linux-kernel - Re: [RFC PATCH 7/8] mm/ioremap: Consider IOREMAP space in generic ioremap

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Message-Id: <276d9794-a1b4-4f0d-a9a3-027823038ecc@app.fastmail.com>
Date:   Mon, 17 Oct 2022 22:52:27 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Michael Ellerman" <mpe@...erman.id.au>,
        "Alexander Gordeev" <agordeev@...ux.ibm.com>
Cc:     "Christophe Leroy" <christophe.leroy@...roup.eu>,
        "Baoquan He" <bhe@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Christoph Hellwig" <hch@...radead.org>,
        wangkefeng.wang@...wei.com, schnelle@...ux.ibm.com,
        "David Laight" <David.Laight@...lab.com>,
        "Stafford Horne" <shorne@...il.com>,
        "Benjamin Herrenschmidt" <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH 7/8] mm/ioremap: Consider IOREMAP space in generic ioremap

On Mon, Oct 17, 2022, at 2:50 PM, Michael Ellerman wrote:
> "Arnd Bergmann" <arnd@...db.de> writes:
>>
>> My guess is that there was no technical reason for this, other
>> than having no reason to change the behavior at the time.
>
> I think the immediate reason for it is that on some CPUs we have to use
> 4K pages in the HPT for IO mappings, but PAGE_SIZE == 64K, and we can
> only have a single page size per segment (256M or 1T).

Right, makes sense. Both the original patch, or the variant with
defining IOREMAP_START everywhere seem fine to me then.

     Arnd