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

[Nicolas Pitre]

On Tue, 7 Sep 2010, saeed bishara wrote:

> On Tue, Sep 7, 2010 at 10:58 AM, saeed bishara <saeed.bishara at gmail.com> wrote:
> > On Mon, Sep 6, 2010 at 5:14 PM, Wolfgang Wegner <ww-ml at gmx.de> wrote:
> >> 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.
> > I'm not sure the writecombine is enabled properly, can you test that on DRAM?
> > you can do that be reserving some memory (mem=<dram size - 8M>), then
> > try to test throughput with and without writecombine.
> >
> also, in order to sent bursts, make sure that the stm instruction is
> used, preferred with 8 registers with address aligned to 8*4 bytes.

The copy_from_user code (through the write() system call) already takes 
care of this with large enough buffers.  Of course with mmap()'d memory 
you are responsible for optimizing this yourself.


Nicolas