Subject: Re: [BUILD FAILURE 01/04] Next June 04:PPC64 randconfig
 [drivers/staging/comedi/drivers.o]
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: subrata@linux.vnet.ibm.com
Cc: fmhess@users.sourceforge.net, Greg KH <greg@kroah.com>,
       Ian Abbott <abbotti@mev.co.uk>, David Miller <davem@davemloft.net>,
       sachinp@linux.vnet.ibm.com, sfr@canb.auug.org.au,
       linux-kernel@vger.kernel.org, Linuxppc-dev@ozlabs.org,
       linux-next@vger.kernel.org, paulus@samba.org,
       Geert.Uytterhoeven@sonycom.com, geert@linux-m68k.org
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Date: Tue, 09 Jun 2009 13:50:18 +1000
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On Sun, 2009-06-07 at 20:06 +0530, Subrata Modak wrote:
> On Sat, 2009-06-06 at 09:36 -0400, Frank Mori Hess wrote:
> > On Saturday 06 June 2009, Greg KH wrote:
> > > Frank and Ian, any thoughts about the vmap call in the
> > > comedi_buf_alloc() call?  Why is it using PAGE_KERNEL_NOCACHE, and what
> > > is the prealloc_buf buffer used for?
> > 
> > It is a circular buffer used to hold data streaming either to or from a 
> > board (for example when producing an analog output waveform).  Reads and 
> > writes to the device files read/write to the circular buffer, plus a few 
> > drivers do dma directly to/from it.  I personally don't have a problem 
> > with requiring drivers to have their own dma buffers and making them copy 
> > data between their private dma buffers and the main circular buffer.  I 
> > guess the original design wanted to support zero-copy dma.
> 
> Great to hear that. How about a patch that solves my build problem on
> PPC64(the problem seems to be existing for long) ? 

In any case, doing PAGE_KERNEL_NOCACHE for DMA memory is incorrect on
many architectures. So at this stage, there's no much option but ifdef I
suspect for now until this is fixed properly.

It does make sense to want to have some memory like that shared between
user space and DMA, though I don't know what the right approach that
works on all archs is at this stage. Worth asking the Alsa guys, I think
they have similar issues :-)

But doing double buffering might do the trick fine for now.

Cheers,
Ben.


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