linux-kernel - Re: [PATCH V5 5/5] of: unittest: Statically apply overlays using fdtoverlay

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Date:   Fri, 22 Jan 2021 15:27:04 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Frank Rowand <frowand.list@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bill Mills <bill.mills@...aro.org>, anmar.oueja@...aro.org
Subject: Re: [PATCH V5 5/5] of: unittest: Statically apply overlays using
 fdtoverlay

On Fri, Jan 22, 2021 at 08:40:49AM +0530, Viresh Kumar wrote:
> On 22-01-21, 10:39, David Gibson wrote:
> > No, it definitely will not work in general.  It might kinda work in a
> > few trivial cases, but it absolutely will not do the neccessary
> > handling in some cases.
> > 
> > > I
> > > did inspect the output dtb (made by merging two overlays) using
> > > fdtdump and it looked okay.
> > 
> > Ok.. but if you're using these bizarre messed up "dtbs" that this test
> > code seems to be, I don't really trust that tells you much.
> 
> I only looked if the changes from the second overlay were present in
> the merge and they were. And so I assumed that it must have worked.
> 
> What about checking the base dtb for /plugin/; in fdtoverlay and fail
> the whole thing in case it is present ? I think it is possible for
> people to get confused otherwise, like I did.

/plugin/ doesn't exist in the dtb, only in the dts.  From the dtb
encoding point of view, there's no difference between a dtb and a
dtbo, a dtbo is just a dtb that follows some conventions for its
content.

If we were doing this from scratch, it would be better for dtbos to
have a different magic number from regular dtbs.  I think I actually
suggested that sometime in the past, but by the time that came up,
dtbos were already in pretty widespread use with the existing format.

-- 
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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