linux-kernel - Re: [PATCH] STAGING: Comedi: Build only on arches providing PAGE_KERNEL

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Date:	Fri, 24 Jun 2011 08:01:39 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Ralf Baechle <ralf@...ux-mips.org>
Cc:	Martyn Welch <martyn.welch@...com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
	David Howells <dhowells@...hat.com>,
	linux-m32r@...linux-m32r.org, linux-m32r-ja@...linux-m32r.org,
	linux-am33-list@...hat.com, Paul Mundt <lethal@...ux-sh.org>,
	linux-sh@...r.kernel.org, Chris Metcalf <cmetcalf@...era.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-arch@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] STAGING: Comedi: Build only on arches providing
 PAGE_KERNEL_NOCACHE

On Thu, 2011-06-23 at 13:02 +0100, Ralf Baechle wrote:
> On Thu, Jun 23, 2011 at 12:53:36PM +0100, Martyn Welch wrote:
> 
> > On 23/06/11 12:45, Ralf Baechle wrote:
> > > On architectures that don't define PAGE_KERNEL_NOCACHE, the Comedi driver turns
> > > into tragedy:
> > > 
> > >   CC [M]  drivers/staging/comedi/drivers.o
> > > drivers/staging/comedi/drivers.c: In function ‘comedi_buf_alloc’:
> > > drivers/staging/comedi/drivers.c:505:41: error: ‘PAGE_KERNEL_NOCACHE’ undeclared (first use in this function)
> > > drivers/staging/comedi/drivers.c:505:41: note: each undeclared identifier is rep orted only once for each function it appears in
> > > make[3]: *** [drivers/staging/comedi/drivers.o] Error 1
> > > 
> > > Restrict the driver to only those architectures that define PAGE_KERNEL_NOCACHE.
> > > 
> > > PAGE_KERNEL_NOCACHE is a kludge - some system architectures such as SGI IP27
> > > are even uable to offer uncached operation - at least in the way an unwitting
> > > driver might assume.  I haven't looked in details how the driver is using
> > > the area vmaped with PAGE_KERNEL_NOCACHE but maybe doing it XFS-style using
> > > cached memory and the flush_kernel_vmap_range / invalidate_kernel_vmap_range
> > > APIs in conjunction with the DMA API is a practical alternative.
> > > 
> > > Signed-off-by: Ralf Baechle <ralf@...ux-mips.org>
> > > 
> > >  drivers/staging/comedi/Kconfig |    1 +
> > >  1 files changed, 1 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/drivers/staging/comedi/Kconfig b/drivers/staging/comedi/Kconfig
> > > index 1502d80..bccdc12 100644
> > > --- a/drivers/staging/comedi/Kconfig
> > > +++ b/drivers/staging/comedi/Kconfig
> > > @@ -2,6 +2,7 @@ config COMEDI
> > >  	tristate "Data acquisition support (comedi)"
> > >  	default N
> > >  	depends on m
> > > +	depends on BROKEN || FRV || M32R || MN10300 || SUPERH || TILE || X86
> > 
> > I'm sure I got comedi to compile on a 32-bit PPC board not that long ago. Has
> > something changed, or is this just not an exhaustive list?

It went away. There's a proper API (pgprot_noncached), but it depends
what this is used for and it's likely to be broken anyways... what is
that driver trying to map non-cached ?

Cheers,
Ben.

> (Adding the PPC folks to cc.)
> 
> A "git grep -w PAGE_KERNEL_NOCACHE arch/powerpc/" doesn't find anything so
> I don't think the driver will build there.  I don't have a PPC toolchain
> to verify that.
> 
>   Ralf
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