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From: David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>
To: Frank Rowand <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: "Geert Uytterhoeven"
	<geert-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.org>,
	"Rob Herring" <robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	"Devicetree Compiler"
	<devicetree-compiler-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"devicetree-spec-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<devicetree-spec-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"Jon Loeliger" <jdl-CYoMK+44s/E@public.gmane.org>,
	"devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org"
	<devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	"Pantelis Antoniou"
	<pantelis.antoniou-OWPKS81ov/FWk0Htik3J/w@public.gmane.org>,
	"Pantelis
	Antomarek.vasut-Re5JQEeQqe/2sr8fMPgRzw@public.gmane.org"
	<panto-wVdstyuyKrO8r51toPun2/C9HSW9iNxf@public.gmane.org>,
	"Grant Likely"
	<grant.likely-s3s/WqlpOiPyB63q8FvJNQ@public.gmane.org>,
	"Marek Vašut"
	<marek.vasut-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	"Tom Rini" <trini-OWPKS81ov/FWk0Htik3J/w@public.gmane.org>,
	"Kyle Evans" <kevans-h+KGxgPPiopAfugRpC6u6w@public.gmane.org>,
	"Alan Tull" <atull-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	"Michael Ellerman" <mpe-Gsx/Oe8HsFggBc27wqDAHg@public.gmane.org>
Subject: Re: [RFC] devicetree: new FDT format version
Date: Sat, 27 Jan 2018 20:00:23 +1100
Message-ID: <20180127090023.GB12900@umbus> (raw)
In-Reply-To: <83008da0-7383-ba2d-a239-e11ad7d1327d-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>


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On Fri, Jan 26, 2018 at 02:08:08PM -0800, Frank Rowand wrote:
> On 01/26/18 00:56, Geert Uytterhoeven wrote:
> > On Thu, Jan 25, 2018 at 1:29 PM, David Gibson
> > <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org> wrote:
> >> On Wed, Jan 24, 2018 at 04:22:15PM -0800, Frank Rowand wrote:
> >>> On 01/24/18 13:16, Frank Rowand wrote:
> >>>> What you say makes sense.  So I'll reverse myself and say that for
> >>>> Linux, we should update the FDT read code to scan all chained
> >>>> overlay FDT(s) as well as the base FDT.
> >>>
> >>> < snip >
> >>>
> >>> What I wrote was somewhat ambiguous.  What I meant by "FDT read
> >>> code" was functions that check for existence of nodes in the
> >>> FDT or read property values from the FDT.
> >>
> >> Oh.. not just FDT unflattening code.
> >>
> >> The trouble with this is that scanning for a specific node or property
> >> in a set of chained overlays is highly nontrivial.  Even if you set
> >> aside the arguably self-imposed design constraints in libfdt, you
> >> can't just do the same lookup in each fragment: along the way you need
> >> to resolve the path at which each fragment applies.  That in itself is
> >> non-trivial.  If you have overlays applying on top of other overlays,
> >> that could involve recursive lookups of things from previous overlays.
> >> It's spectacularly complicated and we have to do it on *every single*
> >> read operation.
> >>
> >> Either fully applying the overlay in flattened form, or (even
> > 
> > Applying the overlays in flattened form sounds like a good idea to me.
> 
> I'm having trouble envisioning what "apply an overlay in flattened form"
> means.

Uh.. exactly what fdt_overlay_apply() does right now.

> I'll provide a simple example to illustrate.
> 
>   - Base devicetree contains one non-root node which contains one property.
>   - Overlay devicetree adds one property to that same node.
> 
> The before and after dt_struct block of the FDTs would contain (vastly
> simplified, ignoring tags, not notating nesting of nodes, ignoring
> root node, etc):
> 
> (1) base FDT dt_struct:
>   [node1] [prop1 value length] [prop1 name offset] [prop1 value]
> 
> (2) overlay FDT dt_struct:
>   [fragment 1 node] [__overlay__ node (for node1)] [prop2 value length] [prop2 name offset] [prop2 value]
> 
> (3) resulting FDT after applying overlay to base:
> 
>   [node1] [prop1 value length] [prop1 name offset] [prop1 value] [prop2 value length] [prop2 name offset] [prop2 value]
> 
>   - dt_string block updated to add the string for prop2 name (if not already in block)
>   - prop2 name offset updated for offset in updated dt_string block
> 
> Is creating (3) from (1) and (2) what is meant by "apply an overlay
> in flattened form"?
> 
> Getting from (1) and (2) to (3) does not look trivial.  Am I missing
> something?

