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

quoted

Sent: 22 February 2024 22:36
To: David Laight <redacted>

On Thu, Feb 22, 2024 at 10:05:04PM +0000, David Laight wrote:

quoted

From: Jason Gunthorpe

quoted

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.

...

quoted

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.

...

quoted

+/*
+ * 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".

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