When doing a PREREAD sync of an mbuf-type dma buffer, do a writeback of

the first cacheline if the buffer start address is not on a cacheline
boundary.  Normally a buffer which is not cacheline-aligned is bounced,
but a special rule applies for mbufs, which are always misaligned due to
the header.  We know the cpu will not write to the header while dma is in
progress (so we've been told anyway), but it may have written to the
header shortly before starting a read, so we need to flush that write out
to memory before invalidating the whole buffer.

In collaboration with Mical Meloun and Svata Kraus.

sys/arm/arm/busdma_machdep-v6.c

@@ -1456,7 +1456,24 @@ _bus_dmamap_sync(bus_dma_tag_t dmat, bus_dmamap_t map, bus_dmasync_op_t op)
 			break;
 
 		case BUS_DMASYNC_PREREAD:
+			/*
+			 * An mbuf may start in the middle of a cacheline. There
+			 * will be no cpu writes to the beginning of that line
+			 * (which contains the mbuf header) while dma is in
+			 * progress.  Handle that case by doing a writeback of
+			 * just the first cacheline before invalidating the
+			 * overall buffer.  Any mbuf in a chain may have this
+			 * misalignment.  Buffers which are not mbufs bounce if
+			 * they are not aligned to a cacheline.
+			 */
 			while (sl != end) {
+				if (sl->vaddr & arm_dcache_align_mask) {
+					KASSERT(map->flags & DMAMAP_MBUF,
+					    ("unaligned buffer is not an mbuf"));
+					cpu_dcache_wb_range(sl->vaddr, 1);
+					l2cache_wb_range(sl->vaddr,
+					    sl->busaddr, 1);
+				}
 				cpu_dcache_inv_range(sl->vaddr, sl->datacount);
 				l2cache_inv_range(sl->vaddr, sl->busaddr, 
 				    sl->datacount);