linux-kernel - Re: [RFC PATCH 32/77] dtc-parser: Introduce last_header

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Message-ID: <aXA8AOuucE5JdJCW@zatzit>
Date: Wed, 21 Jan 2026 13:37:52 +1100
From: David Gibson <david@...son.dropbear.id.au>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Ayush Singh <ayush@...gleboard.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	devicetree-compiler@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree-spec@...r.kernel.org,
	Hui Pu <hui.pu@...ealthcare.com>,
	Ian Ray <ian.ray@...ealthcare.com>,
	Luca Ceresoli <luca.ceresoli@...tlin.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [RFC PATCH 32/77] dtc-parser: Introduce last_header_flags

On Mon, Jan 19, 2026 at 03:11:45PM +0100, Herve Codina wrote:
> On Thu, 15 Jan 2026 17:31:24 +1100
> David Gibson <david@...son.dropbear.id.au> wrote:
> 
> > On Mon, Jan 12, 2026 at 03:19:22PM +0100, Herve Codina wrote:
> > > The parser needs to get header flags value in different places.
> > > 
> > > It relies on the fact that the rule used to parse the dts file is always
> > >   headers memreserves devicetree
> > > 
> > > With that only rule to parse the file, it uses '$<flags>-1' construct to
> > > get the flags value.
> > > 
> > > With the future introduction of import symbols parsing, this rule will
> > > change and the parser couldn't rely anymore on '$<flags>-1' to get flags
> > > value. Indeed, import symbols parsing will add a new optional symbol in
> > > this rule leading to two possible rules (with and without the new
> > > symbol) to parse the source file.
> > > 
> > > Introduce the last_header_flags variable to explicitly keep track of
> > > flags while also being agnostic of the rule structure and use this new
> > > variable instead of '$<flags>-1'.  
> > 
> > I'm not sure this approach is safe: I'm not sure bison guarantees that
> > semantic rules will always be executed in the same order, so using
> > global variables is risky.
> 
> if rules were not executed in the same order '$<flags>-1' construct would
> not work.

Uhh.. I'm pretty sure $<flags>-1 only depends on the parse tree
structure, not the order in which the actual semantic rule code
fragments run.

> The problem is not the order. I don't think the order will change. The
> problem is related to the number of items on stack.
> 
> With import symbols, that the stack will be:
>   header memreserved devicetree
> or
>   header memreserved importsyms devicetree
> 
> Using '$-1' will no more be possible. Indeed, '$-1' from the devicetree
> rule will reference 'header' in one case and 'memreserved' in the other
> case.
> 
> Without a global variable, I don't know how to reference 'header' (or flags
> value) in all cases.
> 
> Any better ideas are welcome.

Ok, I'll try to look into it and see what I can come up with.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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