Remove non-portable bit-field layout assumptions tree-wide

[parthitce]

Problem Description

Many places use a C bit-field struct or union to map a fixed bit layout: a hardware register, a DMA descriptor, or a wire or flash field. The C standard does not fix that layout, so this is not portable.

From: https://github.com/zephyrproject-rtos/zephyr/blob/main/doc/contribute/coding_guidelines/index.rst?plain=1#L38

• C11 6.7.2.1: the bit order within a unit, and whether a field spills into the next unit, are implementation defined.
• Zephyr already forbids relying on this: MISRA Dir 1.1 (Required) maps to CERT EXP11-C, "Do not make assumptions regarding the layout of structures with bit-fields."

__packed does not help. It sets byte padding, not bit order.

Already seen in practice: #113320 fixes a bit-field overlay in irq_multilevel.h that gave wrong values on big-endian. A guideline PR #113584 was closed as already covered by the rule above.

A scan of main at b5fb5c845b9e (2026-07-16) finds 738 sites in 238 files. This is a wide clean-up, so it is an RFC, not one bug.

Proposed Change (Summary)

1. Enforce the rule we already have (EXP11-C). Do not add a new rule. Replace each bit-field overlay with plain integer access using FIELD_GET, FIELD_PREP, and GENMASK, with byte-order helpers (sys_le32_to_cpu and so on) at hardware or wire boundaries.
2. Split the work by tree area. Each area is one small PR set. This issue is the tracking issue.
3. Add a CI check (checkpatch or Coccinelle) so the pattern does not come back.

No behaviour change is intended. The encodings stay the same. Only the access method changes.

The findings fall into four groups:

Group	Meaning	Sites	Files	Confidence
C1	Direct bit-field unions	264	76	deterministic
C2	Transitive bit-field unions	38	32	deterministic
C3	Packed structs with bit-fields	192	78	deterministic
C4	Bit-field struct over raw memory (heuristic)	244	88	heuristic, confirm each

Sites per area:

Area	C1	C2	C3	C4	Total
arch	2	0	0	0	2
drivers	132	6	79	175	392
subsys	12	11	42	19	84
lib	0	0	12	8	20
soc	87	2	8	4	101
include	29	16	46	0	91
boards	1	0	1	0	2
tests	0	3	0	37	40
samples	1	0	4	1	6
Total	264	38	192	244	738

Notes: drivers has the most work. soc is mostly register headers. include holds API headers owned by each subsystem. kernel has no findings.

Proposed Change (Detailed)

The fix pattern:

/* Before: layout is implementation defined */
union {
        uint32_t val;
        struct { uint32_t mode : 2; uint32_t vector : 8; } bits;
} reg;
reg.val      = sys_read32(ADDR);
uint8_t mode = reg.bits.mode;

/* After: portable and explicit */
#define REG_MODE   GENMASK(1, 0)
#define REG_VECTOR GENMASK(9, 2)
uint32_t reg = sys_read32(ADDR);   /* use sys_le32_to_cpu at a LE boundary */
uint8_t  mode = FIELD_GET(REG_MODE, reg);

Steps per PR: add named GENMASK masks, convert reads and writes to FIELD_GET and FIELD_PREP, add the right byte-order helper at each boundary, keep a BUILD_ASSERT on the word size, then remove the bit-field struct or union. Do the big-endian or cross-machine sites (protocol, DMA, flash) first.

C1 to C3 are deterministic and can be treated as a firm list. C4 is heuristic. It keeps only strong signals (register casts, casts of typed byte buffers, memcpy between a bit-field object and a byte buffer). Confirm each C4 site before its PR.

Every site is listed below. Each file is a link to the snapshot. The numbers after it are the lines in that file.

C1: Direct bit-field unions (264 sites, 76 files)

