linux-kernel - Re: [PATCH 1/3] of: always populate a root node

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Message-ID: <20220517093729.214c33a3@fixe.home>
Date:   Tue, 17 May 2022 09:37:29 +0200
From:   Clément Léger <clement.leger@...tlin.com>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Rob Herring <robh@...nel.org>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Allan Nielsen <allan.nielsen@...rochip.com>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Steen Hegelund <steen.hegelund@...rochip.com>,
        Thomas Petazzoni <thomas.petazonni@...tlin.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Mark Brown <broonie@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Andrew Lunn <andrew@...n.ch>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 1/3] of: always populate a root node

Le Mon, 16 May 2022 23:11:03 -0400,
Frank Rowand <frowand.list@...il.com> a écrit :

> On 5/3/22 08:45, Rob Herring wrote:
> > On Wed, Apr 27, 2022 at 11:45:00AM +0200, Clément Léger 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. This commit creates an
> >> empty root node if not present.  
> > 
> > The existing unittest essentially does the same thing for running the 
> > tests on non-DT systems. It should be modified to use this support 
> > instead. Maybe that's just removing the unittest code that set of_root.
> > 
> > I expect Frank will have some comments.  
> 
> My preference would be for unflatten_and_copy_device_tree() to
> use a compiled in FDT that only contains a root node, in the
> case that no valid device tree is found (in other words,
> "if (!initial_boot_params)".

Ok, so basically, instead of creating the root node manually, you
expect a device-tree which contains the following to be builtin the
kernel and unflattened if needed:

/ {

};

Maybe "chosen" and "aliases" nodes should also be provided as empty
nodes since the unittest are creating them anyway and the core DT code
also uses them.

Thanks,

Clément

> 
> unflatten_and_copy_device_tree() calls unittest_unflatten_overlay_base()
> after unflattening the device tree passed into the booting kernel.  This
> step is needed for a specific portion of the unittests.
> 
> I'm still looking at the bigger picture of using overlays for the PCIe
> card, so more comments will be coming about that bigger picture.
> 
> -Frank
> 


-- 
Clément Léger,
Embedded Linux and Kernel engineer at Bootlin
https://bootlin.com