[9front] [patch] cdfs handle block sizes correctly

[Arne Meyer <meyer.arne83@netcologne.de>]

The Readblock enum does not work when you try to read audio cds. 4 cdda blocks are lager than 8192 and the command fails (at least when using an usb cd drive). This sets the block count according to the track block size. With this and the fix for libdisk I can rip audio cds on my el cheapo usb dvd drive. Tested with "Midnight Oil - Blue Sky Mining"
Greetings,
Arne

diff e938acc8ff64a3cbbd6ef7ba88f83e3f03ede681 uncommitted
--- a/sys/src/cmd/cdfs/dat.h
+++ b/sys/src/cmd/cdfs/dat.h
@@ -133,10 +133,11 @@
        DVDNblock = 16,         /* DVD ECC block is 16 sectors */
        BDNblock = 32,          /* BD ECC block (`cluster') is 32 sectors */
        /*
-        * make a single transfer fit in a 9P rpc.  if we don't do this,
-        * remote access (e.g., via /mnt/term/dev/sd*) fails mysteriously.
+        * number of blocks read/written must fit in this. if we don't do this,
+        * remote access (e.g., via /mnt/term/dev/sd* or nusb/disk) fails mysteriously.
+        * see /sys/src/9/port/devmnt.c MAXRPC.
         */
-       Readblock = 8192/BScdrom,
+       Maxrpc = 8192,
 };

 typedef struct Buf Buf;
--- a/sys/src/cmd/cdfs/mmc.c
+++ b/sys/src/cmd/cdfs/mmc.c
@@ -1171,7 +1171,7 @@
        o->track = &drive->track[trackno];
        o->nchange = drive->nchange;
        o->omode = OREAD;
-       o->buf = bopen(mmcread, OREAD, o->track->bs, Readblock);
+       o->buf = bopen(mmcread, OREAD, o->track->bs, Maxrpc/o->track->bs);
        o->buf->otrack = o;

        aux->nropen++;
@@ -1395,7 +1395,7 @@
        o->nchange = drive->nchange;
        o->omode = OWRITE;
        o->track = t;
-       o->buf = bopen(mmcwrite, OWRITE, bs, Readblock);
+       o->buf = bopen(mmcwrite, OWRITE, bs, Maxrpc/bs);
        o->buf->otrack = o;

        aux->nwopen++;