arch/arm64/kernel/vdso/gettimeofday.S - arm/linux - Git at Google

 /*
  * Userspace implementations of gettimeofday() and friends.
  *
  * Copyright (C) 2012 ARM Limited
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  * Author: Will Deacon <will.deacon@arm.com>
  */

 #include <linux/linkage.h>
 #include <asm/asm-offsets.h>
 #include <asm/unistd.h>

 #define NSEC_PER_SEC_LO16	0xca00
 #define NSEC_PER_SEC_HI16	0x3b9a

 vdso_data	.req	x6
 seqcnt		.req	w7
 w_tmp		.req	w8
 x_tmp		.req	x8

 /*
  * Conventions for macro arguments:
  * - An argument is write-only if its name starts with "res".
  * - All other arguments are read-only, unless otherwise specified.
  */

 	.macro	seqcnt_acquire
 9999:	ldr	seqcnt, [vdso_data, #VDSO_TB_SEQ_COUNT]
 	tbnz	seqcnt, #0, 9999b
 	dmb	ishld
 	.endm

 	.macro	seqcnt_check fail
 	dmb	ishld
 	ldr	w_tmp, [vdso_data, #VDSO_TB_SEQ_COUNT]
 	cmp	w_tmp, seqcnt
 	b.ne	\fail
 	.endm

 	.macro	syscall_check fail
 	ldr	w_tmp, [vdso_data, #VDSO_USE_SYSCALL]
 	cbnz	w_tmp, \fail
 	.endm

 	.macro get_nsec_per_sec res
 	mov	\res, #NSEC_PER_SEC_LO16
 	movk	\res, #NSEC_PER_SEC_HI16, lsl #16
 	.endm

 	/*
 	 * Returns the clock delta, in nanoseconds left-shifted by the clock
 	 * shift.
 	 */
 	.macro	get_clock_shifted_nsec res, cycle_last, mult
 	/* Read the virtual counter. */
 	isb
 	mrs	x_tmp, cntvct_el0
 	/* Calculate cycle delta and convert to ns. */
 	sub	\res, x_tmp, \cycle_last
 	/* We can only guarantee 56 bits of precision. */
 	movn	x_tmp, #0xff00, lsl #48
 	and	\res, x_tmp, \res
 	mul	\res, \res, \mult
 	.endm

 	/*
 	 * Returns in res_{sec,nsec} the REALTIME timespec, based on the
 	 * "wall time" (xtime) and the clock_mono delta.
 	 */
 	.macro	get_ts_realtime res_sec, res_nsec, \
 			clock_nsec, xtime_sec, xtime_nsec, nsec_to_sec
 	add	\res_nsec, \clock_nsec, \xtime_nsec
 	udiv	x_tmp, \res_nsec, \nsec_to_sec
 	add	\res_sec, \xtime_sec, x_tmp
 	msub	\res_nsec, x_tmp, \nsec_to_sec, \res_nsec
 	.endm

 	/*
 	 * Returns in res_{sec,nsec} the timespec based on the clock_raw delta,
 	 * used for CLOCK_MONOTONIC_RAW.
 	 */
 	.macro	get_ts_clock_raw res_sec, res_nsec, clock_nsec, nsec_to_sec
 	udiv	\res_sec, \clock_nsec, \nsec_to_sec
 	msub	\res_nsec, \res_sec, \nsec_to_sec, \clock_nsec
 	.endm

 	/* sec and nsec are modified in place. */
 	.macro add_ts sec, nsec, ts_sec, ts_nsec, nsec_to_sec
 	/* Add timespec. */
 	add	\sec, \sec, \ts_sec
 	add	\nsec, \nsec, \ts_nsec

 	/* Normalise the new timespec. */
 	cmp	\nsec, \nsec_to_sec
 	b.lt	9999f
 	sub	\nsec, \nsec, \nsec_to_sec
 	add	\sec, \sec, #1
 9999:
 	cmp	\nsec, #0
 	b.ge	9998f
 	add	\nsec, \nsec, \nsec_to_sec
 	sub	\sec, \sec, #1
 9998:
 	.endm

