From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de (cantor2.suse.de [195.135.220.15]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "mx2.suse.de", Issuer "CAcert Class 3 Root" (verified OK)) by ozlabs.org (Postfix) with ESMTP id F1E81DDDE9 for ; Thu, 13 Sep 2007 03:58:39 +1000 (EST) From: Neil Brown To: Nick Piggin Date: Wed, 12 Sep 2007 19:57:13 +0200 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Message-ID: <18152.10361.790323.494668@notabene.brown> Subject: Re: SYSFS: need a noncaching read In-Reply-To: message from Nick Piggin on Wednesday September 12 References: <1189503798.6674.46.camel@Zeus.EmbLux> <20070912053207.GH23573@pengutronix.de> <20070912100123.GA23182@kroah.com> <200709120519.39960.nickpiggin@yahoo.com.au> Cc: Detlev Zundel , Greg KH , linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org, Heiko Schocher List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wednesday September 12, nickpiggin@yahoo.com.au wrote: > 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) No. Looking in fs/sysfs/file.c, we notice the field "needs_read_fill" in struct sysfs_buffer. sysfs_read_file will only call fill_read_buffer (which calls the ->show routine) if this is 1; It is cleared by fill_read_buffer and set to 1: - at open - by fill_write_buffer (i.e. if you write to the file descriptor) - by sysfs_poll when an event was detected. So currently you cannot simply open a sysfs file an read multiple times. One option would be to call fill_read_buffer if *ppos == 0. I cannot see that being a problem in practice, but maybe there is a reason why it wasn't done that way. Another option might be to call fill_read_buffer also if buffer->event != atomic_read(&attr_sd->s_event) and require drivers to call sysfs_notify when they make a change that should be noticed. But I doubt that is really important. NeilBrown