as cinap noted, the previous patch was wrong.  pass in the sector size
instead of using 512.  the Sfis already has a sector size, but it doesn't
tell us which one we're using.  this is not a problem for the nix version
of the same code, so i'm going to ignore this ugly for now.  /sys/src/9
should adopt the same code.

Reference: /n/atom/patch/applied/pcahci4ks
Date: Sat Jan 25 14:46:19 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/pc/sdiahci.c	Sat Jan 25 14:42:43 2014
+++ /sys/src/9/pc/sdiahci.c	Sat Jan 25 14:42:43 2014
@@ -1507,7 +1507,7 @@
 }
 
 static Alist*
-ahcibuild(Aportm *m, int rw, void *data, uint n, vlong lba)
+ahcibuild(Aportm *m, int rw, void *data, uint ss, uint n, vlong lba)
 {
 	uchar *c;
 	uint flags;
@@ -1519,7 +1519,7 @@
 	flags = Lpref;
 	if(rw == SDwrite)
 		flags |= Lwrite;
-	mkalist(m, flags, data, n);
+	mkalist(m, flags, data, ss*n);
 	return l;
 }
 
@@ -1750,7 +1750,7 @@
 		if(n > max)
 			n = max;
 		qlock(&d->portm);
-		ahcibuild(&d->portm, rw, data, n*u->secsize, lba);
+		ahcibuild(&d->portm, rw, data, d->secsize, n, lba);
 		status = io(d, Pdma, 5000, 0);
 		qunlock(&d->portm);
 		switch(status){