linux-kernel - Re: [PATCH 2/5] dma_map_sg

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Date:	Fri, 21 Dec 2007 09:58:56 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	David Miller <davem@...emloft.net>
Cc:	fujita.tomonori@....ntt.co.jp, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org, jens.axboe@...cle.com
Subject: Re: [PATCH 2/5] dma_map_sg_ring() helper

On Thursday 20 December 2007 18:42:44 David Miller wrote:
> From: FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
> Date: Thu, 20 Dec 2007 16:06:31 +0900
>
> > On Thu, 20 Dec 2007 16:49:30 +1100
> >
> > Rusty Russell <rusty@...tcorp.com.au> wrote:
> > > +/**
> > > + * dma_map_sg_ring - Map an entire sg ring
> > > + * @dev: Device to free noncoherent memory for
> > > + * @sg: The sg_ring
> > > + * @direction: DMA_TO_DEVICE, DMA_FROM_DEVICE or DMA_BIDIRECTIONAL.
> > > + *
> > > + * This returns -ENOMEM if mapping fails.  It's not clear that telling
> > > you + * it failed is useful though.
> > > + */
> > > +int dma_map_sg_ring(struct device *dev, struct sg_ring *sg,
> > > +		     enum dma_data_direction direction)
> > > +{
> > > +	struct sg_ring *i;
> > > +	unsigned int num;
> > > +
> > > +	for (i = sg; i; i = sg_ring_next(i, sg)) {
> > > +		BUG_ON(i->num > i->max);
> > > +		num = dma_map_sg(dev, i->sg, i->num, direction);
> > > +		if (num == 0 && i->num != 0)
> > > +			goto unmap;
> > > +	}
> > > +	return 0;
> >
> > I don't think that this works for IOMMUs that could merge sg entries.
>
> Right, it won't work at all.
>
> The caller has to be told how many DMA entries it really did use to
> compose the mapping, and there has to be a way to properly iterate
> over them.  The assumption that the IOMMU will map the SG entries
> 1-to-1 is invalid.

Good catch.  Indeed, what's missing is one line:

	i->num = num;

Of course, an arch-specific version of this could merge between sg_rings,
too, but that's an optimization.

Thanks,
Rusty.

dma_map_sg_ring() helper

Obvious counterpart to dma_map_sg.  Note that this is arch-independent
code; sg_rings are backwards compatible with simple sg arrays.

Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
---
 drivers/base/dma-mapping.c  |   13 +++++++++++++
 include/linux/dma-mapping.h |    4 ++++
 2 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/drivers/base/dma-mapping.c b/drivers/base/dma-mapping.c
--- a/drivers/base/dma-mapping.c
+++ b/drivers/base/dma-mapping.c
@@ -8,6 +8,7 @@
  */
 
 #include <linux/dma-mapping.h>
+#include <linux/sg_ring.h>
 
 /*
  * Managed DMA API
@@ -162,6 +163,60 @@ void dmam_free_noncoherent(struct device
 }
 EXPORT_SYMBOL(dmam_free_noncoherent);
 
+/**
+ * dma_map_sg_ring - Map an entire sg ring
+ * @dev: Device to free noncoherent memory for
+ * @sg: The sg_ring
+ * @direction: DMA_TO_DEVICE, DMA_FROM_DEVICE or DMA_BIDIRECTIONAL.
+ *
+ * This returns -ENOMEM if mapping fails.  It's not clear that telling you
+ * it failed is useful though.
+ */
+int dma_map_sg_ring(struct device *dev, struct sg_ring *sg,
+		     enum dma_data_direction direction)
+{
+	struct sg_ring *i;
+	unsigned int num;
+
+	for (i = sg; i; i = sg_ring_next(i, sg)) {
+		BUG_ON(i->num > i->max);
+		num = dma_map_sg(dev, i->sg, i->num, direction);
+		if (num == 0 && i->num != 0)
+			goto unmap;
+		i->num = num;
+	}
+	return 0;
+
+unmap:
+	while (sg) {
+		dma_unmap_sg(dev, sg->sg, sg->num, direction);
+		sg = sg_ring_next(sg, i);
+	}
+	return -ENOMEM;
+
+}
+EXPORT_SYMBOL(dma_map_sg_ring);
+
+/**
+ * dma_unmap_sg_ring - Unmap an entire sg ring
+ * @dev: Device to free noncoherent memory for
+ * @sg: The sg_ring
+ * @direction: DMA_TO_DEVICE, DMA_FROM_DEVICE or DMA_BIDIRECTIONAL.
+ *
+ * Call after dma_map_sg_ring() succeeds.
+ */
+void dma_unmap_sg_ring(struct device *dev, struct sg_ring *sg,
+		       enum dma_data_direction direction)
+{
+	struct sg_ring *i;
+
+	for (i = sg; i; i = sg_ring_next(i, sg)) {
+		BUG_ON(i->num > i->max);
+		dma_unmap_sg(dev, i->sg, i->num, direction);
+	}
+}
+EXPORT_SYMBOL(dma_unmap_sg_ring);
+
 #ifdef ARCH_HAS_DMA_DECLARE_COHERENT_MEMORY
 
 static void dmam_coherent_decl_release(struct device *dev, void *res)
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -87,6 +87,12 @@ dma_mark_declared_memory_occupied(struct
 }
 #endif
 
+struct sg_ring;
+extern int dma_map_sg_ring(struct device *dev, struct sg_ring *sg,
+			   enum dma_data_direction direction);
+extern void dma_unmap_sg_ring(struct device *dev, struct sg_ring *sg,
+			      enum dma_data_direction direction);
+
 /*
  * Managed DMA API
  */

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