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@xxxxxxxxxx> > > > > 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".