Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX_copy()

Niklas Schnelle
2024-02-23T11:02:41Z

On Fri, 2024-02-23 at 09:07 +0000, David Laight wrote:
> From: Jason Gunthorpe
> > Sent: 22 February 2024 22:36
> > To: David Laight <David.Laight@ACULAB.COM>
> >
> > On Thu, Feb 22, 2024 at 10:05:04PM +0000, David Laight wrote:
> > > From: Jason Gunthorpe
> > > > Sent: 21 February 2024 01:17
> > > >
> > > > The kernel provides driver support for using write combining IO memory
> > > > through the __iowriteXX_copy() API which is commonly used as an optional
> > > > optimization to generate 16/32/64 byte MemWr TLPs in a PCIe environment.
> > > >
> > > ...
> > > > Implement __iowrite32/64_copy() specifically for ARM64 and use inline
> > > > assembly to build consecutive blocks of STR instructions. Provide direct
> > > > support for 64/32/16 large TLP generation in this manner. Optimize for
> > > > common constant lengths so that the compiler can directly inline the store
> > > > blocks.
> > > ...
> > > > +/*
> > > > + * This generates a memcpy that works on a from/to address which is aligned to
> > > > + * bits. Count is in terms of the number of bits sized quantities to copy. It
> > > > + * optimizes to use the STR groupings when possible so that it is WC friendly.
> > > > + */
> > > > +#define memcpy_toio_aligned(to, from, count, bits) \
> > > > + ({ \
> > > > + volatile u##bits __iomem *_to = to; \
> > > > + const u##bits *_from = from; \
> > > > + size_t _count = count; \
> > > > + const u##bits *_end_from = _from + ALIGN_DOWN(_count, 8); \
> > > > + \
> > > > + for (; _from < _end_from; _from += 8, _to += 8) \
> > > > + __const_memcpy_toio_aligned##bits(_to, _from, 8); \
> > > > + if ((_count % 8) >= 4) {
> > >
> > > If (_count & 4) {
> >
> > That would be obfuscating, IMHO. The compiler doesn't need such things
> > to generate optimal code.
>
> Try it: https://godbolt.org/z/EvvGrTxv3
> And it isn't that obfuscated - no more so than your version.

The godbolt link does "n % 8 > 4" instead of "... >= 4" as in Jason's
original code. With ">=" the compiled code matches that for "n & 4".