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Mon, 22 Aug 2022 15:08:11 GMT Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3A44B4C04A; Mon, 22 Aug 2022 15:08:11 +0000 (GMT) Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D089C4C046; Mon, 22 Aug 2022 15:08:10 +0000 (GMT) Received: from oc-nschnelle.boeblingen.de.ibm.com (unknown [9.155.199.46]) by d06av22.portsmouth.uk.ibm.com (Postfix) with ESMTP; Mon, 22 Aug 2022 15:08:10 +0000 (GMT) Message-ID: Subject: Re: [PATCH v2 09/11] s390: mm: Convert to GENERIC_IOREMAP From: Niklas Schnelle To: Christoph Hellwig , Baoquan He Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, agordeev@linux.ibm.com, wangkefeng.wang@huawei.com, linux-arm-kernel@lists.infradead.org, Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , linux-s390@vger.kernel.org Date: Mon, 22 Aug 2022 17:08:10 +0200 In-Reply-To: References: <20220820003125.353570-1-bhe@redhat.com> <20220820003125.353570-10-bhe@redhat.com> Content-Type: text/plain; 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dmarc=pass (policy=none) header.from=ibm.com X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 91B5080062 X-Rspam-User: X-HE-Tag: 1661180907-215907 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Sun, 2022-08-21 at 00:05 -0700, Christoph Hellwig wrote: > > +void __iomem * > > +arch_ioremap(phys_addr_t *paddr, size_t size, unsigned long *prot_val) > > { > > if (!static_branch_unlikely(&have_mio)) > > + return (void __iomem *) *paddr; > > + return NULL; > > This logic isn't new in the patch, but it could really use a comment > as it is rather non-obvious. Yes, makes sense. Basically we fake MMIO addresses because the s390 architecture doesn't have MMIO as a concept. That is until the PCI MIO instructions introduced pseudo-MMIO though only for specific PCI load/store instructions. Without those PCI BAR spaces as well as config space is accessed with so called function handles. As these are a bad fit for Linux' MMIO based APIs we create fake MMIO addresses (called address cookies) that encode an index into the zpci_iomap_start[] which can be decoded by our implementation of ioread*/iowrite*(). I don't think this is the right place to describe this overall scheme in detail but maybe we can leave a a good bread crumb. Maybe something like below? /* * When PCI MIO instructions are unavailable the "physical" address encodes * a hint for accessing the PCI memory space it represents. Just pass it * unchanged such that ioread/iowrite can decode it. */