Subject: Re: are device names part of sysfs ABI? (was Re: devicename part of LEDs under ethernet MAC / PHY)

From: Marek Behún <[email-masked]>
Date: Mon, 4 Oct 2021 09:04:38 +0200

Hi Greg,

On Mon, 4 Oct 2021 08:37:37 +0200
Greg Kroah-Hartman <[email-masked]> wrote:

> On Sun, Oct 03, 2021 at 10:53:38PM +0200, Marek Behún wrote:
> > Hello Greg,
> > 
> > could you give your opinion on this discussion?  
> 
> What discussion?  Top posting ruins that :(

Sorry, the discussion is here
https://lore.kernel.org/linux-leds/20211001144053.3952474a@thinkpad/T/
But the basic question is below, so you don't need to read the
discussion.

> > Are device names (as returned by dev_name() function) also part of
> > sysfs ABI? Should these names be stable across reboots / kernel
> > upgrades?  
> 
> Stable in what exact way?

Example:
- Board has an ethernet PHYs that is described in DT, and therefore
  has stable sysfs path (derived from DT path), something like
    /sys/devices/.../mdio_bus/f1072004.mdio-mii/f1072004.mdio-mii:01

- The PHY has a subnode describing a LED.
  The LED subsystem has a different naming scheme (it uses DT node name
  as a last resort). When everything is okay, the dev_name() of the LED
  will be something like
    ethphy42:green:link

- Now suppose that the PHY driver is unloaded and loaded again. The PHY
  sysfs path is unchanged, but the LED will now be named
    ethphy43:green:link

Is this OK?

> Numbering of devices (where a dynamic value is part of a name, like the
> "42" in "usb42"), is never guaranteed to be stable, but the non-number
> part of the name (like "usb" is in "usb42") is stable, as that is what
> you have properly documented in the Documentation/ABI/ files defining
> the bus and class devices, right?

It does make sense for removable devices like USB. What I am asking
is whether it is also OK for devices that have stable DT nodes.

Marek