 	.macro clock_gettime_return, shift=0
 	.if \shift == 1
 	lsr	x11, x11, x12
 	.endif
 	stp	x10, x11, [x1, #TSPEC_TV_SEC]
 	mov	x0, xzr
 	ret
 	.endm

 	.macro jump_slot jumptable, index, label
 	.if (. - \jumptable) != 4 * (\index)
 	.error "Jump slot index mismatch"
 	.endif
 	b	\label
 	.endm

 	.text

 /* int __kernel_gettimeofday(struct timeval *tv, struct timezone *tz); */
 ENTRY(__kernel_gettimeofday)
 	.cfi_startproc
 	adr	vdso_data, _vdso_data
 	/* If tv is NULL, skip to the timezone code. */
 	cbz	x0, 2f

 	/* Compute the time of day. */
 1:	seqcnt_acquire
 	syscall_check fail=4f
 	ldr	x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
 	/* w11 = cs_mono_mult, w12 = cs_shift */
 	ldp	w11, w12, [vdso_data, #VDSO_CS_MONO_MULT]
 	ldp	x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
 	seqcnt_check fail=1b

 	get_nsec_per_sec res=x9
 	lsl	x9, x9, x12

 	get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
 	get_ts_realtime res_sec=x10, res_nsec=x11, \
 		clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9

 	/* Convert ns to us. */
 	mov	x13, #1000
 	lsl	x13, x13, x12
 	udiv	x11, x11, x13
 	stp	x10, x11, [x0, #TVAL_TV_SEC]
 2:
 	/* If tz is NULL, return 0. */
 	cbz	x1, 3f
 	ldp	w4, w5, [vdso_data, #VDSO_TZ_MINWEST]
 	stp	w4, w5, [x1, #TZ_MINWEST]
 3:
 	mov	x0, xzr
 	ret
 4:
 	/* Syscall fallback. */
 	mov	x8, #__NR_gettimeofday
 	svc	#0
 	ret
 	.cfi_endproc
 ENDPROC(__kernel_gettimeofday)

 #define JUMPSLOT_MAX CLOCK_MONOTONIC_COARSE

 /* int __kernel_clock_gettime(clockid_t clock_id, struct timespec *tp); */
 ENTRY(__kernel_clock_gettime)
 	.cfi_startproc
 	cmp	w0, #JUMPSLOT_MAX
 	b.hi	syscall
 	adr	vdso_data, _vdso_data
 	adr	x_tmp, jumptable
 	add	x_tmp, x_tmp, w0, uxtw #2
 	br	x_tmp

 	ALIGN
 jumptable:
 	jump_slot jumptable, CLOCK_REALTIME, realtime
 	jump_slot jumptable, CLOCK_MONOTONIC, monotonic
 	b	syscall
 	b	syscall
 	jump_slot jumptable, CLOCK_MONOTONIC_RAW, monotonic_raw
 	jump_slot jumptable, CLOCK_REALTIME_COARSE, realtime_coarse
 	jump_slot jumptable, CLOCK_MONOTONIC_COARSE, monotonic_coarse

 	.if (. - jumptable) != 4 * (JUMPSLOT_MAX + 1)
 	.error	"Wrong jumptable size"
 	.endif

 	ALIGN
 realtime:
 	seqcnt_acquire
 	syscall_check fail=syscall
 	ldr	x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
 	/* w11 = cs_mono_mult, w12 = cs_shift */
 	ldp	w11, w12, [vdso_data, #VDSO_CS_MONO_MULT]
 	ldp	x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
 	seqcnt_check fail=realtime

 	/* All computations are done with left-shifted nsecs. */
 	get_nsec_per_sec res=x9
 	lsl	x9, x9, x12

 	get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
 	get_ts_realtime res_sec=x10, res_nsec=x11, \
 		clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9
 	clock_gettime_return, shift=1

