linux-kernel - Re: [RFC] Upstreaming Linux for Nintendo Wii U

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Date:   Mon, 14 Feb 2022 10:26:47 +1100
From:   Ash Logan <ash@...quark.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "paulus@...ba.org" <paulus@...ba.org>
Cc:     "linkmauve@...kmauve.fr" <linkmauve@...kmauve.fr>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "amd-gfx@...ts.freedesktop.org" <amd-gfx@...ts.freedesktop.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "j.ne@...teo.net" <j.ne@...teo.net>
Subject: Re: [RFC] Upstreaming Linux for Nintendo Wii U

Hi Christophe,

On 12/2/22 00:11, Christophe Leroy wrote:
> Hi Ash,
> 
> Le 11/02/2022 à 12:29, Michael Ellerman a écrit :
>> Ash Logan <ash@...quark.com> writes:
>>> - Like the Wii before it, the Wii U has a small amount of RAM at address
>>> zero, a gap, then a large amount of RAM at a higher address. Instead of
>>> the "map everything and reserve the gap" approach of the Wii, we loop
>>> over each memblock and map only true RAM[9]. This seems to work, but as
>>> far as I can tell is unique amongst powerpc32 platforms, so it's worth
>>> pointing out. (Note: I've been told this doesn't work anymore after some
>>> KUAP changes[10], so this point might be moot; haven't investigated)
>>
>> We'd need more detail on that I guess. Currently all the 32-bit
>> platforms use the flat memory model, which assumes RAM is a single
>> contiguous block. Though that doesn't mean it all has to be used or
>> mapped, like the Wii does. To properly support your layout you should be
>> using sparsemem, but it's possible that's more trouble than it's worth,
>> I'm not sure. How far apart are the low and high blocks of RAM, and what
>> are their sizes?
> 
> Can you provide details on what's happening with KUAP changes ?
> 
> You are pointing to series https://lkml.org/lkml/2021/6/3/204
> 
> Does it work when CONFIG_PPC_KUAP is not selected or doesn't it work
> either ?
> 
> Are you able to bisect which commit of that series is the culprit ?

Emmanuel told me about this during their work on 5.13 which I wasn't 
involved in, and now can't remember any of the details, so I guess I 
don't actually have any more information.
I'm working on getting a baseline setup for 5.15 (just udbg and the 
like), so if there is an issue I should soon find out about it and will 
get back to you.

> Thanks
> Christophe

Thanks,
Ash