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

[Ha N. <han...@gm...>]

On Feb 1, 2008 1:21 PM, Stuart MENEFY <stu...@st...> wrote:
> Folks
>
> Giuseppe Cavallaro 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 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.
>
> Note that there are generally two 'copies' involved in receiving data.
> The first is where the data is written into the network stack's packet
> buffer, the second is where the data is copied into the user's buffer.
> (This is ignoring the checksum step, which ideally is merged with one of
> the other copies).
>
> Giuseppe is referring to removing the first, which involves the DMA engine
> in the NIC writing the data directly into the sk_buff. Ha Nguyen was asking
> about the second, which is very hard to remove while still retaining the
> standard socket API.

Exactly, I already configured the MAC for zero-copy. Based on Stuart's
input, dmaengine should do my work when it is available. I'm curious
to see if how it improves network performance...

Thanks guys for your feedbacks!

/ha