 	ALIGN
 monotonic:
 	seqcnt_acquire
 	syscall_check fail=syscall
 	ldr	x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
 	/* w11 = cs_mono_mult, w12 = cs_shift */
 	ldp	w11, w12, [vdso_data, #VDSO_CS_MONO_MULT]
 	ldp	x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
 	ldp	x3, x4, [vdso_data, #VDSO_WTM_CLK_SEC]
 	seqcnt_check fail=monotonic

 	/* All computations are done with left-shifted nsecs. */
 	lsl	x4, x4, x12
 	get_nsec_per_sec res=x9
 	lsl	x9, x9, x12

 	get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
 	get_ts_realtime res_sec=x10, res_nsec=x11, \
 		clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9

 	add_ts sec=x10, nsec=x11, ts_sec=x3, ts_nsec=x4, nsec_to_sec=x9
 	clock_gettime_return, shift=1

 	ALIGN
 monotonic_raw:
 	seqcnt_acquire
 	syscall_check fail=syscall
 	ldr	x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
 	/* w11 = cs_raw_mult, w12 = cs_shift */
 	ldp	w12, w11, [vdso_data, #VDSO_CS_SHIFT]
 	ldp	x13, x14, [vdso_data, #VDSO_RAW_TIME_SEC]
 	seqcnt_check fail=monotonic_raw

 	/* All computations are done with left-shifted nsecs. */
 	get_nsec_per_sec res=x9
 	lsl	x9, x9, x12

 	get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
 	get_ts_clock_raw res_sec=x10, res_nsec=x11, \
 		clock_nsec=x15, nsec_to_sec=x9

 	add_ts sec=x10, nsec=x11, ts_sec=x13, ts_nsec=x14, nsec_to_sec=x9
 	clock_gettime_return, shift=1

 	ALIGN
 realtime_coarse:
 	seqcnt_acquire
 	ldp	x10, x11, [vdso_data, #VDSO_XTIME_CRS_SEC]
 	seqcnt_check fail=realtime_coarse
 	clock_gettime_return

 	ALIGN
 monotonic_coarse:
 	seqcnt_acquire
 	ldp	x10, x11, [vdso_data, #VDSO_XTIME_CRS_SEC]
 	ldp	x13, x14, [vdso_data, #VDSO_WTM_CLK_SEC]
 	seqcnt_check fail=monotonic_coarse

 	/* Computations are done in (non-shifted) nsecs. */
 	get_nsec_per_sec res=x9
 	add_ts sec=x10, nsec=x11, ts_sec=x13, ts_nsec=x14, nsec_to_sec=x9
 	clock_gettime_return

 	ALIGN
 syscall: /* Syscall fallback. */
 	mov	x8, #__NR_clock_gettime
 	svc	#0
 	ret
 	.cfi_endproc
 ENDPROC(__kernel_clock_gettime)

 /* int __kernel_clock_getres(clockid_t clock_id, struct timespec *res); */
 ENTRY(__kernel_clock_getres)
 	.cfi_startproc
 	cmp	w0, #CLOCK_REALTIME
 	ccmp	w0, #CLOCK_MONOTONIC, #0x4, ne
 	ccmp	w0, #CLOCK_MONOTONIC_RAW, #0x4, ne
 	b.ne	1f

 	ldr	x2, 5f
 	b	2f
 1:
 	cmp	w0, #CLOCK_REALTIME_COARSE
 	ccmp	w0, #CLOCK_MONOTONIC_COARSE, #0x4, ne
 	b.ne	4f
 	ldr	x2, 6f
 2:
 	cbz	w1, 3f
 	stp	xzr, x2, [x1]

