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

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Message-ID: <20131101160329.37f3707b@notabene.brown>
Date:	Fri, 1 Nov 2013 16:03:29 +1100
From:	NeilBrown <neilb@...e.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	devicetree@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>
Subject: Re: Strange location and name for platform  devices when
 device-tree is used.

On Fri, 01 Nov 2013 15:27:34 +1100 Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:

> On Fri, 2013-11-01 at 15:22 +1100, Benjamin Herrenschmidt wrote:
> > On Fri, 2013-11-01 at 14:59 +1100, NeilBrown wrote:
> > > and I wonder how relevant it still is in this context.  As platform devices
> > > are all in the root of the device-tree and hence are siblings, they must have
> > > unique names in the device-tree and  so the platform devices created from
> > > them will also have unique names -- won't they?
> > 
> > I agree about /sys/devices -> /sys/devices/platform, that makes more
> > sense
> > 
> > The problem with names is that we don't *know* that your devices are
> > at the root and unique. They don't have to be. I have platforms that
> > have several "chips" each containing all the same devices. They need to
> > be de-duped.
> > 
> > Maybe the right approach is to build the de-duplication in sysfs
> > itself ?

Do you mean we could allow multiple devices on the one bus to have the same
name, but get sysfs to notice and de-duplicate by mangling one name?  I don't
think I like that but I might have misunderstood.

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" ??

Hoping someone who understands the device model better than me will help.

> 
> BTW. How come you have devices at the root of the tree without "reg" ?
> It's fairly fishy ... 
> 
> The root of the tree is supposed to represent the processor address
> space, and has #address-cells/#size-cells set appropriately. Any MMIO
> mapped device shall thus have a "reg" property and a unit address.
> 
> Only "container" nodes (such as /cpus or /chosen) or virtual devices
> (such as a node used to representing the collection of bits & pieces
> that makes the audio infrastructure) and are thus not per-se MMIO mapped
> entities can ommit the "reg" properties.
> 
> In the case of pwm, it looks like there's another device providing a pwm
> capability, in which case your backlight would indeed be a "virtual
> device" (basically non-mmio device not hanging off any bus). Or it could
> have been represented as a child of pwm if that had been defined that
> way, I am not familiar with the pwm bindings.

The  'backlight' device is a virtual device.  It uses a 'pwm' device to
provide the variable brightness to the back light.

The 'pwm' device itself is virtual, making use of a 'dmtimer' to provide the
timing...  The timer device (timer11 in omap3.dtsi) has a 'reg' property.

A random example from current mainline is 
  arch/arm/boot/dts/am335x-evmsk.dts
which has 'backlight' as a virtual device compatible with pwm-backlight.
It also has  'gpio-keys', 'gpio-leds', and 'regulator-fixed' compatible
virtual devices.  They seem fairly common.

Thanks,
NeilBrown


> 
> Cheers,
> Ben.
> 
> > Cheers,
> > Ben.
> > 
> > > Any help understanding and/or fixing this discrepancy greatly appreciated.
> > > 
> > > The change of name is particularly annoying to me because one of my platform
> > > devices is a pwm_bl.c backlight.  With a boardfile I
> > > get /sys/class/pwm_backlight.  With devicetree the best I can get
> > > is /sys/class/pwm_backlight.23 (or similar).  It would be really nice to have
> > > a more stable and sensible name here.
> > 
> 


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