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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Wed, 3 Feb 2021 08:50:02 +0100
Message-ID: <CAK8P3a1ZLuDTKY6YbPrcM338K4vmAgESeBiNwWN7VaWj2PtKFg@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] arch/arm/configs: Enable VMSPLIT_2G in imx_v6_v7_defconfig
To:     Alistair Francis <alistair23@gmail.com>
Cc:     Shawn Guo <shawnguo@kernel.org>,
        Alistair Francis <alistair@alistair23.me>,
        Arnd Bergmann <arnd@arndb.de>, Olof Johansson <olof@lixom.net>,
        Rob Herring <robh+dt@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Sascha Hauer <kernel@pengutronix.de>,
        Fabio Estevam <festevam@gmail.com>,
        dl-linux-imx <linux-imx@nxp.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        devicetree <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Feb 3, 2021 at 3:37 AM Alistair Francis <alistair23@gmail.com> wrote:
>
> On Thu, Jan 28, 2021 at 11:13 PM Shawn Guo <shawnguo@kernel.org> wrote:
> >
> > On Sun, Jan 17, 2021 at 10:03:01AM -0800, Alistair Francis wrote:
> > > The reMarkable2 requires VMSPLIT_2G, so lets set this in the
> > > imx_v6_v7_defconfig.
> >
> > Hmm, why is VMSPLIT_2G required by reMarkable2?
>
> I'm not too sure. It's difficult to debug problems as I only have a
> UART but without this I don't see any kernel prints so it seems like
> the kernel doesn't get very far. I haven't had any luck with earlycon
> on the device so I don't know how I can get more information.

In the dts file, I can see that the machine has 1GB of RAM at
contiguous addresses. My first guess would be a problem with
highmem, as this configuration means that with VMSPLIT_3G
there are 768MB of lowmem and 256MB of highmem.

Can you try these two things to narrow the problem down
further?

a) disable CONFIG_HIGHMEM when using VMSPLIT_3G
b) use VMSPLIT_3G_OPT

If both of them solve the problem, then highmem is likely
the root cause. One possible issue might be that the boot
loader loads the initramfs or the dtb into a location outside
of the first 768 MB of lowmem where it is unreachable
in the VMSPLIT_3G configuration.

        Arnd