Re: Strange location and name for platform devices when device-tree is used.

[NeilBrown <neilb-l3A5Bk7waGM@public.gmane.org>]

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On Sat, 02 Nov 2013 10:10:25 +1100 Benjamin Herrenschmidt
<benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org> wrote:

> On Fri, 2013-11-01 at 13:47 -0700, Greg Kroah-Hartman wrote:
> 
> > > > On my device I seem to have some platform devices registered through
> > > > device-tree, and some registered through platform_device_add (e.g.
> > > > 'alarmtimer').  Guaranteeing they remain disjoint sets if the kernel is
> > > > allowed to evolve independently of the devicetree might be tricky....
> > > > Maybe we need "/sys/devices/platform" and "/sys/devices/dt_platform" ??
> > > 
> > > No, I think device-tree created platform devices should go
> > > to /sys/devices/platform like the "classic" ones.
> > > 
> > > The problem is really how to deal with potential name duplication. We
> > > could try to register, if we get -EEXIST (assuming sysfs returns the
> > > right stuff), try again with ".1" etc...
> > 
> > How can there be device name collisions?  All platform devices _should_
> > be named uniquely, if not, you have bigger problems...
> 
> The problem is how to create a unique name for a platform device created
> from a device-tree node.
> 
> Device tree nodes aren't necessarily uniquely named. They are unique
> under a given parent but that hierarchy isn't preserved when creating
> corresponding platform devices (and it would be very tricky to do so).
> 
> Currently, we simply append a number to the name when creating them,
> which is obtained from a global counter.
> 
> Neil is unhappy about that because on his specific hardware, the device
> has a unique name and thus we introduce a naming difference between
> device-tree usage and old-style "hard coded" board file usage.

It occurs to me that a different approach could solve my problem.

My problem stems from the fact that the name of the device on the
platform-bus is used as the name of the device in the "backlight" class.

As Greg writes elsewhere, depending on names with /sys/devices is not
supported - we need to accept that bus-names might change.
However names in class devices tend to be a lot more stable.
Several devices allow these to be explicitly set.
 leds have 'label'
 regulators has "regulator-name"
 gpio-keys has 'label'.

I could just teach pwm_bl to allow a 'label' property which would be used in
place of the platform-bus device name when creating the class/backlight
device.

The maxim "you cannot trust names to remain stable in /sys/devices" can
justify both the movement of platform devices into /sys/devices/platform, and
the use of "label" rather than the device-name for creating the class device.

Does that sound convincing?

Thanks,
NeilBrown

> 
> It would be nice if we could do something that only appends the "global
> number" at the end of the name if the name isn't already unique. Thus my
> proposal of trying first with the base name, and trying again if that
> returns -EEXIST in some kind of loop.
> 
> Do you have a better idea ?
> 
> Cheers,
> Ben.
> 


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