Re: [PATCH 02/20] PCI: fix pci_remap_iospace() remap attribute

[Liviu Dudau <liviu@dudau.co.uk>]

On Fri, Mar 17, 2017 at 01:33:21AM +0100, Luis R. Rodriguez wrote:
> On Thu, Mar 16, 2017 at 04:48:44PM -0500, Bjorn Helgaas wrote:
> > [+cc Luis]
> > 
> > On Mon, Feb 27, 2017 at 03:14:13PM +0000, Lorenzo Pieralisi wrote:
> > > According to the PCI local bus specifications (Revision 3.0, 3.2.5),
> > > I/O Address space transactions are non-posted. On architectures where
> > > I/O space is implemented through a chunk of memory mapped space mapped
> > > to PCI address space (ie IA64/ARM/ARM64) the memory mapping for the
> > > region backing I/O Address Space transactions determines the I/O
> > > transactions attributes (before the transactions actually reaches the
> > > PCI bus where it is handled according to the PCI specifications).
> > > 
> > > Current pci_remap_iospace() interface, that is used to map the PCI I/O
> > > Address Space into virtual address space, use pgprot_device() as memory
> > > attribute for the virtual address mapping, that in some architectures
> > > (ie ARM64) provides non-cacheable but write bufferable mappings (ie
> > > posted writes), 
> 
> <sarcasm>
> Gee wiz, I am glad this is so well documented.
> </sarcasm>
> 
> > > which clash with the non-posted write behaviour for I/O
> > > Address Space mandated by the PCI specifications.
> > > 
> > > Update the prot ioremap_page_range() parameter in pci_remap_iospace()
> > > to pgprot_noncached to ensure that the virtual mapping backing
> > > I/O Address Space guarantee non-posted write transactions issued
> > > when addressing I/O Address Space through the MMIO mapping.
> 
> How did we end up with pgprot_device() then in the first place Liviu Dudau [0] ?
> I ask for two reasons:

[replying using personal email as the corporate email system is taking its sweet time
to deliver the email to my inbox]

I've asked the people with the right knowledge about the correct API to use (Hi Catalin!),
and during the review it did not throw any red flags. I guess, given Bjorn's comment,
that everyone assumed AArch64 is the same as all other architectures and pgprot_device
is synonymous to pgprot_noncached.

> 
> a) should we then use a Fixes tag for this patch ?

I'm not aware of issues being reported, but Lorenzo might have more info on this.

> b) it does not seem clear what the semantics for pgprot_device() or even
>    pgprot_noncached(). Can you add some ?
> 
> 8b921acfeffdb ("PCI: Add pci_remap_iospace() to map bus I/O resources")
> 
> Also this patch claims archs can override this call alone, as its __weak.
> So is the right thing to do to change pci_remap_iospace() to pgprot_noncached()
> or is it for archs to add their own pci_remap_iospace()? If so why ? Without
> proper semantics defined for these helpers this is all fuzzy.

That was the initial intention, to let arches / platforms overwrite the whole
pci_remap_iospace(). I guess the reality is that no one needs to overwrite it except
for the AArch64 quirk, so probably easier to remove the __weak and fix the attributes for arm64.

Best regards,
Liviu

> 
> > > Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> > > Cc: Arnd Bergmann <arnd@arndb.de>
> > > Cc: Will Deacon <will.deacon@arm.com>
> > > Cc: Bjorn Helgaas <bhelgaas@google.com>
> > > Cc: Russell King <linux@armlinux.org.uk>
> > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > ---
> > >  drivers/pci/pci.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > index bd98674..bfb3c6e 100644
> > > --- a/drivers/pci/pci.c
> > > +++ b/drivers/pci/pci.c
> > > @@ -3375,7 +3375,7 @@ int pci_remap_iospace(const struct resource *res, phys_addr_t phys_addr)
> > >  		return -EINVAL;
> > >  
> > >  	return ioremap_page_range(vaddr, vaddr + resource_size(res), phys_addr,
> > > -				  pgprot_device(PAGE_KERNEL));
> > > +				  pgprot_noncached(PAGE_KERNEL));
> > 
> > pgprot_device() is equivalent to pgprot_noncached() on all arches
> > except ARM64, and I trust you're doing the right thing on ARM64, so
> > I'm fine with this from a PCI perspective.
> > 
> > I do find this puzzling because I naively expected pgprot_noncached()
> > to match up with ioremap_nocache(), and apparently it doesn't.
> > 
> > For example, ARM64 ioremap_nocache() uses PROT_DEVICE_nGnRE, which
> > doesn't match the MT_DEVICE_nGnRnE in pgprot_noncached().
> > 
> > The point of these patches is to use non-posted mappings.  Apparently
> > you can do that with pgprot_noncached() here, but ioremap_nocache()
> > isn't enough for the config space mappings?
> 
> This is for iospace it seems, so the other patch I think was for
> config space.
> 
>   Luis
> 
> > I suppose that's a consequence of the pgprot_noncached() vs
> > ioremap_nocache() mismatch, but this is all extremely confusing.
> > 
> > >  #else
> > >  	/* this architecture does not have memory mapped I/O space,
> > >  	   so this function should never be called */
> > > -- 
> > > 2.10.0
> > > 
> > > 
> > > _______________________________________________
> > > linux-arm-kernel mailing list
> > > linux-arm-kernel@lists.infradead.org
> > > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> > 
> 
> -- 
> Luis Rodriguez, SUSE LINUX GmbH
> Maxfeldstrasse 5; D-90409 Nuernberg

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