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Date:      Tue, 29 Jul 2014 02:36:41 +0000 (UTC)
From:      Ian Lepore <ian@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r269212 - head/sys/arm/arm
Message-ID:  <201407290236.s6T2af9G074324@svn.freebsd.org>
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Author: ian
Date: Tue Jul 29 02:36:41 2014
New Revision: 269212
URL: http://svnweb.freebsd.org/changeset/base/269212

Log:
  Memory belonging to an mbuf, or allocated by bus_dmamem_alloc(), never
  triggers a need to bounce due to cacheline alignment.  These buffers
  are always aligned to cacheline boundaries, and even when the DMA operation
  starts at an offset within the buffer or doesn't extend to the end of the
  buffer, it's safe to flush the complete cachelines that were only partially
  involved in the DMA.  This is because there's a very strict rule on these
  types of buffers that there will not be concurrent access by the CPU and
  one or more DMA transfers within the buffer.
  
  Reviewed by:	cognet

Modified:
  head/sys/arm/arm/busdma_machdep-v6.c

Modified: head/sys/arm/arm/busdma_machdep-v6.c
==============================================================================
--- head/sys/arm/arm/busdma_machdep-v6.c	Tue Jul 29 02:36:27 2014	(r269211)
+++ head/sys/arm/arm/busdma_machdep-v6.c	Tue Jul 29 02:36:41 2014	(r269212)
@@ -162,6 +162,8 @@ struct bus_dmamap {
 	void		      *callback_arg;
 	int		      flags;
 #define DMAMAP_COHERENT		(1 << 0)
+#define DMAMAP_DMAMEM_ALLOC	(1 << 1)
+#define DMAMAP_MBUF		(1 << 2)
 	STAILQ_ENTRY(bus_dmamap) links;
 	int		       sync_count;
 	struct sync_list       slist[];
@@ -279,12 +281,22 @@ alignment_bounce(bus_dma_tag_t dmat, bus
 }
 
 /*
- * Return true if the buffer start or end does not fall on a cacheline boundary.
+ * Return true if the DMA should bounce because the start or end does not fall
+ * on a cacheline boundary (which would require a partial cacheline flush).
+ * COHERENT memory doesn't trigger cacheline flushes.  Memory allocated by
+ * bus_dmamem_alloc() is always aligned to cacheline boundaries, and there's a
+ * strict rule that such memory cannot be accessed by the CPU while DMA is in
+ * progress (or by multiple DMA engines at once), so that it's always safe to do
+ * full cacheline flushes even if that affects memory outside the range of a
+ * given DMA operation that doesn't involve the full allocated buffer.  If we're
+ * mapping an mbuf, that follows the same rules as a buffer we allocated.
  */
 static __inline int
-cacheline_bounce(bus_addr_t addr, bus_size_t size)
+cacheline_bounce(bus_dmamap_t map, bus_addr_t addr, bus_size_t size)
 {
 
+	if (map->flags & (DMAMAP_DMAMEM_ALLOC | DMAMAP_COHERENT | DMAMAP_MBUF))
+		return (0);
 	return ((addr | size) & arm_dcache_align_mask);
 }
 
@@ -302,8 +314,9 @@ static __inline int
 might_bounce(bus_dma_tag_t dmat, bus_dmamap_t map, bus_addr_t addr, 
     bus_size_t size)
 {
-	return ((dmat->flags & BUS_DMA_COULD_BOUNCE) ||
-	    !((map->flags & DMAMAP_COHERENT) && cacheline_bounce(addr, size)));
+	return ((dmat->flags & BUS_DMA_EXCL_BOUNCE) ||
+	    alignment_bounce(dmat, addr) ||
+	    cacheline_bounce(map, addr, size));
 }
 
 /*
@@ -322,8 +335,7 @@ must_bounce(bus_dma_tag_t dmat, bus_dmam
     bus_size_t size)
 {
 
-	/* Coherent memory doesn't need to bounce due to cache alignment. */
-	if (!(map->flags & DMAMAP_COHERENT) && cacheline_bounce(paddr, size))
+	if (cacheline_bounce(map, paddr, size))
 		return (1);
 
 	/*
@@ -727,7 +739,9 @@ bus_dmamem_alloc(bus_dma_tag_t dmat, voi
 		return (ENOMEM);
 	}
 
+	(*mapp)->flags = DMAMAP_DMAMEM_ALLOC;
 	(*mapp)->sync_count = 0;
+
 	/* We may need bounce pages, even for allocated memory */
 	error = allocate_bz_and_pages(dmat, *mapp);
 	if (error != 0) {
@@ -1080,6 +1094,9 @@ _bus_dmamap_load_buffer(bus_dma_tag_t dm
 	if (segs == NULL)
 		segs = dmat->segments;
 
+	if (flags & BUS_DMA_LOAD_MBUF)
+		map->flags |= DMAMAP_MBUF;
+
 	map->pmap = pmap;
 
 	if (might_bounce(dmat, map, (bus_addr_t)buf, buflen)) {
@@ -1196,6 +1213,7 @@ _bus_dmamap_unload(bus_dma_tag_t dmat, b
 		map->pagesneeded = 0;
 	}
 	map->sync_count = 0;
+	map->flags &= ~DMAMAP_MBUF;
 }
 
 #ifdef notyetbounceuser

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