linux-kernel - Re: [PATCH 11/12] powerpc: wiiu: don't enforce flat memory

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Message-ID: <935b426a-6c64-beb0-907f-8c3f0a089ab7@heyquark.com>
Date:   Fri, 20 May 2022 20:44:04 +1000
From:   Ash Logan <ash@...quark.com>
To:     Pali Rohár <pali@...nel.org>
Cc:     paulus@...ba.org, mpe@...erman.id.au, christophe.leroy@...roup.eu,
        robh+dt@...nel.org, benh@...nel.crashing.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        j.ne@...teo.net
Subject: Re: [PATCH 11/12] powerpc: wiiu: don't enforce flat memory

On 20/5/22 18:04, Pali Rohár wrote:
> On Friday 20 May 2022 13:41:04 Ash Logan wrote:
>> On 14/5/22 08:43, Pali Rohár wrote:
>>> On Wednesday 02 March 2022 15:44:05 Ash Logan wrote:
>>>> pgtable_32.c:mapin_ram loops over each valid memory range, which means
>>>> non-contiguous memory just works.
>>>
>>> Hello! Does it mean that non-contiguous memory works for any 32-bit
>>> powerpc platform, and not only for wiiu? If yes, should not be
>>> non-contiguous memory support enabled for all 32-bit ppc boards then?
>>
>> Hi! Sorry for my delayed response. As best I can tell, it does indeed
>> Just Work, but I have only been able to test on wiiu which is missing a
>> lot of features other boards have (like PCI) - so it's possible there's
>> still an assumption elsewhere in the kernel that I haven't hit.
>>
>> As best I can tell, the Wii and Wii U are the only 32-bit powerpc boards
>> out there where it's even possible to have non-contiguous memory.
> 
> What is the reason that those two boards are the **only**? Is there some
> specific requirement from bootloader or hardware to "enable"
> non-contiguous memory support?

Not that I know of, I was just saying that I was only aware of those two
boards where the memory map isn't contiguous, and that is the only place
where it has been tested. Evidently you know of another board!

> I'm interested in enabling non-contiguous memory support for P2020-based
> board as it has gaps in its 32-bit memory layout and which could be used
> for RAM mapping when 4GB DDR3 module is plugged in (default is 2GB).

If it's like the Wii or Wii U (some memory at 0, a gap for MMIO or
whatever, then more memory at a higher address) then you should try a
patch along these lines, because barring the unknowns I mentioned before
it should work. At least as far as I'm aware ;)

>>>> Signed-off-by: Ash Logan <ash@...quark.com>
>>>> ---
>>>>  arch/powerpc/mm/init_32.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/powerpc/mm/init_32.c b/arch/powerpc/mm/init_32.c
>>>> index 3d690be48e84..59a84629d9a0 100644
>>>> --- a/arch/powerpc/mm/init_32.c
>>>> +++ b/arch/powerpc/mm/init_32.c
>>>> @@ -125,10 +125,10 @@ void __init MMU_init(void)
>>>>  	 * lowmem_end_addr is initialized below.
>>>>  	 */
>>>>  	if (memblock.memory.cnt > 1) {
>>>> -#ifndef CONFIG_WII
>>>> +#if !defined(CONFIG_WII) && !defined(CONFIG_WIIU)
>>>>  		memblock_enforce_memory_limit(memblock.memory.regions[0].size);
>>>>  		pr_warn("Only using first contiguous memory region\n");
>>>> -#else
>>>> +#elif defined(CONFIG_WII)
>>>>  		wii_memory_fixups();
>>>>  #endif
>>>>  	}
>>>> -- 
>>>> 2.35.1
>>>>