[PATCH v4 2/4] bitfield: Add non-constant field_{prep,get}() helpers

Geert Uytterhoeven
2025-10-17T10:54:55Z

The existing FIELD_{GET,PREP}() macros are limited to compile-time
constants. However, it is very common to prepare or extract bitfield
elements where the bitfield mask is not a compile-time constant.

To avoid this limitation, the AT91 clock driver and several other
drivers already have their own non-const field_{prep,get}() macros.
Make them available for general use by consolidating them in
<linux/bitfield.h>, and improve them slightly:
1. Avoid evaluating macro parameters more than once,
2. Replace "ffs() - 1" by "__ffs()",
3. Support 64-bit use on 32-bit architectures.

This is deliberately not merged into the existing FIELD_{GET,PREP}()
macros, as people expressed the desire to keep stricter variants for
increased safety, or for performance critical paths.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Crt Mori <cmo@melexis.com>
---
v4:
- Add Acked-by,
- Rebase on top of commit 7c68005a46108ffa ("crypto: qat - relocate
power management debugfs helper APIs") in v6.17-rc1,
- Convert more recently introduced upstream copies:
- drivers/edac/ie31200_edac.c
- drivers/iio/dac/ad3530r.c

v3:
- Add Acked-by,
- Drop underscores from macro parameters,
- Use __auto_type where possible,
- Correctly cast reg to the mask type,
- Introduces __val and __reg intermediates to simplify the actual
operation,
- Drop unneeded parentheses,
- Clarify having both FIELD_{GET,PREP}() and field_{get,prep}(),

v2:
- Cast val resp. reg to the mask type,
- Fix 64-bit use on 32-bit architectures,
- Convert new upstream users:
- drivers/crypto/intel/qat/qat_common/adf_gen4_pm_debugfs.c
- drivers/gpio/gpio-aspeed.c
- drivers/iio/temperature/mlx90614.c
- drivers/pinctrl/nuvoton/pinctrl-ma35.c
- sound/usb/mixer_quirks.c
- Convert new user queued in renesas-devel for v6.15:
- drivers/soc/renesas/rz-sysc.c
---
drivers/clk/at91/clk-peripheral.c | 1 +
drivers/clk/at91/pmc.h | 3 --
.../intel/qat/qat_common/adf_pm_dbgfs_utils.c | 8 +----
drivers/edac/ie31200_edac.c | 4 +--
drivers/gpio/gpio-aspeed.c | 5 +--
drivers/iio/dac/ad3530r.c | 3 --
drivers/iio/temperature/mlx90614.c | 5 +--
drivers/pinctrl/nuvoton/pinctrl-ma35.c | 4 ---
drivers/soc/renesas/rz-sysc.c | 3 +-
include/linux/bitfield.h | 36 +++++++++++++++++++
sound/usb/mixer_quirks.c | 4 ---
11 files changed, 42 insertions(+), 34 deletions(-)

diff --git a/drivers/clk/at91/clk-peripheral.c b/drivers/clk/at91/clk-peripheral.c
index e700f40fd87f9327..e7208c47268b6397 100644
--- a/drivers/clk/at91/clk-peripheral.c
+++ b/drivers/clk/at91/clk-peripheral.c
@@ -3,6 +3,7 @@
* Copyright (C) 2013 Boris BREZILLON <b.brezillon@overkiz.com>
*/

+#include <linux/bitfield.h>
#include <linux/bitops.h>
#include <linux/clk-provider.h>
#include <linux/clkdev.h>
diff --git a/drivers/clk/at91/pmc.h b/drivers/clk/at91/pmc.h
index 5daa32c4cf2540d7..543d7aee8d248cdb 100644
--- a/drivers/clk/at91/pmc.h
+++ b/drivers/clk/at91/pmc.h
@@ -117,9 +117,6 @@ struct at91_clk_pms {
unsigned int parent;
};

-#define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
-#define field_prep(_mask, _val) (((_val) << (ffs(_mask) - 1)) & (_mask))
-
#define ndck(a, s) (a[s - 1].id + 1)
#define nck(a) (a[ARRAY_SIZE(a) - 1].id + 1)

diff --git a/drivers/crypto/intel/qat/qat_common/adf_pm_dbgfs_utils.c b/drivers/crypto/intel/qat/qat_common/adf_pm_dbgfs_utils.c
index 69295a9ddf0ac92f..4ccc94ed9493a64c 100644
--- a/drivers/crypto/intel/qat/qat_common/adf_pm_dbgfs_utils.c
+++ b/drivers/crypto/intel/qat/qat_common/adf_pm_dbgfs_utils.c
@@ -1,18 +1,12 @@
// SPDX-License-Identifier: GPL-2.0-only
/* Copyright(c) 2025 Intel Corporation */
+#include <linux/bitfield.h>
#include <linux/bitops.h>
#include <linux/sprintf.h>
#include <linux/string_helpers.h>

