/*
 * Copyright (c) 2013-2014 Wind River Systems, Inc.
 * Copyright (c) 2017-2019 Nordic Semiconductor ASA.
 * Copyright (c) 2020 Stephanos Ioannidis <root@stephanos.io>
 *
 * SPDX-License-Identifier: Apache-2.0
 */

/**
 * @file
 * @brief Thread context switching for ARM Cortex-M and Cortex-R
 *
 * This module implements the routines necessary for thread context switching
 * on ARM Cortex-A, Cortex-M and Cortex-R CPUs.
 */

#include <toolchain.h>
#include <linker/sections.h>
#include <offsets_short.h>
#include <arch/cpu.h>
#include <syscall.h>

#if defined(CONFIG_CPU_CORTEX_M)
#include <arch/arm/aarch32/cortex_m/cpu.h>
#endif
_ASM_FILE_PROLOGUE

GTEXT(z_arm_svc)
GTEXT(z_arm_pendsv)
GTEXT(z_do_kernel_oops)
#if defined(CONFIG_USERSPACE)
GTEXT(z_arm_do_syscall)
#endif

GDATA(_kernel)

#if defined(CONFIG_THREAD_LOCAL_STORAGE) && defined(CONFIG_CPU_CORTEX_M)
GDATA(z_arm_tls_ptr)
#endif

/**
 *
 * @brief PendSV exception handler, handling context switches
 *
 * The PendSV exception is the only execution context in the system that can
 * perform context switching. When an execution context finds out it has to
 * switch contexts, it pends the PendSV exception.
 *
 * When PendSV is pended, the decision that a context switch must happen has
 * already been taken. In other words, when z_arm_pendsv() runs, we *know* we
 * have to swap *something*.
 *
 * For Cortex-M, z_arm_pendsv() is invoked with no arguments.
 *
 * For Cortex-R, PendSV exception is not supported by the architecture and this
 * function is directly called either by z_arm_{exc,int}_exit in case of
 * preemption, or z_arm_svc in case of cooperative switching.
 */

SECTION_FUNC(TEXT, z_arm_pendsv)

#ifdef CONFIG_INSTRUMENT_THREAD_SWITCHING
    /* Register the context switch */
    push {r0, lr}
    bl z_thread_mark_switched_out
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    pop {r0, r1}
    mov lr, r1
#else
    pop {r0, lr}
#endif /* CONFIG_ARMV6_M_ARMV8_M_BASELINE */
#endif /* CONFIG_INSTRUMENT_THREAD_SWITCHING */

    /* load _kernel into r1 and current k_thread into r2 */
    ldr r1, =_kernel
    ldr r2, [r1, #_kernel_offset_to_current]

#if defined(CONFIG_ARM_STORE_EXC_RETURN)
    /* Store LSB of LR (EXC_RETURN) to the thread's 'mode' word. */
    strb lr, [r2, #_thread_offset_to_mode_exc_return]
#endif

    /* addr of callee-saved regs in thread in r0 */
    ldr r0, =_thread_offset_to_callee_saved
    add r0, r2

    /* save callee-saved + psp in thread */
#if defined(CONFIG_CPU_CORTEX_M)
    mrs ip, PSP
#endif

#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    /* Store current r4-r7 */
    stmea r0!, {r4-r7}
    /* copy r8-r12 into r3-r7 */
    mov r3, r8
    mov r4, r9
    mov r5, r10
    mov r6, r11
    mov r7, ip
    /* store r8-12 */
    stmea r0!, {r3-r7}
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    stmia r0, {v1-v8, ip}
#ifdef CONFIG_FPU_SHARING
    /* Assess whether switched-out thread had been using the FP registers. */
    tst lr, #_EXC_RETURN_FTYPE_Msk
    bne out_fp_endif

    /* FP context active: set FP state and store callee-saved registers.
     * Note: if Lazy FP stacking is enabled, storing the callee-saved
     * registers will automatically trigger FP state preservation in
     * the thread's stack. This will also clear the FPCCR.LSPACT flag.
     */
    add r0, r2, #_thread_offset_to_preempt_float
    vstmia r0, {s16-s31}

out_fp_endif:
    /* At this point FPCCR.LSPACT is guaranteed to be cleared,
     * regardless of whether the thread has an active FP context.
     */
#endif /* CONFIG_FPU_SHARING */
#elif defined(CONFIG_ARMV7_R) || defined(CONFIG_AARCH32_ARMV8_R) \
	|| defined(CONFIG_ARMV7_A)
    /* Store rest of process context */
    cps #MODE_SYS
    stm r0, {r4-r11, sp}
    cps #MODE_SVC
#else
#error Unknown ARM architecture
#endif /* CONFIG_ARMV6_M_ARMV8_M_BASELINE */

    /* Protect the kernel state while we play with the thread lists */
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    cpsid i
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    movs.n r0, #_EXC_IRQ_DEFAULT_PRIO
    msr BASEPRI_MAX, r0
    isb /* Make the effect of disabling interrupts be realized immediately */
#elif defined(CONFIG_ARMV7_R) || defined(CONFIG_AARCH32_ARMV8_R) \
	|| defined(CONFIG_ARMV7_A)
    /*
     * Interrupts are still disabled from arch_swap so empty clause
     * here to avoid the preprocessor error below
     */
#else
#error Unknown ARM architecture
#endif /* CONFIG_ARMV6_M_ARMV8_M_BASELINE */

