Re: [PATCH v2 4/7] sparc32: Do not select ZONE_DMA

From: Andreas Larsson <andreas@gaisler.com>
To: Arnd Bergmann <arnd@arndb.de>, Sam Ravnborg <sam@ravnborg.org>,
	"Maciej W. Rozycki" <macro@orcam.me.uk>,
	sparclinux@vger.kernel.org, Randy Dunlap <rdunlap@infradead.org>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>,
	linux-parport@lists.infradead.org,
	"David S . Miller" <davem@davemloft.net>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/7] sparc32: Do not select ZONE_DMA
Date: Thu, 7 Mar 2024 13:42:06 +0100	[thread overview]
Message-ID: <74afd65d-85ed-4ee9-b5f2-a6702f9bc01c@gaisler.com> (raw)
In-Reply-To: <5568f26d-9e1a-4530-a086-564659b21666@app.fastmail.com>

On 2024-03-06 19:19, Arnd Bergmann wrote:
> 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?
The amount of RAM can vary greatly between systems, from less that
128 MiB up to 2 GiB. An upcoming design uses the entire 36-bit
physical address space and have the possibility of having up to 60
GiB memory.

Thanks,
Andreas