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List:       linux-arm-kernel
Subject:    Re: perfomance of copy_to/from_user
From:       Nicolas Pitre <nico () cam ! org>
Date:       2004-10-06 17:46:22
Message-ID: Pine.LNX.4.61.0410061343480.25631 () xanadu ! home
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On Wed, 6 Oct 2004, Thomas Wöckinger wrote:

> On Wednesday 06 October 2004 18:28, you wrote:
> > On Wed, 6 Oct 2004, Thomas Wöckinger wrote:
> > > Hi!
> > >
> > > I have some performance Problems using copy_to_user or copy_from_user. I
> > > can only transfer about 22,5 MB/s from or to userspace
> > >
> > > I am using an Intel Xscale processor running on 500MHz pluged into
> > > idxp425 development board. An DSP card (pluged into PCI slot) is writing
> > > directly into a 256K kernel space buffer.
> > >
> > > the kernel version is 2.6.8.1
> > >
> > > My first question is: can i speed up the copy_to/from_user?
> >
> > Hardly.
> >
> > > if not, is it possible on an idxp425 board to access the whole memory
> > > directly, so that i can write to the userspace buffer without using the
> > > xscale.
> >
> > It is possible for you to mmap() the 256k kernel buffer into user space
> > and avoid copying the data entirely.
> 
> sometimes i have to transfer about 1MB (imagedata) beetween the dsp card and 
> the xscale, and it's hard to get such a big continous buffer, especially when 
> i have four dsp card's and each of them want to transfer such an datachunk to 
> the host

So?  How does this relates to mmap()?

You need to allocate the memory regardless of mmap().  And a mmap()'d 
buffer doesn't have to be contigous physical memory.


Nicolas

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