linux-kernel - Re: SYSFS: need a noncaching read

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Message-Id: <200709120519.39960.nickpiggin@yahoo.com.au>
Date:	Wed, 12 Sep 2007 05:19:39 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Greg KH <greg@...ah.com>
Cc:	Robert Schwebel <r.schwebel@...gutronix.de>,
	Heiko Schocher <hs@...x.de>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, Detlev Zundel <dzu@...x.de>
Subject: Re: SYSFS: need a noncaching read

On Wednesday 12 September 2007 20:01, Greg KH wrote:
> On Wed, Sep 12, 2007 at 07:32:07AM +0200, Robert Schwebel wrote:
> > On Tue, Sep 11, 2007 at 11:43:17AM +0200, Heiko Schocher wrote:
> > > I have developed a device driver and use the sysFS to export some
> > > registers to userspace.
> >
> > Uuuh, uggly. Don't do that. Device drivers are there to abstract things,
> > not to play around with registers from userspace.
> >
> > > I opened the sysFS File for one register and did some reads from this
> > > File, but I alwas becoming the same value from the register, whats not
> > > OK, because they are changing. So I found out that the sysFS caches
> > > the reads ... :-(
> >
> > Yes, it does. What you can do is close()ing the file handle between
> > accesses, which makes it work but is slow.
>
> Do an lseek back to 0 and then re-read, you will get called in your
> driver again.

Can you do a pread with offset 0 to avoid the two syscalls? (which some
people seem to be concerned about)
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