Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Feedback-ID: i56a14606:Fastmail
User-Agent: Cyrus-JMAP/3.7.0-alpha0-1047-g9e4af4ada4-fm-20221005.001-g9e4af4ad
Mime-Version: 1.0
Message-Id: <276d9794-a1b4-4f0d-a9a3-027823038ecc@app.fastmail.com>
In-Reply-To: <87sfjmlim3.fsf@mpe.ellerman.id.au>
References: <cover.1665568707.git.christophe.leroy@csgroup.eu>
 <8c7ac4667c6a3cc48f98110117536f60d51ece4a.1665568707.git.christophe.leroy@csgroup.eu>
 <d0acd053-96d3-4e18-a9de-97987d8be14b@app.fastmail.com>
 <Y0u4tN+mIgNSWwdi@li-4a3a4a4c-28e5-11b2-a85c-a8d192c6f089.ibm.com>
 <af8ba7b8-5198-42a1-84c7-9b7d7892ceb3@app.fastmail.com>
 <87sfjmlim3.fsf@mpe.ellerman.id.au>
Date:   Mon, 17 Oct 2022 22:52:27 +0200
From:   "Arnd Bergmann" <arnd@arndb.de>
To:     "Michael Ellerman" <mpe@ellerman.id.au>,
        "Alexander Gordeev" <agordeev@linux.ibm.com>
Cc:     "Christophe Leroy" <christophe.leroy@csgroup.eu>,
        "Baoquan He" <bhe@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Linux-Arch <linux-arch@vger.kernel.org>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Christoph Hellwig" <hch@infradead.org>,
        wangkefeng.wang@huawei.com, schnelle@linux.ibm.com,
        "David Laight" <David.Laight@aculab.com>,
        "Stafford Horne" <shorne@gmail.com>,
        "Benjamin Herrenschmidt" <benh@kernel.crashing.org>
Subject: Re: [RFC PATCH 7/8] mm/ioremap: Consider IOREMAP space in generic ioremap
Content-Type: text/plain
Precedence: bulk

On Mon, Oct 17, 2022, at 2:50 PM, Michael Ellerman wrote:
> "Arnd Bergmann" <arnd@arndb.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