linux-kernel - Re: [RFC] driver-core: Remove dummy 'platform

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Message-ID: <CAPXgP12yK8LV1Qi7ykS-VbaQY-S5+bp4FX-1sOxxUTPG4UE56g@mail.gmail.com>
Date:	Fri, 23 Nov 2012 15:39:57 +0100
From:	Kay Sievers <kay@...y.org>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] driver-core: Remove dummy 'platform_bus'

On Thu, Nov 22, 2012 at 10:20 PM, Grant Likely
<grant.likely@...retlab.ca> wrote:
> On Thu, Nov 22, 2012 at 7:17 PM, Kay Sievers <kay@...y.org> wrote:
>> On Wed, Nov 21, 2012 at 3:52 PM, Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:

>>> If the devices don't show up under platform/ where are they going to be
>>> at now, virtual/ ?  That doesn't sound like a good plan, they should be
>>> somewhere "useful".
>>
>> Just a note to keep in mind: We usually need and want devices to have
>> a bus or class. Devices without a "subsystem" are invisible to udev,
>> and do not get proper coldplug support at bootup.
>
> Note: this patch is only about the "platform_bus" dummy device. It has
> nothing to do with platform_bus_type.

Ah, I see now.

Why do you want to remove the "platform_bus" fake-parent, entirely? I
understand and agree that drivers should not fiddle with that
directly. But I don't see a real reason why it should not be private
to the platform_bus_type. It's nothing really wrong with it, I guess.

I have on x86:
  coretemp.0 -> ../../../devices/platform/coretemp.0
  dock.0 -> ../../../devices/platform/dock.0
  dock.1 -> ../../../devices/platform/dock.1
  efifb.0 -> ../../../devices/platform/efifb.0
  i8042 -> ../../../devices/platform/i8042
  iTCO_wdt -> ../../../devices/pci0000:00/0000:00:1f.0/iTCO_wdt
  pcspkr -> ../../../devices/platform/pcspkr
  serial8250 -> ../../../devices/platform/serial8250
  thinkpad_acpi -> ../../../devices/platform/thinkpad_acpi
  thinkpad_hwmon -> ../../../devices/platform/thinkpad_hwmon

which look pretty reasonable in a "platform parent" instead of ending
up in virtual/.

We should probably remove the explicit assignment in the drivers like
your patch does, unexport "platform_bus" from the core, so nobody can
use it anymore in the future. The /sys/bus/platform/devices/ directory
can then be created on-demand only, with the first registration of a
device without a parent, we do that in other parts of the core
already.

Wouldn't that solve your problem and still do not touch any other
stuff visible in /sys?

Also, it seems there are devices without any subsystem in the list of
things your patch touches? That is really not how things should work.
All devices should have a bus or class, so userspace receives proper
events, and can enumerate them.
The /sys/devices/ hierarchy is not really meant to be used directly,
it can for some devices even change at runtime. It's not considered a
stable or reasonable predictable ABI, all lookups should start in the
flat device symlink lists of the class/ and bus/, and not in the
hierarchical tree in devices/.

Kay
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