Area	File	Lines
arch	arch/arm/core/mmu/arm_mmu_priv.h	L87, L124
drivers	drivers/audio/mic_privacy/intel/mic_privacy_registers.h	L23, L95, L125, L206
drivers	drivers/can/can_mcan.h	L910, L951, L992
drivers	drivers/clock_control/clock_control_mchp_pic32cm_jh.c	L99
drivers	drivers/clock_control/clock_control_mchp_pic32cm_pl.c	L48
drivers	drivers/clock_control/clock_control_mchp_pic32cz_ca.c	L92
drivers	drivers/clock_control/clock_control_mchp_sam_d5x_e5x.c	L118
drivers	drivers/crypto/crypto_intel_sha_priv.h	L58
drivers	drivers/crypto/crypto_intel_sha_registers.h	L12, L36, L45, L54, L62, L70, L78, L86, L94, L101, L109, L116, L129, L137, L144
drivers	drivers/dai/intel/alh/alh.h	L52
drivers	drivers/dai/intel/ssp/dai-params-intel-ipc4.h	L136, L162, L187
drivers	drivers/ethernet/intel/eth_intel_igc_priv.h	L369, L409
drivers	drivers/flash/flash_cadence_nand_ll.h	L334
drivers	drivers/fpga/fpga_altera_agilex_bridge.h	L75, L105, L121
drivers	drivers/gpio/gpio_max14906.h	L93, L107, L121, L135, L149, L163, L177, L191, L205, L218, L228, L238, L252, L265
drivers	drivers/gpio/gpio_max14916.h	L98, L112, L125, L136, L150, L164, L178, L192
drivers	drivers/gpio/gpio_max2219x.c	L91, L105, L119, L132, L145, L156
drivers	drivers/i2c/i2c_dw_registers.h	L16, L72, L94, L120, L132, L148
drivers	drivers/i2c/i2c_wch.c	L40
drivers	drivers/interrupt_controller/intc_clic.h	L46, L60, L69, L78, L101, L114
drivers	drivers/interrupt_controller/intc_intel_vtd.h	L19, L62, L81, L99
drivers	drivers/memc/memc_mspi_aps_z8.h	L43, L55, L65, L75, L86, L96, L104
drivers	drivers/mfd/mfd_infineon_hppass.c	L59, L75, L89, L106, L134, L150
drivers	drivers/mipi_dbi/mipi_dbi_nxp_lcdic.c	L53
drivers	drivers/pcie/host/ptm.h	L15, L29
drivers	drivers/pcie/host/vc.h	L16, L33, L44, L72, L89, L108
drivers	drivers/sensor/bosch/bmi160/bmi160.h	L444
drivers	drivers/sensor/bosch/bmm350/bmm350.h	L454
drivers	drivers/sensor/lm75/lm75.c	L50
drivers	drivers/sensor/lm77/lm77.c	L35
drivers	drivers/sensor/pixart/paa3905/paa3905.h	L25
drivers	drivers/sensor/pni/rm3100/rm3100.h	L31
drivers	drivers/sensor/tdk/icm45686/icm45686.h	L50
drivers	drivers/spi/spi_it8xxx2.c	L71
drivers	drivers/uaol/uaol_intel_adsp.h	L40, L53, L63, L73, L82, L89, L96, L107, L119, L129, L140, L147, L154, L173, L183, L196
drivers	drivers/usb/device/usb_dc_kinetis.c	L56
drivers	drivers/usb/udc/udc_kinetis.c	L55
drivers	drivers/w1/w1_ds2477_85_common.h	L180
drivers	drivers/watchdog/wdt_nxp_fs26.c	L51
subsys	subsys/bluetooth/controller/ll_sw/lll.h	L515, L517
subsys	subsys/bluetooth/controller/ll_sw/lll_conn.h	L90
subsys	subsys/bluetooth/controller/ll_sw/ull_conn_iso_types.h	L95
subsys	subsys/bluetooth/controller/ll_sw/ull_conn_types.h	L170
subsys	subsys/bluetooth/controller/ll_sw/ull_llcp_internal.h	L186
subsys	subsys/lorawan/services/frag_transport.c	L93, L108
subsys	subsys/net/l2/ethernet/gptp/gptp_messages.h	L296
subsys	subsys/net/l2/wifi/wifi_shell.c	L87
subsys	subsys/net/lib/ptp/ds.h	L190
subsys	subsys/portability/posix/options/posix_internal.h	L39
soc	soc/amd/acp_7_0/include/acp70_chip_reg.h	L15, L25, L33, L42, L107, L120, L133, L142, L154
soc	soc/amd/acp_7_0/include/acp70_fw_scratch_mem.h	L54
soc	soc/amd/acp_7_x/include/acp7x_chip_reg.h	L13, L22, L29, L36, L57, L79, L97, L105, L115, L122, L129, L139, L148, L159, L174, L185, L191, L198, L204, L211, L218, L225, L232, L239, L246, L253, L260, L267, L274, L281, L287, L293, L299, L305, L312, L319, L326, L334, L346, L357, L367, L377, L385, L397, L404, L411, L419, L427, L434, L442, L451, L464, L475, L496, L507, L522, L534, L547, L564, L579, L590, L601, L615, L633, L647, L671, L680
soc	soc/amd/acp_7_x/include/acp7x_fw_scratch_mem.h	L54
soc	soc/intel/intel_adsp/common/include/manifest.h	L40
soc	soc/ite/ec/it8xxx2/chip_chipregs.h	L521, L584, L668
soc	soc/renesas/rz/common/pinctrl_rza.h	L18
soc	soc/renesas/rz/common/pinctrl_rzg.h	L18
soc	soc/renesas/rz/common/pinctrl_rzn.h	L20
soc	soc/renesas/rz/common/pinctrl_rzt.h	L20
soc	soc/renesas/rz/common/pinctrl_rzv.h	L18
include	include/zephyr/acpi/acpi.h	L47
include	include/zephyr/arch/arc/v2/arc_connect.h	L120, L133, L154
include	include/zephyr/arch/x86/ia32/segmentation.h	L132, L150, L186
include	include/zephyr/arch/xtensa/mpu.h	L95, L141
include	include/zephyr/canbus/isotp.h	L176
include	include/zephyr/display/mb_display.h	L38
include	include/zephyr/drivers/dac/dac161s997.h	L36
include	include/zephyr/drivers/flash/ra_flash_api_extensions.h	L58
include	include/zephyr/drivers/mic_privacy/intel/mic_privacy.h	L27
include	include/zephyr/drivers/misc/renesas_rx_dtc/renesas_rx_dtc.h	L98
include	include/zephyr/drivers/sensor_data_types.h	L157
include	include/zephyr/drivers/usb_c/usbc_pd.h	L320, L351, L427, L474, L533, L552, L599, L618, L665, L698, L730
include	include/zephyr/irq_multilevel.h	L25
include	include/zephyr/net/ethernet.h	L361
boards	boards/arduino/opta/board.h	L19
samples	samples/subsys/logging/syst/src/main.c	L21

