Re: [PATCH] arm64: Enable PCI write-combine resources under sysfs

Benjamin Herrenschmidt
2020-09-17T00:30:27Z

On Wed, 2020-09-16 at 09:12 -0300, Jason Gunthorpe wrote:
> > Also we could make this a variable rather than a constant and
> > choose
> > a more appropriate set of flags at boot time....
>
> It is a function, so it could check the CPU ID for the known broken
> devices and block them.

Sure, I meant in the abstract way. It's not a hot path so it doesnt
have to be a static key.

> > > > Why would that be a regression ?
> > >
> > > Using the WC submission flow when it doesn't work costs something
> > > like
> > > 10% performance vs using the non-WC flow.
> >
> > You mean the driver uses a different path to the HW which ahs that
> > overhead, not that MMIOs have that overhead right ?
>
> The different path has overhead of doing extra useless MMIOs because
> they don't combine

I see. This might have to end up being a TX2 specific hack until the
end of times...

Cheers,
Ben.