lib: Add support for generic packing operations (554aae35) · Commits · 戴 / test

Documentation/packing.txt

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		================================================
		Generic bitfield packing and unpacking functions
		================================================

		Problem statement
		-----------------

		When working with hardware, one has to choose between several approaches of
		interfacing with it.
		One can memory-map a pointer to a carefully crafted struct over the hardware
		device's memory region, and access its fields as struct members (potentially
		declared as bitfields). But writing code this way would make it less portable,
		due to potential endianness mismatches between the CPU and the hardware device.
		Additionally, one has to pay close attention when translating register
		definitions from the hardware documentation into bit field indices for the
		structs. Also, some hardware (typically networking equipment) tends to group
		its register fields in ways that violate any reasonable word boundaries
		(sometimes even 64 bit ones). This creates the inconvenience of having to
		define "high" and "low" portions of register fields within the struct.
		A more robust alternative to struct field definitions would be to extract the
		required fields by shifting the appropriate number of bits. But this would
		still not protect from endianness mismatches, except if all memory accesses
		were performed byte-by-byte. Also the code can easily get cluttered, and the
		high-level idea might get lost among the many bit shifts required.
		Many drivers take the bit-shifting approach and then attempt to reduce the
		clutter with tailored macros, but more often than not these macros take
		shortcuts that still prevent the code from being truly portable.

		The solution
		------------

		This API deals with 2 basic operations:
		- Packing a CPU-usable number into a memory buffer (with hardware
		constraints/quirks)
		- Unpacking a memory buffer (which has hardware constraints/quirks)
		into a CPU-usable number.

		The API offers an abstraction over said hardware constraints and quirks,
		over CPU endianness and therefore between possible mismatches between
		the two.

		The basic unit of these API functions is the u64. From the CPU's
		perspective, bit 63 always means bit offset 7 of byte 7, albeit only
		logically. The question is: where do we lay this bit out in memory?

		The following examples cover the memory layout of a packed u64 field.
		The byte offsets in the packed buffer are always implicitly 0, 1, ... 7.
		What the examples show is where the logical bytes and bits sit.

		1. Normally (no quirks), we would do it like this:

		63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32
		7 6 5 4
		31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
		3 2 1 0

		That is, the MSByte (7) of the CPU-usable u64 sits at memory offset 0, and the
		LSByte (0) of the u64 sits at memory offset 7.
		This corresponds to what most folks would regard to as "big endian", where
		bit i corresponds to the number 2^i. This is also referred to in the code
		comments as "logical" notation.


		2. If QUIRK_MSB_ON_THE_RIGHT is set, we do it like this:

		56 57 58 59 60 61 62 63 48 49 50 51 52 53 54 55 40 41 42 43 44 45 46 47 32 33 34 35 36 37 38 39
		7 6 5 4
		24 25 26 27 28 29 30 31 16 17 18 19 20 21 22 23 8 9 10 11 12 13 14 15 0 1 2 3 4 5 6 7
		3 2 1 0

		That is, QUIRK_MSB_ON_THE_RIGHT does not affect byte positioning, but
		inverts bit offsets inside a byte.


		3. If QUIRK_LITTLE_ENDIAN is set, we do it like this:

		39 38 37 36 35 34 33 32 47 46 45 44 43 42 41 40 55 54 53 52 51 50 49 48 63 62 61 60 59 58 57 56
		4 5 6 7
		7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8 23 22 21 20 19 18 17 16 31 30 29 28 27 26 25 24
		0 1 2 3

		Therefore, QUIRK_LITTLE_ENDIAN means that inside the memory region, every
		byte from each 4-byte word is placed at its mirrored position compared to
		the boundary of that word.

		4. If QUIRK_MSB_ON_THE_RIGHT and QUIRK_LITTLE_ENDIAN are both set, we do it
		like this:

		32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
		4 5 6 7
		0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
		0 1 2 3


		5. If just QUIRK_LSW32_IS_FIRST is set, we do it like this:

		31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
		3 2 1 0
		63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32
		7 6 5 4

		In this case the 8 byte memory region is interpreted as follows: first
		4 bytes correspond to the least significant 4-byte word, next 4 bytes to
		the more significant 4-byte word.


		6. If QUIRK_LSW32_IS_FIRST and QUIRK_MSB_ON_THE_RIGHT are set, we do it like
		this:

		24 25 26 27 28 29 30 31 16 17 18 19 20 21 22 23 8 9 10 11 12 13 14 15 0 1 2 3 4 5 6 7
		3 2 1 0
		56 57 58 59 60 61 62 63 48 49 50 51 52 53 54 55 40 41 42 43 44 45 46 47 32 33 34 35 36 37 38 39
		7 6 5 4


		7. If QUIRK_LSW32_IS_FIRST and QUIRK_LITTLE_ENDIAN are set, it looks like
		this:

		7 6 5 4 3 2 1 0 15 14 13 12 11 10 9 8 23 22 21 20 19 18 17 16 31 30 29 28 27 26 25 24
		0 1 2 3
		39 38 37 36 35 34 33 32 47 46 45 44 43 42 41 40 55 54 53 52 51 50 49 48 63 62 61 60 59 58 57 56
		4 5 6 7


		8. If QUIRK_LSW32_IS_FIRST, QUIRK_LITTLE_ENDIAN and QUIRK_MSB_ON_THE_RIGHT
		are set, it looks like this:

		0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
		0 1 2 3
		32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
		4 5 6 7


		We always think of our offsets as if there were no quirk, and we translate
		them afterwards, before accessing the memory region.

