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

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Date:	Fri, 1 Nov 2013 14:59:25 +1100
From:	NeilBrown <neilb@...e.de>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	devicetree@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>
Subject: Strange location and name for platform  devices when device-tree is
 used.

My ARM board has a collection of "platform devices".

When I build a kernel using a board file, these platform devices are named
with exactly the names I give them, and they appear in sysfs under
   /sys/devices/platform

all as you might expect.

When I build a kernel using device-tree (and trying to follow the established
patterns for the dts file),  these platform devices appear directly in
"/sys/devices" (not in the "platform" subdirectory) and  they don't have the
names I give them, but instead a sequential number is appended.

The former makes /sys/devices look very untidy.  It can presumably be fixed by
changing of_platform_populate() to replace a parent for 'NULL' with
'platform_bus' as the patch below demonstrates.  Is there any chance that is
correct? (It seems to work, but for all I know it might break something else).

The latter is caused by of_device_make_bus_id():

	/*
	 * No BusID, use the node name and add a globally incremented
	 * counter (and pray...)
	 */
	magic = atomic_add_return(1, &bus_no_reg_magic);
	dev_set_name(dev, "%s.%d", node->name, magic - 1);

This call to prayer dates back to:

commit 9309180f11f0107c9858a61a1ac2b04518a91080
Author: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Date:   Tue Nov 21 14:56:37 2006 +1100

    [POWERPC] powerpc: Workaround for of_platform without "reg" nor "dcr-reg"

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?

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.

Thanks,
NeilBrown

diff --git a/drivers/of/platform.c b/drivers/of/platform.c
index 9b439ac63d8e..af3ef3513cb0 100644
--- a/drivers/of/platform.c
+++ b/drivers/of/platform.c
@@ -476,6 +476,9 @@ int of_platform_populate(struct device_node *root,
 	struct device_node *child;
 	int rc = 0;

+	if (parent == NULL)
+		parent = &platform_bus;
+
 	root = root ? of_node_get(root) : of_find_node_by_path("/");
 	if (!root)
 		return -EINVAL;

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