Date:	Tue, 08 Sep 2015 15:42:40 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	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 Thursday 03 September 2015 03:44:15 Luis R. Rodriguez wrote:
> On Sun, Aug 30, 2015 at 09:30:26PM +0200, Arnd Bergmann wrote:
> > On Friday 28 August 2015 17:17:27 Luis R. Rodriguez wrote:
> > > 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.
> > 
> > Which architectures are you thinking about here?
> 
> Really only S390 would benefit from this now.

Ok

> > > Build tested with allyesconfig on:
> > > 
> > >         * S390
> > >         * x86_64
> > > 
> > > Signed-off-by: Luis R. Rodriguez <mcgrof@...e.com>
> > 
> > It's not really clear to me what the purpose of the patch is, is this 
> > meant as a cleanup, or are you trying to avoid some real-life bugs
> > you ran into?
> 
> Upon adding a new helper into CONFIG_PCI_IOMAP it was only through
> 0-day build testing that I found that I needed to add something for S390.
> This means we fix S390 reactively. With the asm-generic stuff in place
> to return NULL we don't need to do anything but a respective return
> NULL static inline, the moment that is done the author would know some
> architectures may not get support for the functionality they are adding.
> Without this we only find out reactively.

Hmm, my gut feeling tells me that your approach won't solve the problem
in general. s390 PCI is just weird in many ways and it will occasionally
suffer from problems like this (as do other aspects of the s390 architecture
that are unlike the rest of the world).

Maybe Martin and Heiko can comment on this, they may have a preference
from the s390 point of view.

> > The version from lib/iomap.c seems correct for uses of CONFIG_GENERIC_IOMAP,
> > but most architectures can do better without that option.
> 
> By do better do you mean a more optimized solution ?

Yes: most architectures access the PCI I/O space through memory mapped I/O,
so we can return a regular __iomem pointer from ioport_map, rather than
a garbled pointer that the x86 version has to use.

This means we also get to define iowrite32() to be identical to writel()
and can save the conditional for each caller.

The lib/iomap.c version is really only needed for architectures that use
pointer access for PCI memory space, and special instructions for PCI
I/O space, like x86. s390 has special instructions for both, some
architectures do not have any I/O port access at all, and most of them
treat memory and I/O space the same way.

Note that there are still two possible implementations for ioport_map
on those:

a) just return a pointer from ioport_map() that points to the existing
   mapping, define ioport_unmap() as an empty function, and have
   pci_iounmap() check the pointer.

b) create a new ioremap() mapping in ioport_map(), and define both
   ioport_unmap() and pci_iounmap as iounmap.

	Arnd
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