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.

BTW, the throughput I get with a "dd if=bitmap of=/dev/fb0 bs=512"
is the same I get from my test application writing longwords
sequentially to the mmapped frame buffer.

Regards,
Wolfgang