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

[saeed bishara]

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.

saeed