firewire: revert to 4 GB RDMA, fix protocols using Memory Space

Undo a feature introduced in v3.14 by commit fcd46b34425d
"firewire: Enable remote DMA above 4 GB".  That change raised the
minimum address at which protocol drivers and user programs can register
for request reception from 0x0001'0000'0000 to 0x8000'0000'0000.
It turned out that at least one vendor-specific protocol exists which
uses lower addresses:  https://bugzilla.kernel.org/show_bug.cgi?id=76921

For the time being, revert most of commit fcd46b34425d so that affected
protocols work like with kernel v3.13 and before.  Just keep the valid
documentation parts from the regressing commit, and the ability to
identify controllers which could be programmed to accept >32 bit
physical DMA addresses.  The rest of fcd46b34425d should probably be
brought back as an optional instead of default feature.

Reported-by: Fabien Spindler <fabien.spindler@inria.fr>
Cc: <stable@vger.kernel.org> # 3.14+
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

1 files changed, 8 insertions, 5 deletions

diff --git a/Documentation/debugging-via-ohci1394.txt b/Documentation/debugging-via-ohci1394.txt
index fa0151a712f9..5c9a567b3fac 100644
--- a/Documentation/debugging-via-ohci1394.txt
+++ b/Documentation/debugging-via-ohci1394.txt
@@ -25,9 +25,11 @@ using data transfer rates in the order of 10MB/s or more.
 With most FireWire controllers, memory access is limited to the low 4 GB
 of physical address space.  This can be a problem on IA64 machines where
 memory is located mostly above that limit, but it is rarely a problem on
-more common hardware such as x86, x86-64 and PowerPC.  However, at least
-Agere/LSI FW643e and FW643e2 controllers are known to support access to
-physical addresses above 4 GB.
+more common hardware such as x86, x86-64 and PowerPC.
+
+At least LSI FW643e and FW643e2 controllers are known to support access to
+physical addresses above 4 GB, but this feature is currently not enabled by
+Linux.
 
 Together with a early initialization of the OHCI-1394 controller for debugging,
 this facility proved most useful for examining long debugs logs in the printk
@@ -101,8 +103,9 @@ Step-by-step instructions for using firescope with early OHCI initialization:
    compliant, they are based on TI PCILynx chips and require drivers for Win-
    dows operating systems.
 
-   The mentioned kernel log message contains ">4 GB phys DMA" in case of
-   OHCI-1394 controllers which support accesses above this limit.
+   The mentioned kernel log message contains the string "physUB" if the
+   controller implements a writable Physical Upper Bound register.  This is
+   required for physical DMA above 4 GB (but not utilized by Linux yet).
 
 2) Establish a working FireWire cable connection: