firewire: ohci: always use packet-per-buffer mode for isochronous reception

This is a minimal change meant for the short term:  Never set the
ohci->use_dualbuffer flag to true.

There are two reasons to do so:

  - Packet-per-buffer mode and dual-buffer mode do not behave the same
    under certain circumstances, notably if several packets are covered
    by a single fw_cdev_iso_packet descriptor.
    http://marc.info/?l=linux1394-devel&m=124965653718313
    Therefore the driver stack should not silently choose one or the
    other mode but should leave the choice to the high-level driver
    (regardless if kernel driver or userspace driver).  Or simply always
    only offer packet-per-buffer mode, since a considerable number of
    controllers, even current ones, does not offer dual-buffer support.

  - Even under circumstances where packet-per-buffer mode and
    dual-buffer mode behave exactly the same --- notably when used
    through libraw1394, libdc1394, as well as the current two kernel
    drivers which use isochronous reception (firewire-net and firedtv)
    --- we are still faced with the problem that several OHCI 1.1
    controllers have bugs in dual-buffer mode.  Although it looks like
    we have identified most of those buggy controllers by now, we
    cannot be quite sure about that.

So, use packet-per-buffer by default from now on.  This change should
be followed up by a more complete solution:  Either extend the
in-kernel API and the userspace ABI by a choice between the two IR modes
or remove all dual-buffer related code from firewire-ohci.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/ohci.c

@@ -2226,7 +2226,6 @@ static int ohci_queue_iso_receive_dualbuffer(struct fw_iso_context *base,
 	if (rest == 0)
 		return -EINVAL;
 
-	/* FIXME: make packet-per-buffer/dual-buffer a context option */
 	while (rest > 0) {
 		d = context_get_descriptors(&ctx->context,
 					    z + header_z, &d_bus);
@@ -2470,7 +2469,10 @@ static int __devinit pci_probe(struct pci_dev *dev,
 	}
 
 	version = reg_read(ohci, OHCI1394_Version) & 0x00ff00ff;
+#if 0
+	/* FIXME: make it a context option or remove dual-buffer mode */
 	ohci->use_dualbuffer = version >= OHCI_VERSION_1_1;
+#endif
 
 	/* dual-buffer mode is broken if more than one IR context is active */
 	if (dev->vendor == PCI_VENDOR_ID_AGERE &&