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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 24 Jan 2020 17:44:30 +0100
Message-ID: <CAK8P3a19FPc_=RK3_NthZEhryx6t1PmFvj2h_Gzo2M2rJEehhg@mail.gmail.com>
Subject: Re: [PATCH] soc: imx: Makefile: only build soc-imx8 when CONFIG_ARM64
To:     Adam Ford <aford173@gmail.com>
Cc:     Peng Fan <peng.fan@nxp.com>, Abel Vesa <abel.vesa@nxp.com>,
        Anson Huang <anson.huang@nxp.com>,
        "shawnguo@kernel.org" <shawnguo@kernel.org>,
        "s.hauer@pengutronix.de" <s.hauer@pengutronix.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        dl-linux-imx <linux-imx@nxp.com>,
        "kernel@pengutronix.de" <kernel@pengutronix.de>,
        =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        Leonard Crestez <leonard.crestez@nxp.com>,
        "festevam@gmail.com" <festevam@gmail.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
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On Fri, Jan 24, 2020 at 4:54 PM Adam Ford <aford173@gmail.com> wrote:
> On Wed, Jan 15, 2020 at 4:39 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > On Wed, Jan 15, 2020 at 3:38 AM Peng Fan <peng.fan@nxp.com> wrote:
>
> I would be interested in learning more about running the i.MX8M in
> 32-bit mode.  I'm looking at running Linux on a device with < 1GB of
> RAM, so having 32-bit pointers and instructions would be preferred.
> My preference would be to run as ARMv7 for migration purposes, but I'm
> open to alternatives.
>
> Does anyone have any suggestions on where I might find some generic
> stuff for either iMX8M or generic ARMv8 docs for doing something like
> this?

By far the easiest way is to just run a 64-bit kernel and 32-bit user space,
which should give you 90% of the space savings. This requires no further
work, just pick the 32-bit distro of your choice.

The get the last few percent of memory savings and run a 64-bit kernel,
at least a bit of porting effort should be needed. In principle you should
be able to pass the dtb file for your machine to a 32-bit kernel, but
all drivers have to be there and you need to adjust a few dependencies
to allow 32-bit builds such as

 config CLK_IMX8MM
         bool "IMX8MM CCM Clock Driver"
-        depends on ARCH_MXC && ARM64
+       depends on ARCH_MXC && ARM64
         help
             Build the driver for i.MX8MM CCM Clock Driver

and then there will likely be a few bugs.

      Arnd