Rename dcache_dma_preread() to dcache_inv_poc_dma() to make it clear that it

is a dcache invalidate to point of coherency just like dcache_inv_poc(), but
a slightly different version specific to dma operations.  Elaborate the
comment about how and why it's different.

2 files changed, 10 insertions, 8 deletions

diff --git a/sys/arm/arm/busdma_machdep-v6.c b/sys/arm/arm/busdma_machdep-v6.c
index f032504..297b129 100644
--- a/sys/arm/arm/busdma_machdep-v6.c
+++ b/sys/arm/arm/busdma_machdep-v6.c
@@ -1284,7 +1284,7 @@ dma_preread_safe(vm_offset_t va, vm_paddr_t pa, vm_size_t size)
 	if ((va + size) & cpuinfo.dcache_line_mask)
 		dcache_wb_poc(va + size, pa + size, 1);
 
-	dcache_dma_preread(va, pa, size);
+	dcache_inv_poc_dma(va, pa, size);
 }
 
 static void
@@ -1406,7 +1406,7 @@ _bus_dmamap_sync(bus_dma_tag_t dmat, bus_dmamap_t map, bus_dmasync_op_t op)
 		if ((op & BUS_DMASYNC_PREREAD) && !(op & BUS_DMASYNC_PREWRITE)) {
 			bpage = STAILQ_FIRST(&map->bpages);
 			while (bpage != NULL) {
-				dcache_dma_preread(bpage->vaddr, bpage->busaddr,
+				dcache_inv_poc_dma(bpage->vaddr, bpage->busaddr,
 				    bpage->datacount);
 				bpage = STAILQ_NEXT(bpage, links);
 			}
diff --git a/sys/arm/include/cpu-v6.h b/sys/arm/include/cpu-v6.h
index 7465a89..cf845c6 100644
--- a/sys/arm/include/cpu-v6.h
+++ b/sys/arm/include/cpu-v6.h
@@ -471,15 +471,17 @@ dcache_inv_poc(vm_offset_t va, vm_paddr_t pa, vm_size_t size)
 }
 
 /*
- * Discard D-cache lines to PoC, prior to overwrite by DMA engine
+ * Discard D-cache lines to PoC, prior to overwrite by DMA engine.
  *
- * Invalidate caches, discarding data in dirty lines.  This is useful
- * if the memory is about to be overwritten, e.g. by a DMA engine.
- * Invalidate caches from innermost to outermost to follow the flow
- * of dirty cachelines.
+ * Normal invalidation does L2 then L1 to ensure that stale data from L2 doesn't
+ * flow into L1 while invalidating.  This routine is intended to be used only
+ * when invalidating a buffer before a DMA operation loads new data into memory.
+ * The concern in this case is that dirty lines are not evicted to main memory,
+ * overwriting the DMA data.  For that reason, the L1 is done first to ensure
+ * that an evicted L1 line doesn't flow to L2 after the L2 has been cleaned.
  */
 static __inline void
-dcache_dma_preread(vm_offset_t va, vm_paddr_t pa, vm_size_t size)
+dcache_inv_poc_dma(vm_offset_t va, vm_paddr_t pa, vm_size_t size)
 {
 	vm_offset_t eva = va + size;