lib: Add support for generic packing operations

This provides an unified API for accessing register bit fields
regardless of memory layout. The basic unit of data for these API
functions is the u64. The process of transforming an u64 from native CPU
encoding into the peripheral's encoding is called 'pack', and
transforming it from peripheral to native CPU encoding is 'unpack'.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 231 insertions, 0 deletions

diff --git a/lib/Kconfig b/lib/Kconfig
index a9e56539bd11..ac1fcf06d8ea 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -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
 
diff --git a/lib/Makefile b/lib/Makefile
index 3b08673e8881..7d4db18fabf1 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -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
diff --git a/lib/packing.c b/lib/packing.c
new file mode 100644
index 000000000000..50d1e9f2f5a7
--- /dev/null
+++ b/lib/packing.c
@@ -0,0 +1,213 @@
+// SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0
+/* Copyright (c) 2016-2018, NXP Semiconductors
+ * Copyright (c) 2018-2019, Vladimir Oltean <olteanv@gmail.com>
+ */
+#include <linux/packing.h>
+#include <linux/module.h>
+#include <linux/bitops.h>
+#include <linux/errno.h>
+#include <linux/types.h>
+
+static int get_le_offset(int offset)
+{
+	int closest_multiple_of_4;
+
+	closest_multiple_of_4 = (offset / 4) * 4;
+	offset -= closest_multiple_of_4;
+	return closest_multiple_of_4 + (3 - offset);
+}
+
+static int get_reverse_lsw32_offset(int offset, size_t len)
+{
+	int closest_multiple_of_4;
+	int word_index;
+
+	word_index = offset / 4;
+	closest_multiple_of_4 = word_index * 4;
+	offset -= closest_multiple_of_4;
+	word_index = (len / 4) - word_index - 1;
+	return word_index * 4 + offset;
+}
+
+static u64 bit_reverse(u64 val, unsigned int width)
+{
+	u64 new_val = 0;
+	unsigned int bit;
+	unsigned int i;
+
+	for (i = 0; i < width; i++) {
+		bit = (val & (1 << i)) != 0;
+		new_val |= (bit << (width - i - 1));
+	}
+	return new_val;
+}
+
+static void adjust_for_msb_right_quirk(u64 *to_write, int *box_start_bit,
+				       int *box_end_bit, u8 *box_mask)
+{
+	int box_bit_width = *box_start_bit - *box_end_bit + 1;
+	int new_box_start_bit, new_box_end_bit;
+
+	*to_write >>= *box_end_bit;
+	*to_write = bit_reverse(*to_write, box_bit_width);
+	*to_write <<= *box_end_bit;
+
+	new_box_end_bit   = box_bit_width - *box_start_bit - 1;
+	new_box_start_bit = box_bit_width - *box_end_bit - 1;
+	*box_mask = GENMASK_ULL(new_box_start_bit, new_box_end_bit);
+	*box_start_bit = new_box_start_bit;
+	*box_end_bit   = new_box_end_bit;
+}
+
+/**
+ * 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)
+{
+	/* Number of bits for storing "uval"
+	 * also width of the field to access in the pbuf
+	 */
+	u64 value_width;
+	/* Logical byte indices corresponding to the
+	 * start and end of the field.
+	 */
+	int plogical_first_u8, plogical_last_u8, box;
+
+	/* startbit is expected to be larger than endbit */
+	if (startbit < endbit)
+		/* Invalid function call */
+		return -EINVAL;
+
+	value_width = startbit - endbit + 1;
+	if (value_width > 64)
+		return -ERANGE;
+
+	/* Check if "uval" fits in "value_width" bits.
