Remove comment that doesn't seem to be true and add comments describing

what is going on, to replace it. Slight formatting changes
Code here is alredy different to NetBSD.
MFC after:	1 week

1 files changed, 16 insertions, 17 deletions

diff --git a/sys/dev/usb/ohci.c b/sys/dev/usb/ohci.c
index ff343a2..11a5049 100644
--- a/sys/dev/usb/ohci.c
+++ b/sys/dev/usb/ohci.c
@@ -549,32 +549,31 @@ ohci_alloc_std_chain(struct ohci_pipe *opipe, ohci_softc_t *sc,
 		 *
 		 * If/when dma has multiple segments, this will need to
 		 * properly handle fragmenting TD's.
-		 *
-		 * We can describe the above using maxsegsz = 4k and nsegs = 2
-		 * in the future.
+		 * 
+		 * Note that if we are gathering data from multiple SMALL
+		 * segments, e.g. mbufs, we need to do special gymnastics,
+		 * e.g. bounce buffering or data aggregation,
+		 * BEFORE WE GET HERE because a bulk USB transfer must
+		 * consist of maximally sized packets right up to the end.
+		 * A shorter than maximal packet means that it is the end
+		 * of the transfer. If the data transfer length is a
+		 * multiple of the packet size, then a 0 byte
+		 * packet will be the signal of the end of transfer.
+		 * Since packets can't cross TDs this means that
+		 * each TD except the last one must cover an exact multiple
+		 * of the maximal packet length.
 		 */
-		if ((OHCI_PAGE_OFFSET(dataphys) + len) <=
-		    (2 * OHCI_PAGE_SIZE)) {
-			/* we can handle it in this TD */
+		if (OHCI_PAGE_OFFSET(dataphys) + len <= (2 * OHCI_PAGE_SIZE)) {
+			/* We can handle all that remains in this TD */
 			curlen = len;
 		} else {
-			/* XXX The calculation below is wrong and could
-			 * result in a packet that is not a multiple of the
-			 * MaxPacketSize in the case where the buffer does not
-			 * start on an appropriate address (like for example in
-			 * the case of an mbuf cluster). You'll get an early
-			 * short packet.
-			 */
 			/* must use multiple TDs, fill as much as possible. */
 			curlen = 2 * OHCI_PAGE_SIZE -
 				 OHCI_PAGE_OFFSET(dataphys);
 			/* the length must be a multiple of the max size */
 			curlen -= curlen %
 			    UGETW(opipe->pipe.endpoint->edesc->wMaxPacketSize);
-#ifdef DIAGNOSTIC
-			if (curlen == 0)
-				panic("ohci_alloc_std: curlen == 0");
-#endif
+			KASSERT ((curlen == 0), ("ohci_alloc_std: curlen == 0"));
 		}
 		DPRINTFN(4,("ohci_alloc_std_chain: dataphys=0x%08x "
 			    "len=%d curlen=%d\n",