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

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Date:   Fri, 13 Nov 2020 15:36:07 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Max Filippov <jcmvbkbc@...il.com>
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

Max,

On Fri, Nov 13 2020 at 05:50, Max Filippov wrote:
> On Fri, Nov 13, 2020 at 5:40 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> On Fri, Nov 13 2020 at 04:23, Max Filippov wrote:
>> > Fixmap on xtensa grows upwards, i.e. bigger fixmap entry index
>> > corresponds to a higher virtual address. This was lost in highmem
>> > generalization resulting in the following runtime warnings:
>>
>> Sorry for not noticing.
>>
>> > Fix it by adding __ARCH_HAS_POSITIVE_FIXMAP macro and implementing
>> > vaddr_in_fixmap and fixmap_pte primitives differently depending on
>> > whether it is defined or not.
>>
>> What's wrong with just doing the obvious and making the fixmap defines
>> the other way round?
>
> It becomes really awkward when we get to support high memory with
> aliasing data cache: we must think about the actual virtual addresses
> assigned to pages and it feels much simpler when it's done this way.

Feeling are not really a technical argument. Is there any functional
difference which matters?

Thanks,

        tglx