_MAX_LOPS) {
@@ -873,7 +873,7 @@ static int cio2_vb2_buf_init(struct vb2_buffer *vb)
                 b->offset = sg->sgl->offset;
 
         i = j = 0;
-        for_each_sg_page(sg->sgl, &sg_iter, sg->nents, 0) {
+        for_each_sg_dma_page (sg->sgl, &sg_iter, sg->nents, 0) {
                 if (!pages--)
                         break;
                 b->lop[i][j] = sg_page_iter_dma_address(&sg_iter) >> PAGE_SHIFT;
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index b96f0d0b5b8f..b4be960c7e5d 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -339,12 +339,12 @@ int sg_alloc_table_chained(struct sg_table *table, int nents,
 /*
  * sg page iterator
  *
- * Iterates over sg entries page-by-page.  On each successful iteration,
- * you can call sg_page_iter_page(@piter) and sg_page_iter_dma_address(@piter)
- * to get the current page and its dma address. @piter->sg will point to the
- * sg holding this page and @piter->sg_pgoffset to the page's page offset
- * within the sg. The iteration will stop either when a maximum number of sg
- * entries was reached or a terminating sg (sg_last(sg) == true) was reached.
+ * Iterates over sg entries page-by-page.  On each successful iteration, you
+ * can call sg_page_iter_page(@piter) to get the current page and its dma
+ * address. @piter->sg will point to the sg holding this page and
+ * @piter->sg_pgoffset to the page's page offset within the sg. The iteration
+ * will stop either when a maximum number of sg entries was reached or a
+ * terminating sg (sg_last(sg) == true) was reached.
  */
 struct sg_page_iter {
         struct scatterlist      *sg;            /* sg holding the page */
@@ -356,7 +356,19 @@ struct sg_page_iter {
                                                  * next step */
 };
 
+/*
+ * sg page iterator for DMA addresses
+ *
+ * This is the same as sg_page_iter however you can call
+ * sg_page_iter_dma_address(@dma_iter) to get the page's DMA
+ * address. sg_page_iter_page() cannot be called on this iterator.
+ */
+struct sg_dma_page_iter {
+        struct sg_page_iter base;
+};
+
 bool __sg_page_iter_next(struct sg_page_iter *piter);
+bool __sg_page_iter_dma_next(struct sg_dma_page_iter *dma_iter);
 void __sg_page_iter_start(struct sg_page_iter *piter,
                           struct scatterlist *sglist, unsigned int nents,
                           unsigned long pgoffset);
@@ -372,11 +384,13 @@ static inline struct page *sg_page_iter_page(struct sg_page_iter *piter)
 /**
  * sg_page_iter_dma_address - get the dma address of the current page held by
  * the page iterator.
- * @piter:      page iterator holding the page
+ * @dma_iter:   page iterator holding the page
  */
-static inline dma_addr_t sg_page_iter_dma_address(struct sg_page_iter *piter)
+static inline dma_addr_t
+sg_page_iter_dma_address(struct sg_dma_page_iter *dma_iter)
 {
-        return sg_dma_address(piter->sg) + (piter->sg_pgoffset << PAGE_SHIFT);
+        return sg_dma_address(dma_iter->base.sg) +
+               (dma_iter->base.sg_pgoffset << PAGE_SHIFT);
 }
 
 /**
@@ -385,11 +399,28 @@ static inline dma_addr_t sg_page_iter_dma_address(struct sg_page_iter *piter)
  * @piter:      page iterator to hold current page, sg, sg_pgoffset
  * @nents:      maximum number of sg entries to iterate over
  * @pgoffset:   starting page offset
+ *
+ * Callers may use sg_page_iter_page() to get each page pointer.
  */
 #define for_each_sg_page(sglist, piter, nents, pgoffset)                   \
         for (__sg_page_iter_start((piter), (sglist), (nents), (pgoffset)); \
              __sg_page_iter_next(piter);)
 
+/**
+ * for_each_sg_dma_page - iterate over the pages of the given sg list
+ * @sglist:     sglist to iterate over
+ * @dma_iter:   page iterator to hold current page
+ * @dma_nents:  maximum number of sg entries to iterate over, this is the value
+ *              returned from dma_map_sg
+ * @pgoffset:   starting page offset
+ *
+ * Callers may use sg_page_iter_dma_address() to get each page's DMA address.
+ */
+#define for_each_sg_dma_page(sglist, dma_iter, dma_nents, pgoffset)            \
+        for (__sg_page_iter_start(&(dma_iter)->base, sglist, dma_nents,        \
+                                  pgoffset);                                   \
+             __sg_page_iter_dma_next(dma_iter);)
+
 /*
  * Mapping sg iterator
  *
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 9ba349e775ef..739dc9fe2c55 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -625,6 +625,32 @@ bool __sg_page_iter_next(struct sg_page_iter *piter)
 }
 EXPORT_SYMBOL(__sg_page_iter_next);
 
+static int sg_dma_page_count(struct scatterlist *sg)
+{
+        return PAGE_ALIGN(sg->offset + sg_dma_len(sg)) >> PAGE_SHIFT;
+}
+
+bool __sg_page_iter_dma_next(struct sg_dma_page_iter *dma_iter)
+{
+        struct sg_page_iter *piter = &dma_iter->base;
+
+        if (!piter->__nents || !piter->sg)
+                return false;
+
+        piter->sg_pgoffset += piter->__pg_advance;
+        piter->__pg_advance = 1;
+
+        while (piter->sg_pgoffset >= sg_dma_page_count(piter->sg)) {
+                piter->sg_pgoffset -= sg_dma_page_count(piter->sg);
+                piter->sg = sg_next(piter->sg);
+                if (!--piter->__nents || !piter->sg)
+                        return false;
+        }
+
+        return true;
+}
+EXPORT_SYMBOL(__sg_page_iter_dma_next);
+
 /**
  * sg_miter_start - start mapping iteration over a sg list
  * @miter: sg mapping iter to be started