Re: [Xen-devel] [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants

[Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>]

On Fri, 2015-06-26 at 16:24 +0000, Casey Leedom wrote:
>   Thanks for looking into this Ben.  As it stands now, it seems as
> if Write Combined mappings simply aren't supported and/or all
> driver writers trying to utilize Write Combined mappings have to
> "hand roll" their own solutions which really isn't supportable.
> 
>   One thing that might be considered is simply to treat the desire
> to utilize the Write Combining hardware as a separate issue and
> develop writel_wc(), writeq_wc(), etc.  Those could be defined
> as legal only for Write Combined Mappings and would "do the
> right thing" for each architecture.  

The question then is what is "the right thing". In the powerpc case,
we'll have a non-garded mapping, which means we also get no ordering
between load and stores.

> The initial implementations
> could simply be writel(), writeq(), etc. till each architecture'si
> ssues were investigated and understood.
> 
>   Additionally, it would be very useful to have some sort of
> predicate which could be used to determine if an architecture
> supported Write Combining.  Our drivers would use this to
> eliminate a wasteful attempt to use write combining on those
> architectures where it didn't work.
> 
> Casey
> 
> ________________________________________
> From: Benjamin Herrenschmidt [benh@xxxxxxxxxxxxxxxxxxx]
> Sent: Thursday, June 25, 2015 7:02 PM
> To: Casey Leedom
> Cc: Arnd Bergmann; Luis R. Rodriguez; Michael S. Tsirkin; Bjorn Helgaas; 
> Toshi Kani; Andy Lutomirski; Juergen Gross; Tomi Valkeinen; 
> linux-pci@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; 
> xen-devel@xxxxxxxxxxxxxxxxxxx; linux-fbdev; Suresh Siddha; Ingo Molnar; 
> Thomas Gleixner; Daniel Vetter; Dave Airlie; Antonino Daplas; Jean-Christophe 
> Plagniol-Villard; Dave Hansen; venkatesh.pallipadi@xxxxxxxxx; Stefan Bader; 
> ville.syrjala@xxxxxxxxxxxxxxx; David Vrabel; Jan Beulich; Roger Pau MonnÃ
> Subject: Re: [PATCH v7 5/9] PCI: Add pci_iomap_wc() variants
> 
> On Thu, 2015-06-25 at 21:40 +0000, Casey Leedom wrote:
> >
> > Ah, thanks.  I see now that the __raw_*() APIs don't do any of the
> > Endian Swizzling.  Unfortunately the *_relaxed() APIs on PowerPC
> > are just defined as the normal *() routines.  From
> > arch/powerpc/include/asm/io.h:
> >
> >     /*
> >      * We don't do relaxed operations yet, at least not with this
> > semantic
> >      */
> 
> Yes so I was looking at this but there are some difficulties.
> Architecturally, even with I=1 G=1 mappings (normal ioremap), we have no
> guarantee of ordering of load vs. store unless I misunderstood
> something. I think all current implementations provide some of that but
> without barriers in the accessors, we aren't architecturally correct.
> 
> However, having those barriers will cause issues with G=0 (write
> combine). It's unclear whether eieio() will provide the required
> ordering for I=1 G=0 mappings and it will probably break write combine.
> 
> I'm looking into it with our HW guys and will try to come up with a
> solution for power, but it doesn't help that our memory model conflates
> write combining with other relaxations and that all our barriers also
> prevent write combine.
> 
> Maybe we can bias the relaxed accessors toward write, by having no
> barriers in it, and putting extra ones in reads.
> 
> Cheers,
> Ben.
> 
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