Re: userspace regression with hwmon

[Guenter Roeck]

On 06/27/2017 05:23 AM, Sudip Mukherjee wrote:
> 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.

Your code should check for the 'name' attribute in both locations.

Guenter