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

From: Catalin Marinas <[email-masked]>
Date: Tue, 27 Feb 2024 10:37:18 +0000

On Tue, Feb 20, 2024 at 09:17:08PM -0400, Jason Gunthorpe wrote:
> +/*
> + * 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) {                                  \
> +			__const_memcpy_toio_aligned##bits(_to, _from, 4); \
> +			_from += 4;                                       \
> +			_to += 4;                                         \
> +		}                                                         \
> +		if ((_count % 4) >= 2) {                                  \
> +			__const_memcpy_toio_aligned##bits(_to, _from, 2); \
> +			_from += 2;                                       \
> +			_to += 2;                                         \
> +		}                                                         \
> +		if (_count % 2)                                           \
> +			__const_memcpy_toio_aligned##bits(_to, _from, 1); \
> +	})

Do we actually need all this if count is not constant? If it's not
performance critical anywhere, I'd rather copy the generic
implementation, it's easier to read.

Otherwise, apart from the __raw_writeq() typo that Will mentioned, the
patch looks fine to me.

-- 
Catalin