C2: Transitive bit-field unions (38 sites, 32 files)

Area	File	Lines
drivers	drivers/dai/intel/ssp/dai-params-intel-ipc4.h	L315
drivers	drivers/timer/apic_tsc.c	L78
drivers	drivers/usb/device/usb_dc_sam_usbc.c	L74, L78, L82
drivers	drivers/usb/udc/udc_sam_usbc.c	L42
subsys	subsys/bluetooth/audio/cap_internal.h	L163
subsys	subsys/bluetooth/controller/ll_sw/lll.h	L444
subsys	subsys/bluetooth/controller/ll_sw/nordic/lll/pdu_vendor.h	L18
subsys	subsys/bluetooth/controller/ll_sw/pdu.h	L451, L570, L916
subsys	subsys/logging/frontends/log_frontend_stmesp.c	L94
subsys	subsys/net/ip/net_private.h	L31
subsys	subsys/net/lib/ptp/msg.h	L257
subsys	subsys/usb/device_next/class/usbd_cdc_ecm.c	L45
subsys	subsys/usb/device_next/class/usbd_cdc_ncm.c	L50
soc	soc/ite/ec/it8xxx2/chip_chipregs.h	L726
soc	soc/nuvoton/npcx/common/pinctrl_soc.h	L122
include	include/zephyr/arch/x86/ia32/thread.h	L199
include	include/zephyr/bluetooth/mesh/cfg_cli.h	L1739
include	include/zephyr/instrumentation/instrumentation.h	L72
include	include/zephyr/logging/log_frontend_stmesp_demux.h	L80
include	include/zephyr/logging/log_msg.h	L133
include	include/zephyr/net/ethernet.h	L453
include	include/zephyr/net/ethernet_mgmt.h	L140
include	include/zephyr/net/ieee802154_frame.h	L901
include	include/zephyr/net/mqtt.h	L668
include	include/zephyr/net/net_pkt.h	L348
include	include/zephyr/shell/shell.h	L1017, L1025
include	include/zephyr/sys/mpsc_packet.h	L49
include	include/zephyr/usb/usb_ch9.h	L51, L438
include	include/zephyr/usb/usbd.h	L175
tests	tests/bluetooth/controller/ctrl_isoal/src/isoal_test_common.h	L70
tests	tests/drivers/build_all/sensor/src/generic_test.c	L21
tests	tests/lib/mpsc_pbuf/src/main.c	L41

C3: Packed structs with bit-fields (192 sites, 78 files)

