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Date:   Tue, 28 Nov 2023 13:41:51 +0100
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Serge Semin" <fancer.lancer@gmail.com>,
        "Jiaxun Yang" <jiaxun.yang@flygoat.com>
Cc:     "Thomas Bogendoerfer" <tsbogend@alpha.franken.de>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Mike Rapoport" <rppt@kernel.org>,
        "Matthew Wilcox" <willy@infradead.org>,
        "Tiezhu Yang" <yangtiezhu@loongson.cn>,
        "Huacai Chen" <chenhuacai@kernel.org>,
        "Yinglu Yang" <yangyinglu@loongson.cn>,
        "Alexey Malahov" <Alexey.Malahov@baikalelectronics.ru>,
        "Aleksandar Rikalo" <aleksandar.rikalo@syrmia.com>,
        "Aleksandar Rikalo" <arikalo@gmail.com>,
        "Dragan Mladjenovic" <dragan.mladjenovic@syrmia.com>,
        "Chao-ying Fu" <cfu@wavecomp.com>, "Marc Zyngier" <maz@kernel.org>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/7] mips: dmi: Fix early remap on MIPS32
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On Mon, Nov 27, 2023, at 17:23, Serge Semin wrote:
> On Fri, Nov 24, 2023 at 10:03:49PM +0000, Jiaxun Yang wrote:
>> =E5=9C=A82023=E5=B9=B411=E6=9C=8824=E6=97=A5=E5=8D=81=E4=B8=80=E6=9C=88=
 =E4=B8=8B=E5=8D=886:52=EF=BC=8CSerge Semin=E5=86=99=E9=81=93=EF=BC=9A
>> > On Thu, Nov 23, 2023 at 05:33:31PM +0000, Jiaxun Yang wrote:
>> >>=20
>> [...]
>> >> Actually dmi_setup() is called before cpu_cache_init().
>> >
>> > To preliminary sum the discussion, indeed there can be issues on the
>> > platforms which have DMI initialized on the cached region. Here are
>> > several solutions and additional difficulties I think may be caused=
 by
>> > implementing them:
>>=20
>> Thanks for such detailed conclusion!
>> I'd prefer go solution 1, with comments below.
>> >
>> > 1. Use unmapped cached region utilization in the MIPS32 ioremap_pro=
t()
>> > method.
>> > This solution a bit clumsy than it looks on the first glance.
>> > ioremap_prot() can be used for various types of the cachability
>> > mapping. Currently it's a default-cacheable CA preserved in the
>> > _page_cachable_default variable and Write-combined CA saved in
>> > boot_cpu_data.writecombine. Based on that we would have needed to u=
se
>> > the unmapped cached region utilized for the IO-remaps called with t=
he
>> > "_page_cachable_default" mapping flags passed only. The rest of the=
 IO
>> > range mappings, including the write-combined ones, would have been
>> > handled by VM means. This would have made the ioremap_prot() a bit
>> > less maintainable, but still won't be that hard to implement (unles=
s I
>> > miss something):
>> > --- a/arch/mips/mm/ioremap.c
>> > +++ b/arch/mips/mm/ioremap.c
>> >         /*
>> > -        * Map uncached objects in the low 512mb of address space u=
sing KSEG1,
>> > -        * otherwise map using page tables.
>> > +        * Map uncached/default-cached objects in the low 512mb of =
address
>> > +        * space using KSEG1/KSEG0, otherwise map using page tables.
>> >          */
>> > -       if (IS_LOW512(phys_addr) && IS_LOW512(last_addr) &&
>> > -           flags =3D=3D _CACHE_UNCACHED)
>> > -               return (void __iomem *) CKSEG1ADDR(phys_addr);
>> > +       if (IS_LOW512(phys_addr) && IS_LOW512(last_addr)) {
>> > +               if (flags =3D=3D _CACHE_UNCACHED)
>> > +                       return (void __iomem *) CKSEG1ADDR(phys_add=
r);
>> > +               else if (flags =3D=3D _page_cachable_default)
>> > +                       return (void __iomem *) CKSEG0ADDR(phys_add=
r);
>> > +       }
>> >
>> > Currently I can't figure out what obvious problems it may cause. But
>> > It seems suspicious that the cacheable IO-mapping hasn't been
>> > implemented by the unmapped cacheable region in the first place. In
>> > anyway this solution looks more dangerous than solution 2. because =
it
>> > affects all the MIPS32 platforms at once.
>>=20
>> I just made a quick grep in tree, and it seems like we don't have much
>> user of ioremap_cache (as well as ioremap_uc/wc) here so I think it is
>> a safe assumption.
>
> I wouldn't say there aren't much users. ioremap_wc() and it's
> devm-version is widely utilized in the GPU and network and some other
> subsystems. ioremap_cache() isn't widespread indeed. In anyway even a
> single user must be supported in safely calling the method if it's
> provided by the arch-code, otherwise the method could be considered as
> just a bogus stub to have the kernel successfully built. I bet you'll
> agree with that. But that's not the point in this case.

ioremap_wc() is useful for mapping PCI attached memory such as frame
buffers, but ioremap_cache() is generally underspecified because the
resulting pointer is neither safe to dereference nor to pass into
readl()/writel()/memcpy_fromio() on all architectures.

There was an effort to convert the remaining ioremap_cache() calls
into memremap() a few years ago, not sure if that's still being worked
on but it would be the right thing to do.

     Arnd