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From:   Adam Ford <aford173@gmail.com>
Date:   Fri, 24 Jan 2020 09:54:10 -0600
Message-ID: <CAHCN7xKtfKVQeaAg9sjvc3cHjLoAqAX6F-JyvkG5u23w1OG8CA@mail.gmail.com>
Subject: Re: [PATCH] soc: imx: Makefile: only build soc-imx8 when CONFIG_ARM64
To:     Arnd Bergmann <arnd@arndb.de>
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>,
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        <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 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:
> > > Subject: Re: [PATCH] soc: imx: Makefile: only build soc-imx8 when CONFIG_ARM64
> > > On Tue, Jan 14, 2020 at 9:32 AM Peng Fan <peng.fan@nxp.com> wrote:
> > > > > Subject: Re: [PATCH] soc: imx: Makefile: only build soc-imx8 when
> > > >
> > > > There is no SOC_IMX8 currently. Need to introduce one in
> > > > arch/arm64/Kconfig.platforms. But I not see other vendors introduce
> > > > options like SOC_XX. Is this the right direction to add one in
> > > > Kconfig.platforms?
> > >
> > > I think it would be more consistent with the other platforms to have a symbol
> > > in drivers/soc/imx/Kconfig to control whether we build that driver.
> >
> > Ok, I'll add Kconfig entry in drivers/soc/imx/Kconfig for various i.MX SoCs.
>
> I was thinking of one entry for this driver.
>
> > > For some SoCs, we also allow running 32-bit kernels, so it would not be wrong
> > > to allow enabling the symbol on 32-bit ARM as well, but this is probably
> > > something where you want to consider the bigger picture to see if you want
> > > to support that configuration or not.
> >
> > Does the current upstream kernel support 32bit kernels on ARM64 platforms
> > without vendor specific stuff. I recalled that several years ago, NXP people
> > tried to upstream 32bit kernel support, but rejected by you.
>
> We have at least some Broadcom SoCs that are supported this way. As
> long as you can use the same dtb file on a regular multi_v7_defconfig
> I see no problem with doing this.
>
> What I would like to avoid though are ports that require extra code in
> arch/arm/mach-* that is not needed for the 64-bit target, or ports to
> 64-bit hardware that only run in 32-bit mode.
>
> > So Is there any plan to support 32bit kernel on AARCH64 in upstream
> > kernel?
> > Or any suggestions?
>
> I don't think there should be 32-bit kernel running in aarch64-ilp32
> mode. This was discussed way back when the aarch64-ilp32 user
> space patches first appeared.
>
> Generally speaking you are usually better off running an aarch64
> kernel using aarch32 user space, but there may be reasons for
> running an ARMv8 aarch32 kernel on the same hardware and there
> is no technical reason why this shouldn't work for a clean port.
>
> We never really supported ARMv8-aarch32 in arch/arm/ as a
> separate target, but usually building an ARMv7 kernel is close
> enough to ARMv8-aarch32 that things just work. If you would
> like to help out making ARMv7VE and ARMv8-aarch64 proper
> targets for arch/arm/, let me know and we can discuss what parts
> are missing.

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?

adam

>
>      Arnd
>
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