linux-kernel - Re: [PATCH 1/7] mips: dmi: Fix early remap on MIPS32

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Message-Id: <bb13c070-bdfe-47ae-afed-a05e1e55bb94@app.fastmail.com>
Date:   Fri, 01 Dec 2023 00:13:22 +0000
From:   "Jiaxun Yang" <jiaxun.yang@...goat.com>
To:     "Serge Semin" <fancer.lancer@...il.com>
Cc:     "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
        "Arnd Bergmann" <arnd@...db.de>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Mike Rapoport" <rppt@...nel.org>,
        "Matthew Wilcox" <willy@...radead.org>,
        "Tiezhu Yang" <yangtiezhu@...ngson.cn>,
        "Huacai Chen" <chenhuacai@...nel.org>,
        "Yinglu Yang" <yangyinglu@...ngson.cn>,
        "Alexey Malahov" <Alexey.Malahov@...kalelectronics.ru>,
        "Aleksandar Rikalo" <aleksandar.rikalo@...mia.com>,
        "Aleksandar Rikalo" <arikalo@...il.com>,
        "Dragan Mladjenovic" <dragan.mladjenovic@...mia.com>,
        "Chao-ying Fu" <cfu@...ecomp.com>, "Marc Zyngier" <maz@...nel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] mips: dmi: Fix early remap on MIPS32



在2023年11月30日十一月 下午7:16，Serge Semin写道：
> On Tue, Nov 28, 2023 at 03:46:37PM +0000, Jiaxun Yang wrote:
[...]
>
>> I'd say the safest option is to use CKSEG0 or TO_CAC here, 
>
> I would have agreed with you if MIPS didn't have that special
> _page_cachable_default variable which is undefined for some platforms
> and which might be re-defined during the boot-up process, and if
> MIPS64 didn't have ioremap_prot() always mapping to the uncached
> region.  But IMO updating ioremap_prot() currently seems more risky
> than just converting dmi_early_remap() to the uncached version
> especially seeing it won't change anything. MIPS64 always have IO
> remapped to the uncached region. MIPS32 won't be able to have cached
> mapping until VM is available, and paging and slabs are initialized.
> So on the early MIPS32 bootup stages ioremap_cache() wouldn't have
> worked anyway.

I really didn't get that, using CKSEG0 on 32bit system and TO_CAC
on 64bit system won't hurt.

Something like:
#ifdef CONFIG_64BIT
#define dmi_remap(x, l)		(void *)TO_CAC(x)
#else
#define dmi_remap(x, l)		(void *)CKSEG0(x)
#endif

Can help us avoid all the hassle. Since it always ensures we are
using same CCA to access DMI tables. We can always trust Config.K0
left by firmware in this case.

You may add some sanity check on 32 bit to avoid generating invalid
pointer. (And perhaps implement it as ioremap_early.....) 

Thanks
-- 
- Jiaxun