Area	File	Lines
drivers	drivers/adc/adc_ad405x.c	L212
drivers	drivers/adc/adc_max2253x.c	L102
drivers	drivers/adc/adc_max32.c	L86
drivers	drivers/adc/adc_stm32.c	L270
drivers	drivers/can/can_mcan.h	L909, L950, L991, L1033, L1052
drivers	drivers/dai/intel/ssp/dai-params-intel-ipc4.h	L181, L281, L298
drivers	drivers/dma/dma_iproc_pax.h	L411
drivers	drivers/dma/dma_iproc_pax_v1.h	L62, L76, L85, L92, L104
drivers	drivers/dma/dma_iproc_pax_v2.h	L69, L86, L94, L101
drivers	drivers/interrupt_controller/intc_intel_vtd.h	L25, L65, L84, L102
drivers	drivers/sdhc/sdhc_ti_am654.c	L257
drivers	drivers/sensor/adi/adxl345/adxl345.h	L213
drivers	drivers/sensor/adi/adxl362/adxl362.h	L259
drivers	drivers/sensor/adi/adxl367/adxl367.h	L408
drivers	drivers/sensor/adi/adxl372/adxl372.h	L383
drivers	drivers/sensor/adi/max32664c/max32664c.h	L77
drivers	drivers/sensor/ams/ens210/ens210.h	L63, L69, L75, L80, L86, L92, L98
drivers	drivers/sensor/bosch/bma4xx/bma4xx_decoder.h	L17, L24
drivers	drivers/sensor/bosch/bme280/bme280.h	L226
drivers	drivers/sensor/bosch/bme680/bme680.h	L281
drivers	drivers/sensor/bosch/bmm350/bmm350.h	L456
drivers	drivers/sensor/lm75/lm75.c	L52
drivers	drivers/sensor/lm77/lm77.c	L37
drivers	drivers/sensor/memsic/mmc56x3/mmc56x3.h	L125
drivers	drivers/sensor/pixart/paa3905/paa3905.h	L27
drivers	drivers/sensor/pni/rm3100/rm3100.h	L33
drivers	drivers/sensor/st/iis3dwb/iis3dwb.h	L102, L110, L124
drivers	drivers/sensor/st/lis2dux12/lis2dux12_decoder.h	L21, L29, L42
drivers	drivers/sensor/st/lsm6dsv16x/lsm6dsv16x_decoder.h	L21, L28, L44
drivers	drivers/sensor/st/lsm6dsvxxx/lsm6dsvxxx.h	L253, L262, L277
drivers	drivers/sensor/tdk/icm4268x/icm4268x_decoder.h	L35
drivers	drivers/sensor/tdk/icm45686/icm45686.h	L52
drivers	drivers/spi/spi_it8xxx2.c	L73
drivers	drivers/uaol/uaol_intel_adsp.c	L125, L128, L147, L150, L162, L165
drivers	drivers/usb/device/usb_dc_kinetis.c	L55, L59, L68
drivers	drivers/usb/udc/udc_kinetis.c	L54, L58, L67
drivers	drivers/usb/udc/udc_sam0.c	L28, L47
drivers	drivers/wifi/esp_hosted/esp_hosted_wifi.h	L87
subsys	subsys/bluetooth/controller/hal/ccm.h	L8
subsys	subsys/bluetooth/controller/ll_sw/nordic/hal/nrf5/ecb.c	L42, L44
subsys	subsys/bluetooth/controller/ll_sw/nordic/hal/nrf5/radio/radio.c	L2063, L2065, L2552, L2554
subsys	subsys/bluetooth/controller/ll_sw/pdu.h	L397, L400, L420, L443, L474, L553, L812, L836, L895, L977, L1023, L1043, L1075, L1137, L1199, L1200, L1254
subsys	subsys/bluetooth/controller/ll_sw/pdu_df.h	L7
subsys	subsys/bluetooth/host/classic/avdtp_internal.h	L79, L122, L203
subsys	subsys/bluetooth/mesh/cdb.c	L57
subsys	subsys/bluetooth/mesh/net.c	L74
subsys	subsys/bluetooth/mesh/subnet.c	L53
subsys	subsys/logging/frontends/log_frontend_dict_uart.c	L16
subsys	subsys/mem_mgmt/demand_paging/eviction/lru.c	L58
subsys	subsys/mgmt/mcumgr/transport/include/mgmt/mcumgr/transport/smp_internal.h	L22
subsys	subsys/net/ip/icmpv6.h	L73
subsys	subsys/net/ip/ipv4.h	L94
subsys	subsys/net/ip/tcp_private.h	L168
subsys	subsys/net/l2/ethernet/gptp/gptp_messages.h	L273
subsys	subsys/net/lib/ptp/msg.h	L230
subsys	subsys/net/lib/sntp/sntp_pkt.h	L12
subsys	subsys/portability/posix/options/posix_internal.h	L26
subsys	subsys/usb/device_next/class/usbd_msc_scsi.c	L265
lib	lib/net_buf/buf_simple.c	L217, L230, L265, L278, L291, L304, L503, L515, L547, L559, L571, L583
soc	soc/ite/ec/it8xxx2/chip_chipregs.h	L523, L586, L670
soc	soc/nuvoton/npcx/common/pinctrl_soc.h	L72, L91, L108, L121, L128
include	include/zephyr/arch/x86/ia32/segmentation.h	L54, L108
include	include/zephyr/arch/xtensa/mpu.h	L275
include	include/zephyr/bluetooth/hci_types.h	L4315, L4329, L4342, L4360, L4381, L4399, L4419, L4447, L4470
include	include/zephyr/drivers/dac/dac161s997.h	L44
include	include/zephyr/drivers/rtc/mcp7940n.h	L13, L19, L25, L32, L40, L46, L53, L58, L68, L73, L79, L85, L92, L99, L105
include	include/zephyr/drivers/usb/udc.h	L187
include	include/zephyr/instrumentation/instrumentation.h	L50
include	include/zephyr/logging/log_output_dict.h	L39
include	include/zephyr/net/gptp.h	L147
include	include/zephyr/net/ieee802154_frame.h	L194, L200, L424, L531, L550, L571, L594, L621, L674, L824
include	include/zephyr/net/ieee802154_ie.h	L141
include	include/zephyr/net/wifi_mgmt.h	L1181
include	include/zephyr/usb/usb_ch9.h	L30, L406
boards	boards/arduino/opta/board.h	L16
samples	samples/boards/phytec/reel_board/mesh_badge/src/mesh.c	L47, L53
samples	samples/subsys/logging/syst/src/main.c	L17
samples	samples/tfm_integration/psa_crypto/src/util_sformat.h	L20

