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Date:   Wed, 31 Aug 2022 16:59:56 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Niklas Schnelle <schnelle@linux.ibm.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        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
Subject: Re: [PATCH v2 09/11] s390: mm: Convert to GENERIC_IOREMAP
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On 08/22/22 at 05:08pm, Niklas Schnelle wrote:
> 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.
>  */

Thanks. Looks good to me, I will add these to above the code.

>