linux-kernel - Re: DT: of_platform_populate Vs of_platform_bus

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Message-ID: <5091420D.8050603@gmail.com>
Date:	Wed, 31 Oct 2012 10:21:49 -0500
From:	Rob Herring <robherring2@...il.com>
To:	srinivas.kandagatla@...com
CC:	Grant Likely <grant.likely@...retlab.ca>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: DT: of_platform_populate Vs of_platform_bus_probe.

On 10/31/2012 09:54 AM, Srinivas KANDAGATLA wrote:
> 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";

Well simple-bus here is generally not right unless you want to create
devices for children of child@0.

>         ...
>     };
>     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?

The matches is for matching buses, not child devices. It is assumed you
want to create devices for all 1st level children.

> 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.

There should be no new users of of_platform_bus_probe. It is there for
historical reasons on PPC.

> 
> 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).

of_platform_populate is what you should use.

Rob
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