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

[Wolfgang Wegner]

On Mon, Sep 06, 2010 at 03:03:47PM +0100, Russell King - ARM Linux wrote:
> On Mon, Sep 06, 2010 at 12:02:44PM +0200, Wolfgang Wegner wrote:
> > Mapping the PCI memory space via mmap() resulted in some
> > disappointing ~6.5 MBytes/second. I tried to modify page
> > protection to pgprot_writecombine or pgprot_cached, but while
> > this did reproducably change performance, it was only in
> > some sub-percentage range. I am not sure if I understand
> > correctly how other framebuffers handle this, but it seems
> > the "raw" mmapped write performance is not cared about too
> > much or maybe not that bad with most x86 chip sets?
> > However, the idea left over after some trying and looking
> > around is to use the DMA engine to speed up write() (and
> > also read(), but this is not so important) system calls
> > instead of using mmap.
> 
> Framebuffer applications such as Xorg/Qt do not use read/write calls
> to access their buffers because that will be painfully slow.

That was my impression, too. However, I did not figure out
how they do actually speed up write access for mmapped
operation - although in many places the speed/latency of
PCI transactions is complained about.
Some really simple frame buffers have a shadow buffer in system
RAM to avoid read access for framebuffer-internal operations
that operate on the frame buffer like bitblt etc., but this
does not help for the mmapped case.

Finding my way through the framebuffer jungle is not the
easiest thing, so maybe I am missing something really
obvious here, or is it simply a configuration problem on
my board and the write performance when using mmap is 
good when configured properly?

Regards,
Wolfgang