Re: [PATCH RFC 0/3] API for 128-bit IO access

Will Deacon
2018-01-26T18:11:51Z

On Fri, Jan 26, 2018 at 12:05:42PM +0300, Yury Norov wrote:
> On Wed, Jan 24, 2018 at 10:22:13AM +0000, Will Deacon wrote:
> > On Wed, Jan 24, 2018 at 12:05:16PM +0300, Yury Norov wrote:
> > > This series adds API for 128-bit memory IO access and enables it for ARM64.
> > > The original motivation for 128-bit API came from new Cavium network device
> > > driver. The hardware requires 128-bit access to make things work. See
> > > description in patch 3 for details.
> > >
> > > Also, starting from ARMv8.4, stp and ldp instructions become atomic, and
> > > API for 128-bit access would be helpful in core arm64 code.
> >
> > Only for normal, cacheable memory, so they're not suitable for IO accesses
> > as you're proposing here.
>
> Hi Will,
>
> Thanks for clarification.
>
> Could you elaborate, do you find 128-bit read/write API useless, or
> you just correct my comment?
>
> I think that ordered uniform 128-bit access API would be helpful, even
> if not atomic.

Sorry, but I strongly disagree here. Having an IO accessor that isn't
guaranteed to be atomic is a recipe for disaster if it's not called out
explicitly. You're much better off implementing something along the lines
of <linux/io-128-nonatomic-hi-lo.h> using 2x64-bit accessors like we already
have for the 2x32-bit case.

However, that doesn't solve your problem and is somewhat of a distraction.
I'd suggest that in your case, where you have a device that relies on
128-bit atomic access that is assumedly tightly integrated into your SoC,
then the driver just codes it's own local implementation of the accessor,
given that there isn't a way to guarantee the atomicity architecturally
(and even within your SoC it might not be atomic to all endpoints).

Will