ixp4xx dmabounce

[brian@walsh.ws (Brian Walsh)]

On Wed, Sep 23, 2009 at 10:40 AM, Krzysztof Halasa <khc@pm.waw.pl> wrote:
> Mikael Pettersson <mikpe@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.