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Date: Mon, 11 Dec 2017 05:38:03 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Jie Deng <Jie.Deng1@synopsys.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC][PATCH] apparent big-endian bugs in dwc-xlgmac
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On Mon, Dec 11, 2017 at 05:05:20AM +0000, Al Viro wrote:

> What for?  Sure, this variant will work, but why bother with
> 	a = le32_to_cpu(b);
> 	(cpu_to_le32(a) & ....) | ....
> and how is that better than
> 	(b & ...) | ...
> 
> IDGI...  Mind you, I'm not sure if there is any point keeping _var in that thing,
> seeing that we use var only once - might be better off with
> 	((var) & ~cpu_to_le32(GENMASK(_pos + _len - 1, _pos))) | 	\
> 		cpu_to_le32(_val);					\

FWIW, seeing how many drivers end up open-coding that, I'm rather tempted to
add to linux/bitops.h or linux/byteorder/generic.h the following:

static inline __le16 le16_replace_bits(__le16 old, u16 v, int bit, int size)
{
	__le16 mask = cpu_to_le16(GENMASK(bit + size - 1, bit));
	return (old & ~mask) | (cpu_to_le16(v << bit) & mask);
}

static inline __le32 le32_replace_bits(__le32 old, u32 v, int bit, int size)
{
	__le32 mask = cpu_to_le32(GENMASK(bit + size - 1, bit));
	return (old & ~mask) | (cpu_to_le32(v << bit) & mask);
}

static inline __le64 le64_replace_bits(__le64 old, u64 v, int bit, int size)
{
	__le64 mask = cpu_to_le64(GENMASK_ULL(bit + size - 1, bit));
	return (old & ~mask) | (cpu_to_le64(v << bit) & mask);
}

static inline __be16 be16_replace_bits(__be16 old, u16 v, int bit, int size)
{
	__be16 mask = cpu_to_be16(GENMASK(bit + size - 1, bit));
	return (old & ~mask) | (cpu_to_be16(v << bit) & mask);
}

static inline __be32 be32_replace_bits(__be32 old, u32 v, int bit, int size)
{
	__be32 mask = cpu_to_be32(GENMASK(bit + size - 1, bit));
	return (old & ~mask) | (cpu_to_be32(v << bit) & mask);
}

static inline __be64 be64_replace_bits(__be64 old, u64 v, int bit, int size)
{
	__be64 mask = cpu_to_be64(GENMASK_ULL(bit + size - 1, bit));
	return (old & ~mask) | (cpu_to_be64(v << bit) & mask);
}

static inline u16 le16_get_bits(__le16 v, int bit, int size)
{
	return (le16_to_cpu(v) >> bit) & (BIT(size) - 1);
}

static inline u32 le32_get_bits(__le32 v, int bit, int size)
{
	return (le32_to_cpu(v) >> bit) & (BIT(size) - 1);
}

static inline u64 le64_get_bits(__le64 v, int bit, int size)
{
	return (le64_to_cpu(v) >> bit) & (BIT_ULL(size) - 1);
}

static inline u16 be16_get_bits(__be16 v, int bit, int size)
{
	return (be16_to_cpu(v) >> bit) & (BIT(size) - 1);
}

static inline u32 be32_get_bits(__be32 v, int bit, int size)
{
	return (be32_to_cpu(v) >> bit) & (BIT(size) - 1);
}

static inline u64 be64_get_bits(__be64 v, int bit, int size)
{
	return (be64_to_cpu(v) >> bit) & (BIT_ULL(size) - 1);
}

and let drivers use those...