Re: [PATCH v3 0/2] Add capability to append to property

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Dec 26, 2024 at 12:10:04PM +0100, Geert Uytterhoeven wrote:
> On Thu, Dec 26, 2024 at 7:54 AM David Gibson
> <david@gibson.dropbear.id.au> wrote:
> > On Fri, Dec 20, 2024 at 09:00:14PM +0530, Ayush Singh wrote:
> > > On 16/12/24 11:39, David Gibson wrote:
> > > > On Tue, Dec 10, 2024 at 04:34:17PM +0100, Andreas Gnau wrote:
> > > > > If we add /append-property/, why not add /prepend-property/ as well? This is
> > > > > not only "nice for consistency", but it would also enable solving a problem
> > > > > where SoC compatible strings from dtsi [1] need to be repeated in board dts
> > > > > [2], because one cannot prepend to an existing property.
> > > > >
> > > > > ```
> > > > > dts-v1/;
> > > > > / {
> > > > >   compatible = "soc-vendor,soc1234";
> > > > > };
> > > > >
> > > > > / {
> > > > >   /prepend-property/ compatible = "board-vendor,board-xyz";
> > > > > };
> > > > > ```
> > > > >
> > > > > What do you think?
> > > > So, this kind of demonstrates why I don't love /append-property/ as a
> > > > proposed syntax.  The way I'd prefer to do this, ideally, is to allow
> > > > properties to be described as expressions.  We already have integer
> > > > expressions that can be used in < > context, but I had intended to
> > > > extent to string and bytestring expressions - but I've never had the
> > > > time to implement that.
> > > >
> > > > Under that proposal, I'd expect appending to look something like:
> > > >     str-prop = /previous-value/, "1";
> > > >
> > > > Prepending,
> > > >     str-prop = "1", /previous-value/;
> > > >
> > > > .. and you could do both at once in the obvious way.
> > > >
> > > > The downside, of course, is that this is a much more complicated
> > > > proposal to implement.  Parsing that syntax isn't too hard, but I
> > > > think doing it sensibly will need some structural changes in order to
> > > > evaluate property values as expressions, rather than simply
> > > > constructing the properties directly left to right.  In particular the
> > > > interactions between expression syntax and within-property labels (and
> > > > other markers) could be fiddly to get right.
> > >
> > > Do you wish to allow `/previous-value/` to be repeated in the same property?
> > > Something like this:
> > >
> > >     str-prop = "1", /previous-value/, "2", /previous-value/, "3";
> >
> > Yes, I'd expect that to work.  Note that I don't particularly like the
> > name "/previous-value/" - that was just an example to demonstrate what
> > I had in mind.  If anyone can think of a better or more succinct name
> > for this, please suggest it.
> 
> Something like "/./"? (dot is also used in assembler for the current address)
> So e.g.
> 
>     str-prop = "1", /./, "2", /./, "3";

/./ might work.

> Or would that be too short? "/orig/"?

It's not too short but I think "/orig/" is a bad idea. In cases where
the value of a property is modified more than twice, this would refer
to the most recent previous value, not the "original" / first value
assigned.  "/prev/" might work, though.

-- 
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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