linux-kernel - Re: [PATCH 4/6] of: Create of_root if no dtb provided by firmware

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Date: Tue, 16 Jan 2024 17:18:15 -0800
From: Stephen Boyd <sboyd@...nel.org>
To: Rob Herring <robh@...nel.org>
Cc: Frank Rowand <frowand.list@...il.com>, linux-kernel@...r.kernel.org, patches@...ts.linux.dev, linux-um@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, kunit-dev@...glegroups.com, linux-kselftest@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 4/6] of: Create of_root if no dtb provided by firmware

Quoting Rob Herring (2024-01-15 12:32:30)
> On Fri, Jan 12, 2024 at 12:07:47PM -0800, Stephen Boyd wrote:
> > diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
> > index da9826accb1b..9628e48baa15 100644
> > --- a/drivers/of/Kconfig
> > +++ b/drivers/of/Kconfig
> > @@ -54,9 +54,14 @@ config OF_FLATTREE
> >       select CRC32
> >  
> >  config OF_EARLY_FLATTREE
> > -     bool
> > +     bool "Functions for accessing Flat Devicetree (FDT) early in boot"
> 
> I think we could instead just get rid of this kconfig option. Or 
> always enable with CONFIG_OF (except on Sparc). The only cost of 
> enabling it is init section functions which get freed anyways.

Getting rid of it is a more massive change. It can be the default and
kept hidden instead? If it can't be selected on Sparc then it should be
hidden there anyway.

> 
> >       select DMA_DECLARE_COHERENT if HAS_DMA && HAS_IOMEM
> >       select OF_FLATTREE
> > +     help
> > +       Normally selected by platforms that process an FDT that has been
> > +       passed to the kernel by the bootloader.  If the bootloader does not
> > +       pass an FDT to the kernel and you need an empty devicetree that
> > +       contains only a root node to exist, then say Y here.
> >  
> >  config OF_PROMTREE
> >       bool
[...]
> > @@ -195,6 +191,17 @@ static inline int of_node_check_flag(const struct device_node *n, unsigned long
> >       return test_bit(flag, &n->_flags);
> >  }
> >  
> > +/**
> > + * of_have_populated_dt() - Has DT been populated by bootloader
> > + *
> > + * Return: True if a DTB has been populated by the bootloader and it isn't the
> > + * empty builtin one. False otherwise.
> > + */
> > +static inline bool of_have_populated_dt(void)
> > +{
> > +     return of_root != NULL && !of_node_check_flag(of_root, OF_EMPTY_ROOT);
> 
> Just a side comment, but I think many/all callers of this function could 
> just be removed.
> 
> I don't love new flags. Another possible way to handle this would be 
> checking for "compatible" being present in the root node. I guess this 
> is fine as-is for now at least.

Ok. I can add a check for a compatible property. That's probably better
anyway. Should there be a compatible property there to signal that this
DT isn't compatible with anything? I worry about DT overlays injecting a
compatible string into the root node, but maybe that is already
prevented.