linux-kernel - Re: unstable /sys interface from hmon

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Date:   Mon, 30 Jan 2017 15:11:49 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Daniel Walker <danielwa@...co.com>
Cc:     "xe-kernel@...ernal.cisco.com" <xe-kernel@...ernal.cisco.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jean Delvare <khali@...ux-fr.org>, lm-sensors@...sensors.org,
        linux-kernel@...r.kernel.org
Subject: Re: unstable /sys interface from hmon

On Mon, Jan 30, 2017 at 12:37:14PM -0800, Daniel Walker wrote:
> Hi,
> 
> Cisco recently upgraded to kernel 4.4.. We have applications which use the /sys interfaces for some hmon drivers.
> We found that the following patches,
> 
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=bab2243ce1897865e31ea6d59b0478391f51812b
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=c3b7cddc70075d525db6b3068d8b2b9158eedc84
> 
> Altered the interface as seen from userspace. These patches changed the paths in the following way,
> 
> Before,
> /sys/bus/i2c/devices/14-0050/
> 
> After,
> /sys/bus/i2c/devices/14-0050/hwmon/hwmon7
> 

Actually, the stable link to use would be /sys/class/hwmon/hwmon7
in both cases, and to check if the name attribute resides in hwmon7
or in hwmon7/device (which is the real change in your case - it moved
from hwmon7/device to hwmon7, ie from the parent device to the hwmon
device).

> My impression was that /sys was suppose to be stable, and debugfs was unstable. I'm not sure what the current guidelines are
> for this, but we would request that hmon drivers maintain a stable interface from /sys. This change has cause upgrade issues
> for us which we would like to avoid in the future.
>
The key transition is the move from attaching the attributes to the parent
device (here: the i2c device) to the hwmon device. libsensors handles this
transition transparently; I assume you are not using it ?

> A patch to add in a compatibility layer would be welcome.
> 
A compatibility layer would have to link each attribute back to the parent
device. Interesting idea, actually. I'll check if that is possible.

Thanks,
Guenter