C4: Bit-field struct over raw memory (heuristic) (244 sites, 88 files)

Area	File	Lines
drivers	drivers/adc/adc_ad405x.c	L1123, L1138, L1247, L1283
drivers	drivers/adc/adc_max2253x.c	L402, L417, L482
drivers	drivers/adc/adc_max32.c	L307, L317, L519
drivers	drivers/adc/adc_stm32.c	L1431
drivers	drivers/ieee802154/ieee802154_silabs_efr32.c	L160
drivers	drivers/pwm/pwm_renesas_rx_mtu.c	L619
drivers	drivers/sensor/adi/adxl345/adxl345_decoder.c	L105, L191, L266, L279
drivers	drivers/sensor/adi/adxl345/adxl345_rtio.c	L32
drivers	drivers/sensor/adi/adxl345/adxl345_stream.c	L182, L204, L348
drivers	drivers/sensor/adi/adxl355/adxl355_decoder.c	L45, L106, L247, L253, L297
drivers	drivers/sensor/adi/adxl355/adxl355_stream.c	L39, L154, L184, L196, L201, L253, L333, L374
drivers	drivers/sensor/adi/adxl362/adxl362.c	L364
drivers	drivers/sensor/adi/adxl362/adxl362_decoder.c	L70, L259, L353, L366
drivers	drivers/sensor/adi/adxl362/adxl362_rtio.c	L32
drivers	drivers/sensor/adi/adxl362/adxl362_stream.c	L56, L159, L196, L213, L264, L361, L397
drivers	drivers/sensor/adi/adxl367/adxl367.c	L651
drivers	drivers/sensor/adi/adxl367/adxl367_decoder.c	L508, L627, L735, L748
drivers	drivers/sensor/adi/adxl367/adxl367_rtio.c	L33
drivers	drivers/sensor/adi/adxl367/adxl367_stream.c	L80, L204, L278, L372, L423, L514, L550
drivers	drivers/sensor/adi/adxl372/adxl372_decoder.c	L38, L168, L267, L280
drivers	drivers/sensor/adi/adxl372/adxl372_rtio.c	L34
drivers	drivers/sensor/adi/adxl372/adxl372_stream.c	L218, L256, L389
drivers	drivers/sensor/bosch/bma4xx/bma4xx_decoder.c	L51, L304, L390
drivers	drivers/sensor/bosch/bma4xx/bma4xx_rtio_stream.c	L152, L159, L271
drivers	drivers/sensor/bosch/bme280/bme280_async.c	L45
drivers	drivers/sensor/bosch/bme280/bme280_decoder.c	L12, L59
drivers	drivers/sensor/bosch/bme680/bme680_async.c	L106
drivers	drivers/sensor/bosch/bme680/bme680_decoder.c	L21, L71
drivers	drivers/sensor/bosch/bmm350/bmm350.c	L803
drivers	drivers/sensor/bosch/bmm350/bmm350_decoder.c	L140, L175, L219, L297
drivers	drivers/sensor/bosch/bmm350/bmm350_stream.c	L132
drivers	drivers/sensor/broadcom/afbr_s50/afbr_s50_decoder.c	L44, L91, L157
drivers	drivers/sensor/memsic/mmc56x3/mmc56x3_async.c	L43
drivers	drivers/sensor/memsic/mmc56x3/mmc56x3_decoder.c	L12, L71
drivers	drivers/sensor/pixart/paa3905/paa3905.c	L75
drivers	drivers/sensor/pixart/paa3905/paa3905_decoder.c	L84, L135, L199, L232
drivers	drivers/sensor/pixart/pat9136/pat9136.c	L113
drivers	drivers/sensor/pixart/pat9136/pat9136_decoder.c	L153, L210, L296, L329
drivers	drivers/sensor/pni/rm3100/rm3100.c	L74
drivers	drivers/sensor/pni/rm3100/rm3100_decoder.c	L38, L90, L153, L225
drivers	drivers/sensor/pni/rm3100/rm3100_stream.c	L89
drivers	drivers/sensor/st/iis3dwb/iis3dwb.c	L155
drivers	drivers/sensor/st/iis3dwb/iis3dwb_decoder.c	L79, L80, L109, L167, L304, L370, L403
drivers	drivers/sensor/st/iis3dwb/iis3dwb_stream.c	L239, L306, L397, L444, L445
drivers	drivers/sensor/st/lis2dux12/lis2dux12_decoder.c	L134, L135, L164, L221, L420, L487, L521