It's not trivial, but it's not awful.  Maxime Ripard already
implemented it for libfdt.

-- 
David Gibson			| 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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  parent reply index

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-22  8:09 Frank Rowand
     [not found] ` <b96829f9-2e8b-fdc5-5090-58591e2260cf-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-22  8:14   ` Frank Rowand
     [not found]     ` <ec68cfb3-4702-ba00-e926-3ad63b20b199-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-22 20:08       ` Frank Rowand
2018-01-22 20:08   ` Frank Rowand
     [not found]     ` <bc44f051-835d-1c8d-a928-be0fd4ef80b5-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-25  0:27       ` Frank Rowand
     [not found]         ` <b5a72db1-45b2-f21f-9afd-0991b288840e-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-27  8:48           ` David Gibson
2018-01-29  8:08             ` Frank Rowand
     [not found]               ` <2c352396-154e-ea75-c50b-0f2bfafd19da-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-29 10:56                 ` David Gibson
2018-01-30  1:29                   ` Frank Rowand
2018-01-22 21:01   ` Rob Herring
     [not found]     ` <CAL_JsqJR2y7bMw_-9TBAGWZ_kf7_sZo5qvqvRowJ8jiy=4G0Bg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-01-23 12:42       ` David Gibson
2018-01-23 21:17         ` Frank Rowand
     [not found]           ` <20328477-e511-e875-7dc4-253640f2219e-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-24 15:47             ` Rob Herring
     [not found]               ` <CAL_Jsq+fvFrGhqO0zbEUE_i23FkU=G4Z1-e0vnXHi4KbS2oK0A-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-01-24 21:16                 ` Frank Rowand
     [not found]                   ` <e1bec77d-4dac-200b-34be-23573bf738f0-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-24 22:27                     ` Alan Tull
2018-01-25  0:22                     ` Frank Rowand
     [not found]                       ` <e986435b-481b-2629-7600-10d9e21ac58e-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-25 12:29                         ` David Gibson
2018-01-25 20:01                           ` Frank Rowand
     [not found]                             ` <72d30756-a963-92c9-1838-3e3f80c57e39-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-29 18:32                               ` Grant Likely
     [not found]                                 ` <CACxGe6tUB3NhNhOtqzoJfThx=RE1_=TSDQz+wQUm=sdE5JirZQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-01-29 23:15                                   ` David Gibson
2018-01-26  8:56                           ` Geert Uytterhoeven
     [not found]                             ` <CAMuHMdV0jdj90uzqMx_wtvz=-KaagJG2_UQTm1DW3gzt6cNG6A-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2018-01-26  8:59                               ` Geert Uytterhoeven
2018-01-26 22:08                               ` Frank Rowand
     [not found]                                 ` <83008da0-7383-ba2d-a239-e11ad7d1327d-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-27  9:00                                   ` David Gibson [this message]
2018-01-27  8:56                               ` David Gibson
2018-01-25 23:11                     ` Frank Rowand
2018-01-25 12:22                 ` David Gibson
2018-01-25  9:14             ` Marek Vasut
     [not found]               ` <90983180-ae3b-5a31-9dc0-b62b978a0fba-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-25 12:37                 ` David Gibson
2018-01-27 20:30                   ` Marek Vasut
     [not found]                     ` <5a943937-3b59-514b-3939-df25daea5470-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2018-01-29  0:53                       ` David Gibson
2018-01-23 12:05   ` David Gibson
2018-01-23 21:28     ` Frank Rowand

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