cvs commit: src/share/man/man9 bus_dma.9

[M. Warner Losh]

Here's what I came up with earlier in the week.

Index: bus_dma.9
===================================================================
RCS file: /home/ncvs/src/share/man/man9/bus_dma.9,v
retrieving revision 1.31
diff -u -r1.31 bus_dma.9
--- bus_dma.9	13 Dec 2005 17:07:52 -0000	1.31
+++ bus_dma.9	2 Mar 2006 21:14:40 -0000
@@ -265,14 +265,41 @@
 .It Dv BUS_DMASYNC_PREREAD
 Perform any synchronization required prior to an update of host memory by the
 DMA read operation.
+.Pp
+In other words, the driver is about to command the device to start
+a DMA operation that transfers data from the
+device to the memory described by this map.
+And pending writes to this part of memory that the CPU might have queued
+up will either be discarded or be completed after this operation completes.
 .It Dv BUS_DMASYNC_PREWRITE
 Perform any synchronization required after an update of host memory by the CPU
 and prior to DMA write operations.
+.Pp
+In other words, the CPU flushes any cached writes to the area of memory
+described by this map so they are visible when the device does its
+DMA READ operation on the memory described by the map.
+An implementation may also use this to implement bounce buffers and
+memory barriers.
 .It Dv BUS_DMASYNC_POSTREAD
 Perform any synchronization required after DMA read operations and prior to
 CPU access to host memory.
+.Pp
+In other words, the CPU discards the cached values for the memory range
+described by the map.
+Future reads to the memory range described by the map will fetch the values
+from memory.
+An implementation is free to reload the cache before any driver initiated
+reads to this memory.
+An implementation may also use this to implement bounce buffers and
+memory barriers.
 .It Dv BUS_DMASYNC_POSTWRITE
 Perform any synchronization required after DMA write operations.
+.Pp
+In other words, this is completely useless.
+The driver writer has noticed that the device has indicated the DMA READ
+operation has completed.
+It is theoretically possible that this would cause the device to do something
+but it is unclear what.
 .El
 .It Vt bus_dma_lock_t
 Client specified lock/mutex manipulation method.