drivers	drivers/sensor/st/lis2dux12/lis2dux12_rtio.c	L42
drivers	drivers/sensor/st/lis2dux12/lis2dux12_rtio_stream.c	L181, L249, L343, L392, L393
drivers	drivers/sensor/st/lsm6dsv16x/lsm6dsv16x_decoder.c	L154, L155, L191, L278, L569, L659
drivers	drivers/sensor/st/lsm6dsv16x/lsm6dsv16x_rtio.c	L42
drivers	drivers/sensor/st/lsm6dsv16x/lsm6dsv16x_rtio_stream.c	L379, L454, L553, L606, L607
drivers	drivers/sensor/st/lsm6dsvxxx/lsm6dsvxxx.c	L196
drivers	drivers/sensor/st/lsm6dsvxxx/lsm6dsvxxx_decoder.c	L117, L118, L156, L271, L302, L333, L361, L400, L430, L457, L595, L718
drivers	drivers/sensor/st/lsm6dsvxxx/lsm6dsvxxx_stream.c	L245, L317, L411, L485, L486
drivers	drivers/sensor/tdk/icm4268x/icm4268x_decoder.c	L197, L361, L578, L667, L679, L680, L771
drivers	drivers/sensor/tdk/icm4268x/icm4268x_rtio.c	L68
drivers	drivers/sensor/tdk/icm4268x/icm4268x_rtio_stream.c	L136
drivers	drivers/sensor/tdk/icm45686/icm45686_decoder.c	L377
drivers	drivers/serial/uart_async_rx.c	L22
subsys	subsys/bluetooth/controller/ll_sw/ull_adv_aux.c	L1963
subsys	subsys/bluetooth/controller/ll_sw/ull_sync.c	L1903
subsys	subsys/bluetooth/host/classic/a2dp.c	L1332
subsys	subsys/logging/frontends/log_frontend_dict_uart.c	L163
subsys	subsys/mgmt/mcumgr/smp/src/smp.c	L100
subsys	subsys/mgmt/mcumgr/smp_client/src/client.c	L265, L278, L305
subsys	subsys/modem/modem_cmux.c	L1913, L1935, L1967, L1996
subsys	subsys/net/ip/ipv6.h	L127
subsys	subsys/net/l2/ethernet/gptp/gptp_messages.c	L66, L80
subsys	subsys/net/l2/ieee802154/ieee802154.c	L111
subsys	subsys/net/l2/ieee802154/ieee802154_frame.c	L44, L219
subsys	subsys/net/lib/sntp/sntp.c	L66
lib	lib/net_buf/buf_simple.c	L507, L519, L551, L563, L575, L587
lib	lib/utils/ring_buffer.c	L162, L202
soc	soc/intel/intel_adsp/ace/include/ace20_lnl/adsp_power.h	L32, L48
soc	soc/intel/intel_adsp/ace/include/adsp_memory.h	L139, L195
tests	tests/subsys/mgmt/mcumgr/cb_notifications/src/smp_test_util.c	L36
tests	tests/subsys/mgmt/mcumgr/enum_mgmt/src/smp_test_util.c	L40, L60, L82, L118
tests	tests/subsys/mgmt/mcumgr/fs_mgmt_hash_supported/src/main.c	L77
tests	tests/subsys/mgmt/mcumgr/img_mgmt_slot_info/src/smp_test_util.c	L40
tests	tests/subsys/mgmt/mcumgr/mcumgr_client/src/smp_stub.c	L60, L78
tests	tests/subsys/mgmt/mcumgr/os_mgmt_datetime/src/smp_test_util.c	L40, L58
tests	tests/subsys/mgmt/mcumgr/os_mgmt_info/src/smp_test_util.c	L38
tests	tests/subsys/mgmt/mcumgr/os_mgmt_mpstat/src/smp_test_util.c	L37
tests	tests/subsys/mgmt/mcumgr/settings_mgmt/src/main.c	L184, L238, L314, L368, L455, L730, L869, L1032, L1150, L1480, L1717, L1809, L1924, L2039
tests	tests/subsys/mgmt/mcumgr/settings_mgmt/src/smp_test_util.c	L40, L62, L81, L97, L113, L130, L150
tests	tests/subsys/mgmt/mcumgr/smp_client/src/smp_transport_stub.c	L22
tests	tests/subsys/mgmt/mcumgr/smp_version/src/smp_test_util.c	L39
tests	tests/subsys/mgmt/mcumgr/transport_raw_uart/src/smp_test_util.c	L38
samples	samples/bluetooth/channel_sounding/src/distance_estimation.c	L223

