Re: [PATCH v3 00/32] PCI: fix config and I/O Address space memory mappings

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Wed, Apr 12, 2017 at 11:51:59PM +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2017-04-12 at 12:31 +0100, Russell King - ARM Linux wrote:
> > default implementation should fail if it's not supportable on all
> > architectures.  However, when we have existing drivers using an
> > interface that doesn't provide the semantics they already require,
> > then it makes no sense to effectively break these drivers on a range
> > of existing architectures.
> > 
> > The question really is - what's the best way to solve the problem
> > with
> > existing drivers without breaking them.  I suspect that, sadly, the
> > only realistic way forward here is via the litter-drivers-with-ifdefs
> > approach since you don't like providing a default implementation that
> > is compatible with what these drivers are already doing.
> 
> Then make ioremap_nopost return NULL when the arch doesn't have 
> the right semantic.
> 
> The driver than can *chose* to either silently fallback to ioremap,
> which has served us well for a long time despite being theorically in
> violation of the spec, or do funny things like read back some register
> after every config write to ensure ordering etc...
> 
> I much prefer that approach than having some generic ioremap function
> that exposes a semantic that silently provides a weaker one on some
> architecture.
> 
> At least we make the failure explicit, and the driver can take
> alternate (possibly sub-optimal) action if it chooses to do so.

The same points apply to things like pgprot_writecombine(),
pgprot_noncached(), pgprot_device().  Then there's also pgprot_nonposted()
that this series also introduces.

If ioremap_nopost() is not possible on an architecture, then
pgprot_nonposted() won't be possible either - but you've made no
mention of that so far.

Just like the proposed ioremap_nopost(), pgprot_nonposted() is given a
default implementation that uses pgprot_noncached().  Maybe we should
also make pci_remap_iospace() fail if pgprot_nonposted() is not defined
by the architecture?

-- 
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