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

[Stuart MENEFY <stuart.menefy@st.com>]

Hi Ha

Ha Nguyen wrote:
> 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 not sure what you're doing here. Do you mean you have kernel code
which is calling sys_recv()? In which case your code would never have
any visibility of the sk_buff.

 >                                                        I'm wondering if
> using dma for mem-to-mem copy can improve data transfer performance
> instead of using copy_to_user()???

If you're trying to optimise the copy to user you might like to have a
look at the infrastructure which was put in for Intel IOAT and
CONFIG_NET_DMA. Whether this will be a performance win I'm just not
sure. The DMA can probably move data about more efficiently, but has
to be programmed at the start and an interrupt and reschedule take place
at the end, and caches have to be managed.

As was mentioned a few days ago, once the dmaengine API is in place,
this should be a much easier experiment to try.

> If yes, could someone gives me some hints to go with it. I'm currently
> working on ST7109 and stlinux 2.6.17.14. I have tried the guide at
> http://www.stlinux.com/docs/manual/distribution/fdma_legacy.php and
> http://www.stlinux.com/docs/manual/distribution/fdma_sh_exmpl1.php but
> not succeeded, kernel crashed with the following error:
> 
> Unable to handle kernel NULL pointer dereference at virtual address 00000000
> pc = 86cd56aa
> *pde = 00000000
> Oops: 0001 [#1]
> 
> Pid : 818, Comm: udhcpc
> PC is at create_llu_list+0x86/0x8c
> PC : 86cd56aa SP : 8f633d38 SR : 40008001 TEA : c0103a64 Not tainted
> R0 : 00000000 R1 : 00000000 R2 : 8f633d80 R3 : 00000001
> R4 : 8f633d80 R5 : 00000000 R6 : 00000000 R7 : 00000000
> R8 : 00000001 R9 : 8f633d40 R10 : 00000030 R11 : 86d6616c
> R12 : 0f516ae0 R13 : 86d6616c R14 : 8fbc6000
> MACH: 00000000 MACL: 00000000 GBR : 295737e8 PR : 86cd696c

The DMA code for the ST40 based chips is very specific, so you're
better off posting questions about that to the STLinux bugzilla
(bugzilla.stlinux.com). However the 2.6.17 kernel is from the 2.2
distribution, so you shouldn't be using the FDMA legacy documentation.
The slightly more up to date documentation is at:
http://www.stlinux.com/docs/manual/distribution/distribution_guide14.php

But your best be would be to look at one of the existing drivers which
uses the FDMA, such as the IDE PIO-DMA or ALSA.

Stuart