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

[Grant Likely]

On Sat, 2 Nov 2013 10:45:05 +1100, NeilBrown <neilb@xxxxxxx> wrote:
> On Sat, 02 Nov 2013 10:10:25 +1100 Benjamin Herrenschmidt
> <benh@xxxxxxxxxxxxxxxxxxx> 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?

The problem with a property in the node in this case is that there is no
guarantee of uniqueness. An alternative however would be to look for an
alias in /aliases. Those are unique.

g.
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