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

[Wolfgang Wegner]

Hi Leon,

On Mon, Sep 13, 2010 at 07:10:59PM +0200, Leon Woestenberg wrote:
> 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.

this is what I read all around, however, I do not see how this
can improve anything when using an mmap()ed frame buffer for
pixel-oriented operations...
This is why I thought about reverting to write() and simply transfer
complete frames, which would be sufficient for about 90% of my
application scenarios - and for the other 10% I could live with
the lower performance.

Regards,
Wolfgang