Re: [PATCH 05/20] ARM: implement pci_remap_cfgspace() interface

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

Hi Russell,

On Mon, Mar 20, 2017 at 04:43:55PM +0000, Russell King - ARM Linux wrote:
> On Mon, Feb 27, 2017 at 03:14:16PM +0000, Lorenzo Pieralisi wrote:
> > The PCI bus specifications (rev 3.0, 3.2.5 "Transaction Ordering
> > and Posting") define rules for PCI configuration space transactions
> > ordering and posting, that state that configuration writes have to
> > be non-posted transactions.
> > 
> > Current ioremap interface on ARM provides mapping functions that
> > provide "bufferable" writes transactions (ie ioremap uses MT_DEVICE
> > memory type) aka posted writes, so PCI host controller drivers have
> > no arch interface to remap PCI configuration space with memory
> > attributes that comply with the PCI specifications for configuration
> > space.
> > 
> > Implement an ARM specific pci_remap_cfgspace() interface that allows to
> > map PCI config memory regions with MT_UNCACHED memory type (ie strongly
> > ordered - non-posted writes), providing a remap function that complies
> > with PCI specifications for config space transactions.
> 
> Doesn't this have the side effect that configuration writes can bypass
> writes to device memory if there isn't an intervening dsb?  (They
> probably can do on some CPUs today anyway.)

I assumed that in plain terms, the difference between MT_DEVICE and
MT_UNCACHED is write posting (aka bufferable) behaviour (across CPU
architecture versions) and that does not affect write ordering rules.

You and Catalin have more insights into ARM 32-bit memory types so
I definitely need your input here to be comprehensive and avoid
pitfalls, let me know if you have some specific CPUs in mind on
which this may trigger a regression.

> So, in any case, this looks like an improvement:
> 
> Acked-by: Russell King <rmk+kernel@armlinux.org.uk>

Thank you !

Lorenzo