From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from canuck.infradead.org (canuck.infradead.org [209.217.80.40]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTP id 8B118DDDF3 for ; Wed, 12 Sep 2007 20:01:35 +1000 (EST) Date: Wed, 12 Sep 2007 03:01:23 -0700 From: Greg KH To: Robert Schwebel Subject: Re: SYSFS: need a noncaching read Message-ID: <20070912100123.GA23182@kroah.com> References: <1189503798.6674.46.camel@Zeus.EmbLux> <20070912053207.GH23573@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20070912053207.GH23573@pengutronix.de> Cc: linuxppc-dev@ozlabs.org, Heiko Schocher , linux-kernel@vger.kernel.org, Detlev Zundel List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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