ixp4xx dmabounce

[Mikael Pettersson]

Brian Walsh writes:
 > On Wed, Sep 23, 2009 at 10:40 AM, Krzysztof Halasa <khc at pm.waw.pl> wrote:
 > > Mikael Pettersson <mikpe at it.uu.se> writes:
 > >
 > >> I strongly suspect that something on the USB or networking side
 > >> is allocating I/O buffers without observing the correct DMA APIs.
 > >
 > > At least the network stack allocates buffers ignoring the DMA masks.
 > > The buffers may be allocated by one device (driver) and passed to
 > > another device. The only plausible way to fix it is IMHO limiting all
 > > skb allocations to the common mask (drivers would be free to either
 > > handle or drop skbs outside of their mask).
 > >
 > > This is relatively easy to implement and I'm going to try it, when time
 > > permits.
 > >
 > >> I think Krzysztof Halasa mentioned running ixp4xx devices with 128MB
 > >> RAM and a kernel hacked so kernel-private allocations would always be
 > >> served from memory below 64MB. I think he mentioned doing that because
 > >> of networking components that would ignore PCI DMA mask constraints.
 > >
 > > Right. This works fine for network buffers because they aren't that
 > > large. The current patch is suboptimal, though.
 > > --
 > > Krzysztof Halasa
 > >
 > 
 > I tried Krzysztof's patch and it had no noticeable affect.  I am still getting
 > about 6.3 Mbps IP data throughput when only using the ohci controller and
 > about 3.6 Mbps when the device is attached to the ehci controller.  This
 > device works fine when running the same testing attached to an x86
 > configured machine and gets about 18 Mbps IP data throughput.

If your application can operate in 64MB RAM, you may want to try
a kernel that includes only my ixp4xx disable dmabounce patch,
and boot it with mem=64M. (Look in the kernel boot log and verify
that it only sees 64M of RAM.)

If performance increases, then your performance loss is due to bounces.