linux-kernel - Re: [PATCH V7 1/3] kbuild: Add generic rule to apply fdtoverlay

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Date:   Fri, 12 Feb 2021 10:11:23 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Anmar Oueja <anmar.oueja@...aro.org>,
        Bill Mills <bill.mills@...aro.org>,
        David Gibson <david@...son.dropbear.id.au>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Michal Simek <michal.simek@...inx.com>
Subject: Re: [PATCH V7 1/3] kbuild: Add generic rule to apply fdtoverlay

On 12-02-21, 12:07, Masahiro Yamada wrote:
> BTW, I do not know how to use overlay.
> Do we apply overlay in the build time?

Ideally it can be applied at both build time and runtime, but we
haven't allowed the runtime way until now in kernel. This patchset is
all about applying it at build time.

> If so, I do not know what the benefit of overlay is.
> Or is this just for build testing?

For now the main benefit of using them is that we can keep stuff in
separate files without including each other. For example a primary
board may or may not have an extension board connected to it.

Without overlays we will have this many dtbs for this simple case:
1. primary.dtb
2. extension.dtb
3. primary-includes-extension.dtb

With overlays we will have the first two. Now the same extension can
be applied to lots of boards and multiple extensions can be applied to
the same primary board. This just complicates the process of managing
dtbs.

> I just thought this was done in the boot time,
> for example, in U-Boot or something.

Yes, bootloader can do it as well.

-- 
viresh