#include "adf_pm_dbgfs_utils.h"

-/*
- * This is needed because a variable is used to index the mask at
- * pm_scnprint_table(), making it not compile time constant, so the compile
- * asserts from FIELD_GET() or u32_get_bits() won't be fulfilled.
- */
-#define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
-
#define PM_INFO_MAX_KEY_LEN 21

static int pm_scnprint_table(char *buff, const struct pm_status_row *table,
diff --git a/drivers/edac/ie31200_edac.c b/drivers/edac/ie31200_edac.c
index 5a080ab65476dacf..dfc9a9cecd74207d 100644
--- a/drivers/edac/ie31200_edac.c
+++ b/drivers/edac/ie31200_edac.c
@@ -44,6 +44,7 @@
* but lo_hi_readq() ensures that we are safe across all e3-1200 processors.
*/

+#include <linux/bitfield.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/pci.h>
@@ -139,9 +140,6 @@
#define IE31200_CAPID0_DDPCD BIT(6)
#define IE31200_CAPID0_ECC BIT(1)

-/* Non-constant mask variant of FIELD_GET() */
-#define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
-
static int nr_channels;
static struct pci_dev *mci_pdev;
static int ie31200_registered = 1;
diff --git a/drivers/gpio/gpio-aspeed.c b/drivers/gpio/gpio-aspeed.c
index 7953a9c4e36d7550..3da999334971d501 100644
--- a/drivers/gpio/gpio-aspeed.c
+++ b/drivers/gpio/gpio-aspeed.c
@@ -5,6 +5,7 @@
* Joel Stanley <joel@jms.id.au>
*/

+#include <linux/bitfield.h>
#include <linux/cleanup.h>
#include <linux/clk.h>
#include <linux/gpio/aspeed.h>
@@ -31,10 +32,6 @@
#include <linux/gpio/consumer.h>
#include "gpiolib.h"

-/* Non-constant mask variant of FIELD_GET() and FIELD_PREP() */
-#define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
-#define field_prep(_mask, _val) (((_val) << (ffs(_mask) - 1)) & (_mask))
-
#define GPIO_G7_IRQ_STS_BASE 0x100
#define GPIO_G7_IRQ_STS_OFFSET(x) (GPIO_G7_IRQ_STS_BASE + (x) * 0x4)
#define GPIO_G7_CTRL_REG_BASE 0x180
diff --git a/drivers/iio/dac/ad3530r.c b/drivers/iio/dac/ad3530r.c
index 6134613777b8e1d4..b97b46090d808ee7 100644
--- a/drivers/iio/dac/ad3530r.c
+++ b/drivers/iio/dac/ad3530r.c
@@ -53,9 +53,6 @@
#define AD3530R_MAX_CHANNELS 8
#define AD3531R_MAX_CHANNELS 4

-/* Non-constant mask variant of FIELD_PREP() */
-#define field_prep(_mask, _val) (((_val) << (ffs(_mask) - 1)) & (_mask))
-
enum ad3530r_mode {
AD3530R_NORMAL_OP,
AD3530R_POWERDOWN_1K,
diff --git a/drivers/iio/temperature/mlx90614.c b/drivers/iio/temperature/mlx90614.c
index 8a44a00bfd5ece38..1ad21b73e1b44cb0 100644
--- a/drivers/iio/temperature/mlx90614.c
+++ b/drivers/iio/temperature/mlx90614.c
@@ -22,6 +22,7 @@
* the "wakeup" GPIO is not given, power management will be disabled.
*/

+#include <linux/bitfield.h>
#include <linux/delay.h>
#include <linux/err.h>
#include <linux/gpio/consumer.h>
@@ -68,10 +69,6 @@
#define MLX90614_CONST_SCALE 20 /* Scale in milliKelvin (0.02 * 1000) */
#define MLX90614_CONST_FIR 0x7 /* Fixed value for FIR part of low pass filter */