		Intended use
		------------

		Drivers that opt to use this API first need to identify which of the above 3
		quirk combinations (for a total of 8) match what the hardware documentation
		describes. Then they should wrap the packing() function, creating a new
		xxx_packing() that calls it using the proper QUIRK_* one-hot bits set.

		The packing() function returns an int-encoded error code, which protects the
		programmer against incorrect API use. The errors are not expected to occur
		durring runtime, therefore it is reasonable for xxx_packing() to return void
		and simply swallow those errors. Optionally it can dump stack or print the
		error description.

MAINTAINERS

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		@@ -11673,6 +11673,14 @@ L: linux-i2c@vger.kernel.org
		S: Orphan
		F: drivers/i2c/busses/i2c-pasemi.c

		PACKING
		M: Vladimir Oltean <olteanv@gmail.com>
		L: netdev@vger.kernel.org
		S: Supported
		F: lib/packing.c
		F: include/linux/packing.h
		F: Documentation/packing.txt

		PADATA PARALLEL EXECUTION MECHANISM
		M: Steffen Klassert <steffen.klassert@secunet.com>
		L: linux-crypto@vger.kernel.org

include/linux/packing.h

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		/* SPDX-License-Identifier: BSD-3-Clause
		* Copyright (c) 2016-2018, NXP Semiconductors
		* Copyright (c) 2018-2019, Vladimir Oltean <olteanv@gmail.com>
		*/
		#ifndef _LINUX_PACKING_H
		#define _LINUX_PACKING_H

		#include <linux/types.h>
		#include <linux/bitops.h>

		#define QUIRK_MSB_ON_THE_RIGHT BIT(0)
		#define QUIRK_LITTLE_ENDIAN BIT(1)
		#define QUIRK_LSW32_IS_FIRST BIT(2)

		enum packing_op {
		PACK,
		UNPACK,
		};

		/**
		* packing - Convert numbers (currently u64) between a packed and an unpacked
		* format. Unpacked means laid out in memory in the CPU's native
		* understanding of integers, while packed means anything else that
		* requires translation.
		*
		* @pbuf: Pointer to a buffer holding the packed value.
		* @uval: Pointer to an u64 holding the unpacked value.
		* @startbit: The index (in logical notation, compensated for quirks) where
		* the packed value starts within pbuf. Must be larger than, or
		* equal to, endbit.
		* @endbit: The index (in logical notation, compensated for quirks) where
		* the packed value ends within pbuf. Must be smaller than, or equal
		* to, startbit.
		* @op: If PACK, then uval will be treated as const pointer and copied (packed)
		* into pbuf, between startbit and endbit.
		* If UNPACK, then pbuf will be treated as const pointer and the logical
		* value between startbit and endbit will be copied (unpacked) to uval.
		* @quirks: A bit mask of QUIRK_LITTLE_ENDIAN, QUIRK_LSW32_IS_FIRST and
		* QUIRK_MSB_ON_THE_RIGHT.
		*
		* Return: 0 on success, EINVAL or ERANGE if called incorrectly. Assuming
		* correct usage, return code may be discarded.
		* If op is PACK, pbuf is modified.
		* If op is UNPACK, uval is modified.
		*/
		int packing(void pbuf, u64 uval, int startbit, int endbit, size_t pbuflen,
		enum packing_op op, u8 quirks);

		#endif

lib/Kconfig

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		@@ -18,6 +18,23 @@ config RAID6_PQ_BENCHMARK
		Benchmark all available RAID6 PQ functions on init and choose the
		fastest one.

		config PACKING
		bool "Generic bitfield packing and unpacking"
		default n
		help
		This option provides the packing() helper function, which permits
		converting bitfields between a CPU-usable representation and a
		memory representation that can have any combination of these quirks:
		- Is little endian (bytes are reversed within a 32-bit group)
		- The least-significant 32-bit word comes first (within a 64-bit
		group)
		- The most significant bit of a byte is at its right (bit 0 of a
		register description is numerically 2^7).
		Drivers may use these helpers to match the bit indices as described
		in the data sheets of the peripherals they are in control of.

		When in doubt, say N.

		config BITREVERSE
		tristate

lib/Makefile

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		@@ -108,6 +108,7 @@ obj-$(CONFIG_DEBUG_LIST) += list_debug.o
		obj-$(CONFIG_DEBUG_OBJECTS) += debugobjects.o

		obj-$(CONFIG_BITREVERSE) += bitrev.o
		obj-$(CONFIG_PACKING) += packing.o
		obj-$(CONFIG_RATIONAL) += rational.o
		obj-$(CONFIG_CRC_CCITT) += crc-ccitt.o
		obj-$(CONFIG_CRC16) += crc16.o

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