PERFORCE change 107239 for review

[Warner Losh]

http://perforce.freebsd.org/chv.cgi?CH=107239

Change 107239 by imp at imp_lighthouse on 2006/10/04 05:36:42

	Don't need a stop command for the simple case of reading a single
	block.  This reduces the number of round trips (commands) by 16 for
	a 8k transfer.  We're down from 34 to 16, or a 53% savings.
	Multi-block will be the next big win, where we can go all the way
	down to 2 (start multi-block ... stop at the end).
	
	Since we went from 200kB/s to 220kB/s when I doubled the transfer
	speed, this tells me that 80% of the time isn't affected by the
	transfer.  I hope that the 53% savings will cut the 80% down to 40%
	which should result in bootsting the speed to something like 400kB/s.
	We'll see tomorrow if this actually is the case or not.  This
	is a 4 to 1 overhead still.  And it is 10x slower than the max
	transfer rate based solely on clock speed (3.75MB/s).  This large
	factor makes me think that some part of this analysis is flawed.
	It can't be too fast to do the byte-swapping after the transfers
	either.
	
	4 bit bus will also help, once I've implemented it, but so long as
	the non-data portion of the transfers dominates so much, it does
	little good for the overall numbers.  I might be able to get it up
	to 500k/s by enabling 4-bit bus.  This is still well short of
	the 15MB/s that the clock rate suggests these cards top out
	at.  I'd imagine that multi-block would be a big win...

Affected files ...

.. //depot/projects/arm/src/sys/dev/mmc/mmcsd.c#9 edit

Differences ...

==== //depot/projects/arm/src/sys/dev/mmc/mmcsd.c#9 (text+ko) ====

@@ -173,7 +173,7 @@
 			memset(&stop, 0, sizeof(stop));
 			req.cmd = &cmd;
 			cmd.data = &data;
-			req.stop = &stop;
+//			req.stop = &stop;
 			if (bp->bio_cmd == BIO_READ)
 				cmd.opcode = MMC_READ_SINGLE_BLOCK;
 			else
@@ -189,9 +189,9 @@
 				data.flags = MMC_DATA_READ;
 			else
 				data.flags = MMC_DATA_WRITE;
-			stop.opcode = MMC_STOP_TRANSMISSION;
-			stop.arg = 0;
-			stop.flags = MMC_RSP_R1B | MMC_CMD_AC;
+//			stop.opcode = MMC_STOP_TRANSMISSION;
+//			stop.arg = 0;
+//			stop.flags = MMC_RSP_R1B | MMC_CMD_AC;
 			MMCBUS_WAIT_FOR_REQUEST(device_get_parent(dev), dev,
 			    &req);
 			// XXX error handling