 3:	/* res == NULL. */
 	mov	w0, wzr
 	ret

 4:	/* Syscall fallback. */
 	mov	x8, #__NR_clock_getres
 	svc	#0
 	ret
 5:
 	.quad	CLOCK_REALTIME_RES
 6:
 	.quad	CLOCK_COARSE_RES
 	.cfi_endproc
 ENDPROC(__kernel_clock_getres)
	/*
	* Userspace implementations of gettimeofday() and friends.
	*
	* Copyright (C) 2012 ARM Limited
	*
	* This program is free software; you can redistribute it and/or modify
	* it under the terms of the GNU General Public License version 2 as
	* published by the Free Software Foundation.
	*
	* This program is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program. If not, see <http://www.gnu.org/licenses/>.
	*
	* Author: Will Deacon <will.deacon@arm.com>
	*/

	#include <linux/linkage.h>
	#include <asm/asm-offsets.h>
	#include <asm/unistd.h>

	#define NSEC_PER_SEC_LO16 0xca00
	#define NSEC_PER_SEC_HI16 0x3b9a

	vdso_data .req x6
	seqcnt .req w7
	w_tmp .req w8
	x_tmp .req x8

	/*
	* Conventions for macro arguments:
	* - An argument is write-only if its name starts with "res".
	* - All other arguments are read-only, unless otherwise specified.
	*/

	.macro seqcnt_acquire
	9999: ldr seqcnt, [vdso_data, #VDSO_TB_SEQ_COUNT]
	tbnz seqcnt, #0, 9999b
	dmb ishld
	.endm

	.macro seqcnt_check fail
	dmb ishld
	ldr w_tmp, [vdso_data, #VDSO_TB_SEQ_COUNT]
	cmp w_tmp, seqcnt
	b.ne \fail
	.endm

	.macro syscall_check fail
	ldr w_tmp, [vdso_data, #VDSO_USE_SYSCALL]
	cbnz w_tmp, \fail
	.endm

	.macro get_nsec_per_sec res
	mov \res, #NSEC_PER_SEC_LO16
	movk \res, #NSEC_PER_SEC_HI16, lsl #16
	.endm

	/*
	* Returns the clock delta, in nanoseconds left-shifted by the clock
	* shift.
	*/
	.macro get_clock_shifted_nsec res, cycle_last, mult
	/* Read the virtual counter. */
	isb
	mrs x_tmp, cntvct_el0
	/* Calculate cycle delta and convert to ns. */
	sub \res, x_tmp, \cycle_last
	/* We can only guarantee 56 bits of precision. */
	movn x_tmp, #0xff00, lsl #48
	and \res, x_tmp, \res
	mul \res, \res, \mult
	.endm

	/*
	* Returns in res_{sec,nsec} the REALTIME timespec, based on the
	* "wall time" (xtime) and the clock_mono delta.
	*/
	.macro get_ts_realtime res_sec, res_nsec, \
	clock_nsec, xtime_sec, xtime_nsec, nsec_to_sec
	add \res_nsec, \clock_nsec, \xtime_nsec
	udiv x_tmp, \res_nsec, \nsec_to_sec
	add \res_sec, \xtime_sec, x_tmp
	msub \res_nsec, x_tmp, \nsec_to_sec, \res_nsec
	.endm

	/*
	* Returns in res_{sec,nsec} the timespec based on the clock_raw delta,
	* used for CLOCK_MONOTONIC_RAW.
	*/
	.macro get_ts_clock_raw res_sec, res_nsec, clock_nsec, nsec_to_sec
	udiv \res_sec, \clock_nsec, \nsec_to_sec
	msub \res_nsec, \res_sec, \nsec_to_sec, \clock_nsec
	.endm

	/* sec and nsec are modified in place. */
	.macro add_ts sec, nsec, ts_sec, ts_nsec, nsec_to_sec
	/* Add timespec. */
	add \sec, \sec, \ts_sec
	add \nsec, \nsec, \ts_nsec

