linux-kernel - DT: of_platform_populate Vs of_platform_bus

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Date:	Wed, 31 Oct 2012 14:54:59 +0000
From:	Srinivas KANDAGATLA <srinivas.kandagatla@...com>
To:	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <robherring2@...il.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: DT: of_platform_populate Vs of_platform_bus_probe.

Hi All, 
I have few queries on of_platform_populate and of_platform_bus_probe functions.

Use-case is, I want to explicitly register platform devices from some nodes at post-core or late-init level(like child@1).
And I don't want of_platform_populate to register platform devices for that node again, so I pass "simple-bus" in match-table.
Problem is that exiting code for of_platform_populate probes it.

Looking at the function documentation, which states
of_platform_bus_probe will only create children of the root which are
selected by the @matches argument.

of_platform_populate walks the device tree and creates devices from
nodes.  It differs in that it follows the modern convention of requiring
all device nodes to have a 'compatible' property, and it is suitable for
creating devices which are children of the root node.

Lets say If we call of_platform_populate(NULL, match_table, NULL, NULL)
on a device trees like the below with
struct of_device_id match_table[] = {
    { .compatible = "simple-bus", }
    {}
};

parent@0{
    compatible    = "xxx,parent1", "simple-bus";
    ...
    child@0 {
        compatible    = "xxx,child0", "simple-bus";
        ...
    };
    child@1 {
        compatible    = "xxx,child1";
        ...
    };
    child@2 {
        compatible    = "xxx,child2", "simple-bus";
        ...
    };
};

of_platform_bus_probe would create platform-devices for parent@0,
child@0 and child@2
where as
of_platform_populate would create platform-devices for parent@0,
child@0, child@1 and child@2 nodes.

So the question is
why do we need to have @matches argument to of_platform_populate in the
first place, if it creates all the devices by walking the dt nodes?

It is bit confusing, As some platforms use of_platform_populate(NULL,
of_default_bus_match_table, NULL, NULL) assuming that only matching
nodes will end up having platform device.
Also
some platforms use of_platform_bus_probe(NULL, match_table, NULL), 
where match table is of_default_bus_match_table.

Am not 100% sure what is the right solution, but I think lot of platforms would want behavior like of_platform_bus_probe which takes lookups aswell.
If the suggestion is to use of_platform_bus_probe, Which I can't use as It does not take Auxdata(lookups).

Thanks,
srini

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