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

[Leon Romanovsky <leon@kernel.org>]

On Thu, Sep 10, 2020 at 10:46:00AM +0100, Lorenzo Pieralisi wrote:
> [+Jason]
>
> On Tue, Sep 08, 2020 at 09:33:42AM +1000, Benjamin Herrenschmidt wrote:
> > 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 ?
>
> Hi Jason,
>
> I was wondering if you could help us with this question, we are trying
> to understand what enabling arch_can_pci_mmap_wc() on arm64 would cause
> in mellanox drivers wrt mappings and whether there is an expected
> behaviour behind them, in particular whether there is an implicit
> reliance on x86 write-combine arch/interconnect details.

Sorry, somehow I missed this email.

The arch_can_pci_mmap_wc() used in IB representors, special mode where
we can't perform write-combine test below.

The commit 1f3db161881b ("IB/mlx5: Generally use the WC auto detection test result")
describes it.

I don't see any problem with enabling arch_can_pci_mmap_wc() on ARM.

Thanks

>
> Thanks,
> Lorenzo