linux-kernel - Re: [PATCH v2 4/7] sparc32: Do not select ZONE

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Date: Wed, 06 Mar 2024 19:19:12 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Andreas Larsson" <andreas@...sler.com>,
 "Sam Ravnborg" <sam@...nborg.org>, "Maciej W. Rozycki" <macro@...am.me.uk>,
 sparclinux@...r.kernel.org, "Randy Dunlap" <rdunlap@...radead.org>
Cc: "Miquel Raynal" <miquel.raynal@...tlin.com>,
 linux-parport@...ts.infradead.org, "David S . Miller" <davem@...emloft.net>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/7] sparc32: Do not select ZONE_DMA

On Wed, Mar 6, 2024, at 17:22, Arnd Bergmann wrote:
> On Wed, Mar 6, 2024, at 16:31, Andreas Larsson wrote:

> On a relate note, it does seem odd to have such a small
> lowmem area, and I wonder if that could be extended.
> The 192MB lowmem limit comes from 
>
> #define SRMMU_MAXMEM            0x0c000000
>
> but I don't understand if that is a hardware limitation
> or a design choice that can be changed, and if it is
> even valid on leon or only on the old sun machines.

I had another look and found that this is a result of

arch/sparc/include/asm/page_32.h:#define PAGE_OFFSET    0xf0000000

which gives 3840MiB to userspace addresses, leaving only
256MiB for kernel lowmem and vmalloc space, which is
less than any other architectures.

I still don't know the history behind this choice, but I
see this was already configured the same when arch/sparc/
was originally merged. You can probably change it to a more
sensible 0xc0000000 or 0x80000000 like on other
architectures and run without highmem on anything with
less than 2GB of total RAM.

How much RAM do Leon machines have typically, or at the
maximum?

     Arnd