Subject: SYSFS: need a noncaching read
From: Heiko Schocher <hs@denx.de>
Reply-To: hs@denx.de
To: linux-kernel@vger.kernel.org
Cc: Detlev Zundel <dzu@denx.de>, linuxppc-dev@ozlabs.org
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Organization: DENX
Date: Tue, 11 Sep 2007 11:43:17 +0200
Message-Id: <1189503798.6674.46.camel@Zeus.EmbLux>
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Sender: linux-kernel-owner@vger.kernel.org
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Hello,

I have developed a device driver and use the sysFS to export some
registers to 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 ... :-(

Is there a way to retrigger the reads (in that way, that the sysFS
rereads the values from the driver), without closing and opening the
sysFS Files? Or must I better use the ioctl () Driver-interface for
exporting these registers?

I am asking this, because I must read every 10 ms 2 registers, so
doing a open/read/close for reading one registers is a little bit too
much overhead.

I made a sysFS seek function, which retriggers the read, and that works
fine, but I have again 2 syscalls, whats also is not optimal.

Or can we make a open () with a (new?)Flag, that informs the sysFS to
always reread the values from the underlying driver?

Or a new flag in the "struct attribute_group" in include/linux/sysfs.h,
which let the sysfs rereading the values?

suggestions are welcome

thanks
Heiko
-- 
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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