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

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Message-ID: <20070912100123.GA23182@kroah.com>
Date:	Wed, 12 Sep 2007 03:01:23 -0700
From:	Greg KH <greg@...ah.com>
To:	Robert Schwebel <r.schwebel@...gutronix.de>
Cc:	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 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.

Not that this is a good thing to do for this kind of thing, as others
have already said.

thanks,

greg k-h
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