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: Greg KH <greg@kroah.com>, fmhess@users.sourceforge.net,
       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: Wed, 10 Jun 2009 08:34:39 +1000
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> > 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.
> 
> Ok. But, i am not sure whether Greg will agree to this. If, Ok, is the
> following patch i sent earlier Ok ?
> 
> http://lkml.org/lkml/2009/6/5/462,

Not really.

You probably want to use a constant (call it MY_DMA_MAP_PGPROT), and
in a header, you have a bunch of ifdef's that set it to PAGE_KERNEL,
PAGE_KERNEL_NOCACHE or PAGE_KERNEL_NC depending on what's needed.

Today, you can pretty much assume that

 - x86*, sparc*, ia64*, alpha, ... needs PAGE_KERNEL
 - powerpc needs PAGE_KERNEL if !CONFIG_NOT_COHERENT_CACHE
 - powerpc needs PAGE_KERNEL_NC if CONFIG_NOT_COHERENT_CACHE
 - ARM and MIPS, I think, needs PAGE_KERNEL_NOCACHE
 - ... others I don't know.

Cheers,
Ben.

> Regards--
> Subrata
> 
> > 
> > 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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