netdev - Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX

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Message-ID: <61931c626d9bc54369d73fda8f8ee59bbb5e95bc.camel@linux.ibm.com>
Date: Fri, 23 Feb 2024 12:01:24 +0100
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: David Laight <David.Laight@...LAB.COM>,
        "'Jason Gunthorpe'"
 <jgg@...dia.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andrew Morton
 <akpm@...ux-foundation.org>,
        Christian Borntraeger
 <borntraeger@...ux.ibm.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen
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        Eric Dumazet <edumazet@...gle.com>,
        Gerald Schaefer
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        Vasily Gorbik <gor@...ux.ibm.com>, Heiko
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        "H. Peter Anvin" <hpa@...or.com>,
        Justin
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        Mark Rutland
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        Michael Guralnik <michaelgur@...lanox.com>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>,
        Will Deacon
 <will@...nel.org>
Subject: Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX_copy()

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@...LAB.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".