linux-kernel - Re: [PATCH v2 09/11] s390: mm: Convert to GENERIC

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Date:   Mon, 22 Aug 2022 17:08:10 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Christoph Hellwig <hch@...radead.org>, Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, agordeev@...ux.ibm.com,
        wangkefeng.wang@...wei.com, linux-arm-kernel@...ts.infradead.org,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>, linux-s390@...r.kernel.org
Subject: Re: [PATCH v2 09/11] s390: mm: Convert to GENERIC_IOREMAP

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.
 */