Dependencies

This touches many maintainer areas: arch, drivers, subsys, soc, lib, include, boards, tests, and samples. See the per-area table in the summary for the size of each area. include headers are owned by their subsystems, not one owner. No external modules and no on-wire or on-flash encodings change. Only the access method in the code changes.

Concerns and Unresolved Questions

• Some sites run only on little-endian GCC or Clang and work today. Do we fix all of them, or fix the big-endian, protocol, DMA, and flash sites first and treat the rest as low priority?
• C4 is heuristic. It may hold a few false positives. Confirm each before change.
• Which CI tool for the guard: checkpatch (simple) or Coccinelle (semantic)? How strict, so it does not warn on internal bit-fields where layout is never assumed?
• Scope: do we also convert reserved or padding-only fields, or only fields whose value crosses a boundary?
• Order: land the CI guard first, or the clean-up first?

Alternatives Considered

• Do nothing and rely on the current compiler. Rejected. It breaks the adopted rule and it already broke (PR irq: multilevel: avoid bit-field layout dependency #113320).
• Byte-order guards only. Rejected. They fix endianness but not the bit order or the spill behaviour.
• Add a new coding guideline. Tried in PR doc: contribute: coding guidelines: add coding guideline for bit-fields #113584 and closed as already covered by MISRA Dir 1.1 and EXP11-C. So this RFC proposes enforcement, not new policy.
• Compiler-specific packing attributes. Non-standard and still do not fix the bit order.
• Use FIELD_GET and FIELD_PREP over a plain word. Chosen. Standard C, already common in the tree, same idea as Linux linux/bitfield.h.

Also to add, Linux documents mmap'd bitfield structs over hardware as non-portable and steers toward shift/mask instead: https://docs.kernel.org/core-api/packing.html#problem-statement (and FIELD_GET/FIELD_PREP in include/linux/bitfield.h is the concrete pattern).

Reports can be generated using the attached python script: bitfield_layout_audit.py

NOTE: Issue details and tree wide search is done using Claude Opus 4.8 LLM. Although the data is cross checked multiple time before posting here, please be aware the data could be error-prone.

[parthitce]

List of pull request in open state with Authors tagged:

PR	Line number details	Author
#113509	drivers/wifi/esp_hosted/esp_hosted_wifi.h:L83 (packed struct)	@iabdalkader
#112993	drivers/usb/udc/udc_mchp_g1.c:L62 (packed struct) drivers/usb/udc/udc_mchp_g1.c:L95 (packed struct)	@shijas-mayan
#112964	drivers/sensor/bosch/bmi270/bmi270_decoder.h:L16 (packed struct)	@mbarchie
#112582	drivers/clock_control/clock_control_mchp_pic32cm_sg_gc.c:L91 (union)	@sunil-abraham
#112362	drivers/sensor/tdk/icm566xx/icm566xx.h:L76 (union)	@akim-invn
#111476	include/zephyr/drivers/clock_control/mchp_xec_clock_control.h:L34 (union)	@Manimaran-A
#109976	drivers/dac/dac_infineon_hppass_csg.c:L54 (union)	@jsbatch
#109967	drivers/dac/dac_infineon_hppass_csg.c:L53 (union)	@jsbatch
#109879	drivers/comparator/comparator_infineon_hppass_csg.c:L99 (union)	@jsbatch
#109632	drivers/adc/adc_infineon_hppass_sar.c:L171 (union) drivers/adc/adc_infineon_hppass_sar.c:L188 (union)	@jsbatch
#109456	drivers/i2c/i2c_dw_con.h:L21 (union)	@puneetdixit200
#109130	drivers/clock_control/clock_control_mchp_pic32ck_sg_gc.c:L105 (union)	@Sountharya-MCHP
#106624	soc/egis/et171/et171_otp.c:L16 (union)	@jackylee-go
#106326	include/zephyr/drivers/clock_control/mchp_xec_clock_control.h:L21 (union)	@scottwcpg
#104603	include/zephyr/net/rtp.h:L73 (union) include/zephyr/net/rtp.h:L88 (union)	@javlands
#104178	drivers/sensor/bosch/bma400/bma400_decoder.h:L20 (packed struct) drivers/sensor/bosch/bma400/bma400_decoder.h:L31 (packed struct)	@lucagessi
#97791	drivers/adc/adc_mcp342x.c:L33 (union)	@AUtech-dev
#89478	include/zephyr/drivers/counter/ti_am3352_eqep.h:L104 (packed struct) include/zephyr/drivers/counter/ti_am3352_eqep.h:L132 (packed struct) include/zephyr/drivers/counter/ti_am3352_eqep.h:L196 (packed struct)	@natto1784

Note: This data is grep'd and generated by LLM and manually verified, please ignore / flag if it's false positive.

