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Subject: Re: [PATCH V5 5/5] of: unittest: Statically apply overlays using
 fdtoverlay
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     David Gibson <david@gibson.dropbear.id.au>,
        Rob Herring <robh+dt@kernel.org>,
        Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        Bill Mills <bill.mills@linaro.org>, anmar.oueja@linaro.org
References: <cover.1611124778.git.viresh.kumar@linaro.org>
 <696c137461be8ec4395c733c559c269bb4ad586e.1611124778.git.viresh.kumar@linaro.org>
 <20210121005145.GF5174@yekko.fritz.box>
 <7d6adfd9-da1e-d4ca-3a04-b192f0cf36b0@gmail.com>
 <20210121053426.4dw5oqz7qb4y7hvm@vireshk-i7>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <1cbafa2a-fe9e-04ec-35cc-d675a781a5b6@gmail.com>
Date:   Wed, 20 Jan 2021 23:45:23 -0600
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On 1/20/21 11:34 PM, Viresh Kumar wrote:
> On 20-01-21, 23:14, Frank Rowand wrote:
>> It is a convenient FDT to use because it provides the frame that the overlays
>> require to be applied.  It is fortunate that fdtoverlay does not reject the use
>> of an FDT with overlay metadata as the base blob.
> 
>> This is probably a good idea instead of depending on the leniency of fdtoverlay.
> 
> I believe fdtoverlay allows that intentionally, that would be required
> for the cases where we have a hierarchy of extension boards or
> overlays.
> 
> A platform can have a base dtb (with /plugin/;), then we can have an
> overlay (1) for an extension board (with /plugin/;) and then an
> overlay (2) for an extension board for the previous extension board.
> 
> In such a case overlay-(2) can't be applied directly to the base dtb
> as it may not find all the nodes it is trying to update. And so
> overlay-(2) needs to be applied to overlay-(1) and then the output of
> this can be applied to the base dtb.

I have only the most surface knowledge of fdtoverlay, mostly from
"fdtoverlay --help", but you can apply multiple overlays with a
single invocation of fdtoverlay.  My _assumption_ was that the
overlays would be applied in order, and after any given overlay
was applied, subsequent overlays could reference the previously
applied overlay.

Is my assumption incorrect?

> 
> This is very similar to what I tried with the intermediate.dtb
> earlier.
>