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

Benjamin Herrenschmidt
2020-09-07T23:34:14Z

On Thu, 2020-09-03 at 12:08 +0100, Lorenzo Pieralisi wrote:
> > It's been what other architectures have been doing for mroe than a
> > decade without significant issues... I don't think you should worry
> > too
> > much about this.
>
> Minus what I wrote above, I agree with you. I'd still be able to
> understand what this patch changes in the mellanox driver HW
> handling though - not sure what they expect from
> arch_can_pci_mmap_wc()
> returning 1.

I don't know enough to get into the finer details but looking a bit it
seems when this is set, they allow extra ioctls to create buffers
mapped with pgprot_writecombine().

I suppose this means faster MMIO backet buffers for small packets (ie,
non-DMA use case).

Also note that mlx5_ib_test_wc() only uses arch_can_pci_mmap_wc() for a
non-ROCE ethernet port on a PF... For anyting else, it just seems to
actually try to do it and see what happens :-)

Leon: Can you clarify the use of arch_can_pci_mmap_wc() in mlx5 and
whether you see an issue with enabling this on arm64 ?

Cheers,
Ben.