-/* Non-constant mask variant of FIELD_GET() and FIELD_PREP() */
-#define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
-#define field_prep(_mask, _val) (((_val) << (ffs(_mask) - 1)) & (_mask))
-
struct mlx_chip_info {
/* EEPROM offsets with 16-bit data, MSB first */
/* emissivity correction coefficient */
diff --git a/drivers/pinctrl/nuvoton/pinctrl-ma35.c b/drivers/pinctrl/nuvoton/pinctrl-ma35.c
index cdad01d68a37e365..8d71dc53cc1de1f8 100644
--- a/drivers/pinctrl/nuvoton/pinctrl-ma35.c
+++ b/drivers/pinctrl/nuvoton/pinctrl-ma35.c
@@ -81,10 +81,6 @@
#define MVOLT_1800 0
#define MVOLT_3300 1

-/* Non-constant mask variant of FIELD_GET() and FIELD_PREP() */
-#define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
-#define field_prep(_mask, _val) (((_val) << (ffs(_mask) - 1)) & (_mask))
-
static const char * const gpio_group_name[] = {
"gpioa", "gpiob", "gpioc", "gpiod", "gpioe", "gpiof", "gpiog",
"gpioh", "gpioi", "gpioj", "gpiok", "gpiol", "gpiom", "gpion",
diff --git a/drivers/soc/renesas/rz-sysc.c b/drivers/soc/renesas/rz-sysc.c
index 9f79e299e6f41641..73eaf8b9d69f7208 100644
--- a/drivers/soc/renesas/rz-sysc.c
+++ b/drivers/soc/renesas/rz-sysc.c
@@ -5,6 +5,7 @@
* Copyright (C) 2024 Renesas Electronics Corp.
*/

+#include <linux/bitfield.h>
#include <linux/cleanup.h>
#include <linux/io.h>
#include <linux/mfd/syscon.h>
@@ -16,8 +17,6 @@

#include "rz-sysc.h"

-#define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
-
/**
* struct rz_sysc - RZ SYSC private data structure
* @base: SYSC base address
diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 7ff817bdae19b468..c999fe70076f6684 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -220,4 +220,40 @@ __MAKE_OP(64)
#undef __MAKE_OP
#undef ____MAKE_OP

+/**
+ * field_prep() - prepare a bitfield element
+ * @mask: shifted mask defining the field's length and position
+ * @val: value to put in the field
+ *
+ * field_prep() masks and shifts up the value. The result should be
+ * combined with other fields of the bitfield using logical OR.
+ * Unlike FIELD_PREP(), @mask is not limited to a compile-time constant.
+ */
+#define field_prep(mask, val) \
+ ({ \
+ __auto_type __mask = (mask); \
+ typeof(mask) __val = (val); \
+ unsigned int __shift = sizeof(mask) <= 4 ? \
+ __ffs(__mask) : __ffs64(__mask); \
+ (__val << __shift) & __mask; \
+ })
+
+/**
+ * field_get() - extract a bitfield element
+ * @mask: shifted mask defining the field's length and position
+ * @reg: value of entire bitfield
+ *
+ * field_get() extracts the field specified by @mask from the
+ * bitfield passed in as @reg by masking and shifting it down.
+ * Unlike FIELD_GET(), @mask is not limited to a compile-time constant.
+ */
+#define field_get(mask, reg) \
+ ({ \
+ __auto_type __mask = (mask); \
+ typeof(mask) __reg = (reg); \
+ unsigned int __shift = sizeof(mask) <= 4 ? \
+ __ffs(__mask) : __ffs64(__mask); \
+ (__reg & __mask) >> __shift; \
+ })
+
#endif
diff --git a/sound/usb/mixer_quirks.c b/sound/usb/mixer_quirks.c
index 828af3095b86ee0a..6eee89cbc0867f2b 100644
--- a/sound/usb/mixer_quirks.c
+++ b/sound/usb/mixer_quirks.c
@@ -3311,10 +3311,6 @@ static int snd_bbfpro_controls_create(struct usb_mixer_interface *mixer)
#define RME_DIGIFACE_REGISTER(reg, mask) (((reg) << 16) | (mask))
#define RME_DIGIFACE_INVERT BIT(31)

-/* Nonconst helpers */
-#define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
-#define field_prep(_mask, _val) (((_val) << (ffs(_mask) - 1)) & (_mask))
-
static int snd_rme_digiface_write_reg(struct snd_kcontrol *kcontrol, int item, u16 mask, u16 val)
{
struct usb_mixer_elem_list *list = snd_kcontrol_chip(kcontrol);
--
2.43.0