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

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Message-ID: <20070912113907.GA24087@kroah.com>
Date:	Wed, 12 Sep 2007 04:39:07 -0700
From:	Greg KH <greg@...ah.com>
To:	Heiko Schocher <hs@...x.de>
Cc:	Robert Schwebel <r.schwebel@...gutronix.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 01:13:32PM +0200, Heiko Schocher wrote:
> Hello Greg
> 
> Am Mittwoch, den 12.09.2007, 03:01 -0700 schrieb Greg KH:
> > 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.
> 
> No thats not true. I thought this too, but if I make a:
> 
> seek (fd, 0L, SEEK_SET);
> 
> in Userspace, there is no retrigger in the sysFS, my driver is *not*
> called again. So I made a own sysfs_seek function, which does retrigger
> the driver ...

Hm, are you sure?  Otherwise the poll() stuff would not work at all.

> Is this really wanted in the sysFS, that there is no way to retrigger a
> read?

Yes, use the sysfs poll/select stuff to do that.

And "sysfs" has no upper-case letters :)

thanks,

greg k-h
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