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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 13 Mar 2020 11:23:34 +0100
Message-ID: <CAK8P3a0r1stgYw2DGtsHpMWdBN7GM9miAsUo20NaJxwasQy4iA@mail.gmail.com>
Subject: Re: [PATCH] ARM: dts: Makefile: build arm64 device tree
To:     Peng Fan <peng.fan@nxp.com>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Shawn Guo <shawnguo@kernel.org>,
        DTML <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        NXP Linux Team <linux-imx@nxp.com>
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On Fri, Mar 13, 2020 at 4:38 AM <peng.fan@nxp.com> wrote:
>
> From: Peng Fan <peng.fan@nxp.com>
>
> To support aarch32 mode linux on aarch64 hardware, we need
> build the device tree, so include the arm64 device tree path.
>
> Signed-off-by: Peng Fan <peng.fan@nxp.com>
> ---

There are a few other platforms with similar requirements, in
particular bcm2837,
so maybe try doing it the same way they do, see
arch/arm64/boot/dts/broadcom/bcm2837-rpi-3-b.dts

> V1:
>  This is just the device tree part. Besides this,
>  I am not sure whether need to create a standalone defconfig under arm32
>  for aarch32 mode linux on aarch64 hardware, or use multi_v7_defconfig.
>  multi_v7_defconfig should be ok, need to include LPAE config.

I'd rather not have a standalone defconfig for it, given that we have a
single defconfig for all armv6/armv7/armv7hf i.mx machines.

There was a suggestion to use a fragment for enabling an LPAE
multi_v7_defconfig recently, which I think is still under discussion but
should also help here, both with imx_v6_v7_defconfig and multi_v7_defconfig.

Can you remind us why this platform needs LPAE? Is it only needed to
support more than 4GB of RAM, or something else on top of that?
Note that users that actually have 4GB or more on i.mx8 should
really run a 64-bit kernel anyway, even if they prefer using 32-bit user
space.

Turning on LPAE not only disables imx3 and imx5 but also the Cortex-A9
based imx6 variants.

      Arnd