Kirkwood PCI(e) write performance and DMA engine support for copy_{to, from}_user?

[Leon Woestenberg]

Hello Wolfgang,

On Thu, Sep 9, 2010 at 6:21 PM, Wolfgang Wegner <ww-ml at gmx.de> wrote:
> On Wed, Sep 08, 2010 at 10:35:58AM +0200, Wolfgang Wegner wrote:
>>
> With the FPGA evaluation board I get:
> - around 38 MBytes/second with Nicolas' inline assembly code
> - around 6 MBytes/second with any other C code (mmapped) as
>  well as write() via dd
>
> So the main problem seems to be either our board implementation
> of the PCIe->PCI bridge or the FPGA. However, I am still wondering
> how a framebuffer-based application can attain reasonable performance,
>
Having implemented a framebuffer demo on an FPGA recently using PCI
Express, I think the main performance gain is made by having the DMA
done by the endpoint (FPGA) rather than by the CPU.

> PS: another PCI device I tried via the PCIe->PCI bridge was
>    a Intel 82574L GBit NIC, which was able to reach >600MBit/s
>    throughput when tested with netio or netperf
>
Such devices (typically) use endpoint initiated DMA, i.e. they do not
involve much overhead/latencies on the CPU/host/root complex side.

Regards,

Leon.