	/* Normalise the new timespec. */
	cmp \nsec, \nsec_to_sec
	b.lt 9999f
	sub \nsec, \nsec, \nsec_to_sec
	add \sec, \sec, #1
	9999:
	cmp \nsec, #0
	b.ge 9998f
	add \nsec, \nsec, \nsec_to_sec
	sub \sec, \sec, #1
	9998:
	.endm

	.macro clock_gettime_return, shift=0
	.if \shift == 1
	lsr x11, x11, x12
	.endif
	stp x10, x11, [x1, #TSPEC_TV_SEC]
	mov x0, xzr
	ret
	.endm

	.macro jump_slot jumptable, index, label
	.if (. - \jumptable) != 4 * (\index)
	.error "Jump slot index mismatch"
	.endif
	b \label
	.endm

	.text

	/* int __kernel_gettimeofday(struct timeval tv, struct timezone tz); */
	ENTRY(__kernel_gettimeofday)
	.cfi_startproc
	adr vdso_data, _vdso_data
	/* If tv is NULL, skip to the timezone code. */
	cbz x0, 2f

	/* Compute the time of day. */
	1: seqcnt_acquire
	syscall_check fail=4f
	ldr x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
	/* w11 = cs_mono_mult, w12 = cs_shift */
	ldp w11, w12, [vdso_data, #VDSO_CS_MONO_MULT]
	ldp x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
	seqcnt_check fail=1b

	get_nsec_per_sec res=x9
	lsl x9, x9, x12

	get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
	get_ts_realtime res_sec=x10, res_nsec=x11, \
	clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9

	/* Convert ns to us. */
	mov x13, #1000
	lsl x13, x13, x12
	udiv x11, x11, x13
	stp x10, x11, [x0, #TVAL_TV_SEC]
	2:
	/* If tz is NULL, return 0. */
	cbz x1, 3f
	ldp w4, w5, [vdso_data, #VDSO_TZ_MINWEST]
	stp w4, w5, [x1, #TZ_MINWEST]
	3:
	mov x0, xzr
	ret
	4:
	/* Syscall fallback. */
	mov x8, #__NR_gettimeofday
	svc #0
	ret
	.cfi_endproc
	ENDPROC(__kernel_gettimeofday)

	#define JUMPSLOT_MAX CLOCK_MONOTONIC_COARSE

	/* int __kernel_clock_gettime(clockid_t clock_id, struct timespec tp); /
	ENTRY(__kernel_clock_gettime)
	.cfi_startproc
	cmp w0, #JUMPSLOT_MAX
	b.hi syscall
	adr vdso_data, _vdso_data
	adr x_tmp, jumptable
	add x_tmp, x_tmp, w0, uxtw #2
	br x_tmp

	ALIGN
	jumptable:
	jump_slot jumptable, CLOCK_REALTIME, realtime
	jump_slot jumptable, CLOCK_MONOTONIC, monotonic
	b syscall
	b syscall
	jump_slot jumptable, CLOCK_MONOTONIC_RAW, monotonic_raw
	jump_slot jumptable, CLOCK_REALTIME_COARSE, realtime_coarse
	jump_slot jumptable, CLOCK_MONOTONIC_COARSE, monotonic_coarse

	.if (. - jumptable) != 4 * (JUMPSLOT_MAX + 1)
	.error "Wrong jumptable size"
	.endif

	ALIGN
	realtime:
	seqcnt_acquire
	syscall_check fail=syscall
	ldr x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
	/* w11 = cs_mono_mult, w12 = cs_shift */
	ldp w11, w12, [vdso_data, #VDSO_CS_MONO_MULT]
	ldp x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
	seqcnt_check fail=realtime

	/* All computations are done with left-shifted nsecs. */
	get_nsec_per_sec res=x9
	lsl x9, x9, x12

	get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
	get_ts_realtime res_sec=x10, res_nsec=x11, \
	clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9
	clock_gettime_return, shift=1

