linux-kernel - Re: [PATCH] highmem: fix highmem for xtensa

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Date:   Mon, 16 Nov 2020 06:47:03 -0800
From:   Max Filippov <jcmvbkbc@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>, Chris Zankel <chris@...kel.net>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] highmem: fix highmem for xtensa

Hi Thomas,

On Fri, Nov 13, 2020 at 2:34 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> I agree that the top down mechanics is not really intuitive, but that
> does not justify the ifdeffery in the generic code.

But then maybe xtensa did the right thing where everyone else just
copied the not really intuitive implementation? If nobody else cares
then maybe generic fix_to_virt/virt_to_fix can be changed for positive
indexing?

> xtensa can just use the generic fix_to_virt/virt_to_fix mechanics. All
> it needs is to adjust the mapping defines and to adjust the color offset
> to
>
>         NR_COLORS - color
>
> which is not an unreasonable ask. As a side effect all highmem inflicted
> systems which do not have the cache aliasing problem can just use the
> generic code as is. See untested patch below.

Thanks. I'll test this patch and post the result.
But still this change doesn't look like a step in the right direction to me:
I can't find the reason why fixmap must be indexed backwards.

> It builds for some configs, but the smp_lx200_defconfig (which has the
> aliasing) it fails to build even without this patch (highmem.o at least
> builds).
>
> Toolchain is the one from https://mirrors.edge.kernel.org/pub/tools/crosstool/

xtensa toolchain must match the selected CPU core. For smp_lx200_defconfig
the toolchain is available here:
https://github.com/foss-xtensa/toolchain/releases/download/2020.07/x86_64-2020.07-xtensa-test_mmuhifi_c3-elf.tar.gz

-- 
Thanks.
-- Max