Re: mem-to-mem copy using dma ...

[Giuseppe C. <pep...@gm...>]

Hi,

> I have to read UDP packets from sk_buff to my module in kernel space and
> the udp stack uses copy_to_user() to do my task. I'm wondering if using
> dma for mem-to-mem copy can improve data transfer performance instead of
> using copy_to_user()???

Often to take the best performance by using a DMA engine, e.g. in a network
driver,
the zero-copy mechanism is used because It essentially avoids memcpy.
Abut how to implement the zero-copy mechanism you ought to start looking at
the network device
drivers within the kernel source tree.

>If yes, could someone gives me some hints to go with it. I'm currently
>working on ST7109 and stlinux 2.6.17.14 <http://2.6.17.14><http://2.6.17.14/>.
I have tried
>the guide at
> http://www.stlinux.com/docs/manual/distribution/fdma_legacy.php<http://www.stlinux.com/docs/manual/distribution/fdma_legacy.php>but not
>succeeded, kernel crashed with the following error:

Concerning the 7109; it has an embedded MAC (core by Synopsys) with a own
DMA (no FDMA).
You could directly browse the driver source (called stmmac within the
STLinux kernels) where zero-copy is
fully implemented.
For more information also visit:
http://stlinux.com/docs/manual/distribution/distribution_guide5.php
and STLinux Bugzilla.