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

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Date:	Mon, 17 Sep 2007 07:22:28 +0200
From:	Tejun Heo <htejun@...il.com>
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

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.

There should be an intervening sysfs_notify() call from kernel side to
make sysfs re-populate its cache on read again.  sysfs bin files buffer
the result but don't cache the result but this again doesn't really fit
the usage case.

Thanks.

-- 
tejun

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