linux-kernel - Re: [PATCH v3 1/2] of: create of

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Date:   Tue, 28 Feb 2023 20:05:58 -0600
From:   Frank Rowand <frowand.list@...il.com>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Clément Léger <clement.leger@...tlin.com>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lizhi Hou <lizhi.hou@...inx.com>,
        Allan Nielsen <allan.nielsen@...rochip.com>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Steen Hegelund <steen.hegelund@...rochip.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v3 1/2] of: create of_root if no dtb provided

On 2/27/23 11:17, Rob Herring wrote:
> On Thu, Feb 23, 2023 at 3:34 PM Frank Rowand <frowand.list@...il.com> wrote:
>>
>> When enabling CONFIG_OF on a platform where of_root is not populated by
>> firmware, we end up without a root node. In order to apply overlays and
>> create subnodes of the root node, we need one. Create this root node
>> by unflattening an empty builtin dtb.
>>
>> If firmware provides a flattened device tree (FDT) then the FDT is
>> unflattened via setup_arch().  Otherwise, setup_of() which is called
>> immediately after setup_arch(), and will create the default root node
>> if it does not exist.
> 
> Why do we need a hook after setup_arch() rather than an initcall?
> 
> Rob

It might work as an initcall today.  Maybe not in the future as other
initcalls are added.

But my main stream of thinking is that before the patch "we know" that
the device tree data structure exists when setup_arch() returns.
Adding setup_of() immediately after setup_arch() retains that
guarantee, but one line later in start_kernel().

I could have instead put the call to setup_of() into each architectures'
setup_arch(), but that would just be duplicating the same code for each
architecture, which did not seem like a good choice.

-Frank