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Message-ID: <a381b32861b106afb209628e094260caf5fe9f7b.camel@linux.ibm.com>
Subject: Re: [PATCH v2 09/11] s390: mm: Convert to GENERIC_IOREMAP
From:   Niklas Schnelle <schnelle@linux.ibm.com>
To:     Christoph Hellwig <hch@infradead.org>, Baoquan He <bhe@redhat.com>
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 <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>, linux-s390@vger.kernel.org
Date:   Mon, 22 Aug 2022 17:08:10 +0200
In-Reply-To: <YwHZRhpwL37yLb/o@infradead.org>
References: <20220820003125.353570-1-bhe@redhat.com>
         <20220820003125.353570-10-bhe@redhat.com> <YwHZRhpwL37yLb/o@infradead.org>
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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.
 */