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

Jason Gunthorpe
2020-09-16T14:36:03Z

On Wed, Sep 16, 2020 at 03:09:14PM +0100, Lorenzo Pieralisi wrote:
> On Wed, Sep 16, 2020 at 09:12:26AM -0300, Jason Gunthorpe wrote:
>
> [...]
>
> > > 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
>
> For my own information, this is IB user space driver code, correct ?

Yes, maybe DPDK too

> It tries to mmap buffer as WC and if it succeeds write into it in an
> optimized fashion (that is just pure overhead on platforms where
> normal NC memory - ie WC on arm64 - does not do what the
> _architecture_ defines it should).

Right, pure overhead if large PCI-E TLPs are not delivered to the
device.

Jason