Re: [RFC] Upstreaming Linux for Nintendo Wii U

[Ash Logan]

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@xxxxxxxxxxxx> 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