	ALIGN
	monotonic:
	seqcnt_acquire
	syscall_check fail=syscall
	ldr x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
	/* w11 = cs_mono_mult, w12 = cs_shift */
	ldp w11, w12, [vdso_data, #VDSO_CS_MONO_MULT]
	ldp x13, x14, [vdso_data, #VDSO_XTIME_CLK_SEC]
	ldp x3, x4, [vdso_data, #VDSO_WTM_CLK_SEC]
	seqcnt_check fail=monotonic

	/* All computations are done with left-shifted nsecs. */
	lsl x4, x4, x12
	get_nsec_per_sec res=x9
	lsl x9, x9, x12

	get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
	get_ts_realtime res_sec=x10, res_nsec=x11, \
	clock_nsec=x15, xtime_sec=x13, xtime_nsec=x14, nsec_to_sec=x9

	add_ts sec=x10, nsec=x11, ts_sec=x3, ts_nsec=x4, nsec_to_sec=x9
	clock_gettime_return, shift=1

	ALIGN
	monotonic_raw:
	seqcnt_acquire
	syscall_check fail=syscall
	ldr x10, [vdso_data, #VDSO_CS_CYCLE_LAST]
	/* w11 = cs_raw_mult, w12 = cs_shift */
	ldp w12, w11, [vdso_data, #VDSO_CS_SHIFT]
	ldp x13, x14, [vdso_data, #VDSO_RAW_TIME_SEC]
	seqcnt_check fail=monotonic_raw

	/* All computations are done with left-shifted nsecs. */
	get_nsec_per_sec res=x9
	lsl x9, x9, x12

	get_clock_shifted_nsec res=x15, cycle_last=x10, mult=x11
	get_ts_clock_raw res_sec=x10, res_nsec=x11, \
	clock_nsec=x15, nsec_to_sec=x9

	add_ts sec=x10, nsec=x11, ts_sec=x13, ts_nsec=x14, nsec_to_sec=x9
	clock_gettime_return, shift=1

	ALIGN
	realtime_coarse:
	seqcnt_acquire
	ldp x10, x11, [vdso_data, #VDSO_XTIME_CRS_SEC]
	seqcnt_check fail=realtime_coarse
	clock_gettime_return

	ALIGN
	monotonic_coarse:
	seqcnt_acquire
	ldp x10, x11, [vdso_data, #VDSO_XTIME_CRS_SEC]
	ldp x13, x14, [vdso_data, #VDSO_WTM_CLK_SEC]
	seqcnt_check fail=monotonic_coarse

	/* Computations are done in (non-shifted) nsecs. */
	get_nsec_per_sec res=x9
	add_ts sec=x10, nsec=x11, ts_sec=x13, ts_nsec=x14, nsec_to_sec=x9
	clock_gettime_return

	ALIGN
	syscall: /* Syscall fallback. */
	mov x8, #__NR_clock_gettime
	svc #0
	ret
	.cfi_endproc
	ENDPROC(__kernel_clock_gettime)

	/* int __kernel_clock_getres(clockid_t clock_id, struct timespec res); /
	ENTRY(__kernel_clock_getres)
	.cfi_startproc
	cmp w0, #CLOCK_REALTIME
	ccmp w0, #CLOCK_MONOTONIC, #0x4, ne
	ccmp w0, #CLOCK_MONOTONIC_RAW, #0x4, ne
	b.ne 1f

	ldr x2, 5f
	b 2f
	1:
	cmp w0, #CLOCK_REALTIME_COARSE
	ccmp w0, #CLOCK_MONOTONIC_COARSE, #0x4, ne
	b.ne 4f
	ldr x2, 6f
	2:
	cbz w1, 3f
	stp xzr, x2, [x1]

	3: /* res == NULL. */
	mov w0, wzr
	ret

	4: /* Syscall fallback. */
	mov x8, #__NR_clock_getres
	svc #0
	ret
	5:
	.quad CLOCK_REALTIME_RES
	6:
	.quad CLOCK_COARSE_RES
	.cfi_endproc
	ENDPROC(__kernel_clock_getres)