[PATCH 3/5] firewire: ohci: always use packet-per-buffer mode for isochronous reception

Stefan Richter
2009-12-26T00:35:28Z

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 | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux-2.6.33-rc2/drivers/firewire/ohci.c
===================================================================
--- linux-2.6.33-rc2.orig/drivers/firewire/ohci.c
+++ linux-2.6.33-rc2/drivers/firewire/ohci.c
@@ -2226,7 +2226,6 @@ static int ohci_queue_iso_receive_dualbu
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 pc
}

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 &&