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

Richard GENOUD
2025-10-17T12:50:20Z

Le 17/10/2025 à 14:33, Nuno Sá a écrit :
> On Fri, 2025-10-17 at 12:54 +0200, Geert Uytterhoeven wrote:
>> 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>
>> ---
>
> Hopefully this gets merged soon. About time to have these variants (I do have a
> driver submitted - in review - which is adding yet another variant of this)
>
> Acked-by: Nuno Sá <nuno.sa@analog.com>

Same here, I would happily drop field_{get,set} from my series under
review to include bitfield.h
Thanks Geert!

Acked-by: Richard Genoud <richard.genoud@bootlin.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);

--
Richard Genoud, Bootlin
Embedded Linux and Kernel engineering