linux-kernel - Re: [PATCH] UIO: only call pgprot

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Date:	Wed, 5 Nov 2008 19:27:44 +0100
From:	"Hans J. Koch" <hjk@...utronix.de>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Greg KH <gregkh@...e.de>, "Hans J. Koch" <hjk@...utronix.de>,
	Mike Frysinger <vapier@...too.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] UIO: only call pgprot_noncached if defined

On Wed, Nov 05, 2008 at 12:33:37PM -0500, Mike Frysinger wrote:
> On Wed, Nov 5, 2008 at 11:33, Greg KH wrote:
> > On Wed, Nov 05, 2008 at 12:36:11PM +0100, Hans J. Koch wrote:
> >> There seem to be archs that cannot easily implement a sensible
> >> pgprot_noncached() function, so we should merge this patch. UIO doesn't
> >> compile on these archs right now.
> >
> > No, we should fix those arches to have that function at least NULLed
> > out.  Isn't there only one, Blackfin?  Putting #ifdefs in .c files is
> > not something we really want to do if at all possible.
> 
> that was my question.  this function isnt documented.  if the hardware
> doesnt support it, is the right thing really for the arch to lie to
> drivers and not actually give back cached settings even though it
> asked for non-cached ?

Well, if there's no possibility to map device memory in a non-cached
way, it's very likely you don't want to use UIO at all. If there's a few
seconds between a write and the value actually appearing in the hardware
register, you will certainly don't see the results you expect.

If Blackfin can't mmap non-cached memory, we should make UIO depend on
!BLACKFIN.

Hans
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