[natto1784]

This is something I am interested to hear opinions on. So far all the drivers I have written (and the drivers I am writing) use bitfields for descriptors and such and yes, there are pros and cons to both approaches. So it would be great to reach a consensus here.

[aescolar]

@parthitce This RFC is based on a set of presumptions that I do not believe hold.

Background: We do not follow all MISRA rules, and for the ones we do, we do not do it dogmatically. We follow those that we consider useful, where we consider them useful.

Principal:

That MISRA rule states:
"Any implementation-defined behaviour on which the output of the program depends shall be documented and understood"
That is, we can (and certainly do, as we need to, as we are writing low level code) write some code whose behaviour is implementation defined given the C standard.

In the documentation for Dir 1.1, there is a link to CERT C rule for reference. That does not imply we need to follow that CERT C rule.

Already seen in practice: #113320

That is a just a plain bug. Just because somebody makes a mistake while using a C language feature it does not mean we now need to ban that C language feature. That particular bug can be fixed trivially. And the use in that code is not the most typical use of bitfields.

A guideline PR #113584 was closed as already covered by the rule above.

That PR had been rejected by two people on the principle of the PR.

...finds 738 sites in 238 files...

The fact that bitfields are used in hundredths of places/files, would be an indication that maybe we should not just forbid their use wholesale.

Proposed Change (Summary)

I fundamentally reject this proposal as misguided.
a) It would create a lot of churn (for us and downstreams, complicate backports), result in less readable code, and with all likelihood introduced bugs for no practical benefit.
b) It would create an unnecessary constraint for vendors / driver contributors who are trying to contribute perfectly valid code. Similarly it would prevent us from importing to the tree perfectly valid libraries.

Steps per PR: add named GENMASK masks,

This would mean adding thousands of such macros, effectively replacing HALs headers that use bitfields.

Do nothing and rely on the current compiler. Rejected.

"Rejected" by you ;). Not by others.
We rely on knowing what the compiler does with bitfields. This is not a "one compiler does this in this version and may change at any point". This is: all compilers do the same in these cases, and no compiler designer will change this behavior as they would break tons of existing code.

Byte-order guards only. Rejected. They fix endianness but not the bit order or the spill behaviour.

I fail to see what you mean with that.
What do you mean with "spill behaviour"? One does not use bitfields to access across word/bus access boundaries, because that does not represent proper HW registers. And I doubt anybody would expect a compiler to handle it properly for data packages.
The "bit order" is what we fix with the bigendian/little endian check. In big endian architectures the compiler places bitfields from MSbit to LSBit. While in little endian it starts from LSBit.

Also to add, Linux..

We are not Linux, and we have constraints Linux does not have.

[henrikbrixandersen]

@parthitce Thank you for putting in the legwork performing this analysis.

I will be taking care of the identified issues in the drivers/can/can_mcan.h file and driver.

[teburd]

I wonder if cocci could detect such patterns and reject them?

[nashif]

wait, MISRA Dir 1.1 says:

"Any implementation-defined behaviour on which the output of the program depends shall be documented and understood."

The directive does not say:

• implementation-defined behavior shall not be used;
• bit-field representation shall not affect output;
• bit-fields may not represent hardware registers;
• all bit-field overlays must be replaced by masks and shifts.
• ....

It says the dependency must be documented and understood.

Compliance may be achieved by documenting and understanding that:

• the code is restricted to a particular architecture;
• a particular ABI defines the layout;
• ...

@RobertoBagnara Can you please weigh in?

[henrikbrixandersen]

EXP11-C, which we refer to from MISRA directive 1.1 in our coding guidelines, states the following:

Consequently, it is impossible to write portable safe code that makes assumptions regarding the layout of bit-field structure members.

I do not believe we can say we document nor fully understand where the current usage of bit-fields structure members in Zephyr, as outlined in this PR, can lead to issues or has certain dependencies on the compiler ABI or CPU architecture.

[github-actions]

This issue has been marked as stale because it has been open (more than) 30 days with no activity. Remove the stale label or add a comment saying that you would like to have the label removed otherwise this issue will automatically be closed in 7 days. Note, that you can always re-open a closed issue at any time.