+	 * If value_width is 64, the check will fail, but any
+	 * 64-bit uval will surely fit.
+	 */
+	if (op == PACK && value_width < 64 && (*uval >= (1ull << value_width)))
+		/* Cannot store "uval" inside "value_width" bits.
+		 * Truncating "uval" is most certainly not desirable,
+		 * so simply erroring out is appropriate.
+		 */
+		return -ERANGE;
+
+	/* Initialize parameter */
+	if (op == UNPACK)
+		*uval = 0;
+
+	/* Iterate through an idealistic view of the pbuf as an u64 with
+	 * no quirks, u8 by u8 (aligned at u8 boundaries), from high to low
+	 * logical bit significance. "box" denotes the current logical u8.
+	 */
+	plogical_first_u8 = startbit / 8;
+	plogical_last_u8  = endbit / 8;
+
+	for (box = plogical_first_u8; box >= plogical_last_u8; box--) {
+		/* Bit indices into the currently accessed 8-bit box */
+		int box_start_bit, box_end_bit, box_addr;
+		u8  box_mask;
+		/* Corresponding bits from the unpacked u64 parameter */
+		int proj_start_bit, proj_end_bit;
+		u64 proj_mask;
+
+		/* This u8 may need to be accessed in its entirety
+		 * (from bit 7 to bit 0), or not, depending on the
+		 * input arguments startbit and endbit.
+		 */
+		if (box == plogical_first_u8)
+			box_start_bit = startbit % 8;
+		else
+			box_start_bit = 7;
+		if (box == plogical_last_u8)
+			box_end_bit = endbit % 8;
+		else
+			box_end_bit = 0;
+
+		/* We have determined the box bit start and end.
+		 * Now we calculate where this (masked) u8 box would fit
+		 * in the unpacked (CPU-readable) u64 - the u8 box's
+		 * projection onto the unpacked u64. Though the
+		 * box is u8, the projection is u64 because it may fall
+		 * anywhere within the unpacked u64.
+		 */
+		proj_start_bit = ((box * 8) + box_start_bit) - endbit;
+		proj_end_bit   = ((box * 8) + box_end_bit) - endbit;
+		proj_mask = GENMASK_ULL(proj_start_bit, proj_end_bit);
+		box_mask  = GENMASK_ULL(box_start_bit, box_end_bit);
+
+		/* Determine the offset of the u8 box inside the pbuf,
+		 * adjusted for quirks. The adjusted box_addr will be used for
+		 * effective addressing inside the pbuf (so it's not
+		 * logical any longer).
+		 */
+		box_addr = pbuflen - box - 1;
+		if (quirks & QUIRK_LITTLE_ENDIAN)
+			box_addr = get_le_offset(box_addr);
+		if (quirks & QUIRK_LSW32_IS_FIRST)
+			box_addr = get_reverse_lsw32_offset(box_addr,
+							    pbuflen);
+
+		if (op == UNPACK) {
+			u64 pval;
+
+			/* Read from pbuf, write to uval */
+			pval = ((u8 *)pbuf)[box_addr] & box_mask;
+			if (quirks & QUIRK_MSB_ON_THE_RIGHT)
+				adjust_for_msb_right_quirk(&pval,
+							   &box_start_bit,
+							   &box_end_bit,
+							   &box_mask);
+
+			pval >>= box_end_bit;
+			pval <<= proj_end_bit;
+			*uval &= ~proj_mask;
+			*uval |= pval;
+		} else {
+			u64 pval;
+
+			/* Write to pbuf, read from uval */
+			pval = (*uval) & proj_mask;
+			pval >>= proj_end_bit;
+			if (quirks & QUIRK_MSB_ON_THE_RIGHT)
+				adjust_for_msb_right_quirk(&pval,
+							   &box_start_bit,
+							   &box_end_bit,
+							   &box_mask);
+
+			pval <<= box_end_bit;
+			((u8 *)pbuf)[box_addr] &= ~box_mask;
+			((u8 *)pbuf)[box_addr] |= pval;
+		}
+	}
+	return 0;
+}
+EXPORT_SYMBOL(packing);
+
+MODULE_LICENSE("GPL v2");
+MODULE_DESCRIPTION("Generic bitfield packing and unpacking");