linux-kernel - Re: [RFC] asm-generic/pci_iomap.h: make custom PCI BAR requirements explicit

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Message-ID: <20150903014506.GR8051@wotan.suse.de>
Date:	Thu, 3 Sep 2015 03:45:07 +0200
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>, arnd@...db.de,
	linux-arch@...r.kernel.org, mingo@...nel.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	linux-s390@...r.kernel.org, bp@...e.de, linux@...ck-us.net,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	rostedt@...dmis.org
Subject: Re: [RFC] asm-generic/pci_iomap.h: make custom PCI BAR requirements
 explicit

On Sat, Aug 29, 2015 at 08:25:05AM -0700, Christoph Hellwig wrote:
> On Fri, Aug 28, 2015 at 05:17:27PM -0700, Luis R. Rodriguez wrote:
> > From: "Luis R. Rodriguez" <mcgrof@...e.com>
> > 
> > The S390 architecture requires a custom pci_iomap() implementation
> > as the asm-generic implementation assumes there are disjunctions
> > between PCI BARs, and on S390 PCI BAR are not disjunctive, S390 requires
> > the bar parameter in order to find the corresponding device and create
> > the mapping cookie.
> > 
> > This clash with the asm-generic pci_iomap() implementation is implicit,
> > there are no current semantics to make this incompatability explicit.
> > Make the S390 PCI BAR non-disjunction incompatibility explicit, and
> > also pave the way for alternative incompatibilities to be defined.
> > 
> > While at it, as with the ioremap*() variants, since we have no clear
> > semantics yet well defined provide a solution for them that returns
> > NULL. This allows architectures to move forward by defining pci_ioremap*()
> > variants without requiring immediate changes to all architectures. Each
> > architecture then can implement their own solution as needed and
> > when they get to it.
> 
> Now that you have the config symbol available why not move the S390
> implementation to generic code based on that can kill of
> asm/pci_iomap.h?  Seems like we're really not dealing with something
> inherent to the architecture, but two possible implementations based
> on architecture constraints.

That would be the other approach to take too, but perhaps that can wait
until yet another architecture with similar requirement pops up.

 Luis
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