linux-kernel - Re: userspace regression with hwmon

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Message-ID: <20170627122300.GA14227@sudip-tp>
Date:   Tue, 27 Jun 2017 13:23:00 +0100
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Jean Delvare <jdelvare@...e.de>, linux-kernel@...r.kernel.org,
        Fenghua Yu <fenghua.yu@...el.com>, linux-hwmon@...r.kernel.org
Subject: Re: userspace regression with hwmon

On Mon, Jun 26, 2017 at 10:06:11AM -0700, Guenter Roeck wrote:
> On Mon, Jun 26, 2017 at 03:33:20PM +0100, Sudip Mukherjee wrote:
> > Hi Guenter, Jean,
> > 
> > The patch d72d19c26c41 ("hwmon: (coretemp) Convert to use
> > devm_hwmon_device_register_with_groups") has changed the location of the
> > sysfs nodes from /sys/devices/platform/coretemp.0/ to
> > /sys/devices/platform/coretemp.0/hwmon/hwmon0/ and that has broken
> > some of our usespace applications when we have updated the kernel from
> > v3.8 to v4.4. For now I am reverting that said patch in our production
> > kernel (based on v4.4) but that is not an elegant solution.
> > Do you have any other reports like this? I am sure changing the paths
> > upstream will now break some other userspace. So, what can be an elegant
> > solution to our problem? or do we always need to carry an out-of-tree
> > patch for our production kernel?
> > 
> 
> The problem only occurs if attributes are accessed using the raw path,
> and if user space does not take into account that attributes may reside
> in the hwmon directory (/sys/class/hwmon/hwmonX) _or_ in the device
> directory (/sys/class/hwmon/hwmonX/device). There is no guarantee that
> the raw path doesn't change from release to release; that is what the
> symlinks are for. Actually, I have seen the raw platform device path
> change over time, so I am surprised that this never affected you.

Well, using the raw platform device path was a bad design decision.
But I still see a difference. In v3.8 all the temp* nodes were in
/sys/class/hwmon/hwmon0/device/ but in v4.4 I can see that they are in
/sys/class/hwmon/hwmon0/. So if we do modify the code, then we still
need to have two versions of userspace code based on the kernel.

--
Regards
Sudip