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Date: Tue, 9 Jun 2009 20:54:54 +0200
Message-ID: <10f740e80906091154p60babd99t8452696ec7ccc8f3@mail.gmail.com>
Subject: Re: [BUILD FAILURE 01/04] Next June 04:PPC64 randconfig 
	[drivers/staging/comedi/drivers.o]
From: Geert Uytterhoeven <geert@linux-m68k.org>
To: subrata@linux.vnet.ibm.com
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       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
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On Tue, Jun 9, 2009 at 20:34, Subrata Modak<subrata@linux.vnet.ibm.com> wrote:
> On Tue, 2009-06-09 at 13:50 +1000, Benjamin Herrenschmidt wrote:
>> 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.
>
> 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,

Your patch helps powerpc only. Compilation is still broken on most
other architectures.

>> 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.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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