    /*
     * Prepare to clear PendSV with interrupts unlocked, but
     * don't clear it yet. PendSV must not be cleared until
     * the new thread is context-switched in since all decisions
     * to pend PendSV have been taken with the current kernel
     * state and this is what we're handling currently.
     */
#if defined(CONFIG_CPU_CORTEX_M)
    ldr v4, =_SCS_ICSR
    ldr v3, =_SCS_ICSR_UNPENDSV
#endif

    /* _kernel is still in r1 */

    /* fetch the thread to run from the ready queue cache */
    ldr r2, [r1, #_kernel_offset_to_ready_q_cache]

    str r2, [r1, #_kernel_offset_to_current]

    /*
     * Clear PendSV so that if another interrupt comes in and
     * decides, with the new kernel state based on the new thread
     * being context-switched in, that it needs to reschedule, it
     * will take, but that previously pended PendSVs do not take,
     * since they were based on the previous kernel state and this
     * has been handled.
     */

    /* _SCS_ICSR is still in v4 and _SCS_ICSR_UNPENDSV in v3 */
#if defined(CONFIG_CPU_CORTEX_M)
    str v3, [v4, #0]
#endif

#if defined(CONFIG_THREAD_LOCAL_STORAGE)
    /* Grab the TLS pointer */
    ldr r4, =_thread_offset_to_tls
    adds r4, r2, r4
    ldr r0, [r4]

#if defined(CONFIG_CPU_AARCH32_CORTEX_R)
    /* Store TLS pointer in the "Process ID" register.
     * This register is used as a base pointer to all
     * thread variables with offsets added by toolchain.
     */
    mcr 15, 0, r0, cr13, cr0, 3
#endif

#if defined(CONFIG_CPU_CORTEX_M)
    /* For Cortex-M, store TLS pointer in a global variable,
     * as it lacks the process ID or thread ID register
     * to be used by toolchain to access thread data.
     */
    ldr r4, =z_arm_tls_ptr
    str r0, [r4]
#endif

#endif

#if defined(CONFIG_ARM_STORE_EXC_RETURN)
    /* Restore EXC_RETURN value. */
    ldrsb lr, [r2, #_thread_offset_to_mode_exc_return]
#endif

    /* Restore previous interrupt disable state (irq_lock key)
     * (We clear the arch.basepri field after restoring state)
     */
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE) && (_thread_offset_to_basepri > 124)
    /* Doing it this way since the offset to thread->arch.basepri can in
     * some configurations be larger than the maximum of 124 for ldr/str
     * immediate offsets.
     */
    ldr r4, =_thread_offset_to_basepri
    adds r4, r2, r4

    ldr r0, [r4]
    movs.n r3, #0
    str r3, [r4]
#else
    ldr r0, [r2, #_thread_offset_to_basepri]
    movs r3, #0
    str r3, [r2, #_thread_offset_to_basepri]
#endif

#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    /* BASEPRI not available, previous interrupt disable state
     * maps to PRIMASK.
     *
     * Only enable interrupts if value is 0, meaning interrupts
     * were enabled before irq_lock was called.
     */
    cmp r0, #0
    bne _thread_irq_disabled
    cpsie i
_thread_irq_disabled:

#if defined(CONFIG_MPU_STACK_GUARD) || defined(CONFIG_USERSPACE)
    /* Re-program dynamic memory map */
    push {r2,lr}
    mov r0, r2
    bl z_arm_configure_dynamic_mpu_regions
    pop {r2,r3}
    mov lr, r3
#endif

#ifdef CONFIG_USERSPACE
    /* restore mode */
    ldr r3, =_thread_offset_to_mode
    adds r3, r2, r3
    ldr r0, [r3]
    mrs r3, CONTROL
    movs.n r1, #1
    bics r3, r1
    orrs r3, r0
    msr CONTROL, r3

    /* ISB is not strictly necessary here (stack pointer is not being
     * touched), but it's recommended to avoid executing pre-fetched
     * instructions with the previous privilege.
     */
    isb

#endif

    ldr r4, =_thread_offset_to_callee_saved
    adds r0, r2, r4

    /* restore r4-r12 for new thread */
    /* first restore r8-r12 located after r4-r7 (4*4bytes) */
    adds r0, #16
    ldmia r0!, {r3-r7}
    /* move to correct registers */
    mov r8, r3
    mov r9, r4
    mov r10, r5
    mov r11, r6
    mov ip, r7
    /* restore r4-r7, go back 9*4 bytes to the start of the stored block */
    subs r0, #36
    ldmia r0!, {r4-r7}
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    /* restore BASEPRI for the incoming thread */
    msr BASEPRI, r0

#ifdef CONFIG_FPU_SHARING
    /* Assess whether switched-in thread had been using the FP registers. */
    tst lr, #_EXC_RETURN_FTYPE_Msk
    beq in_fp_active
    /* FP context inactive for swapped-in thread:
     * - reset FPSCR to 0
     * - set EXC_RETURN.F_Type (prevents FP frame un-stacking when returning
     *   from pendSV)
     */
    movs.n r3, #0
    vmsr fpscr, r3
    b in_fp_endif

in_fp_active:
    /* FP context active:
     * - clear EXC_RETURN.F_Type
     * - FPSCR and caller-saved registers will be restored automatically
     * - restore callee-saved FP registers
     */
    add r0, r2, #_thread_offset_to_preempt_float
    vldmia r0, {s16-s31}
in_fp_endif:
    /* Clear CONTROL.FPCA that may have been set by FP instructions */
    mrs r3, CONTROL
    bic r3, #_CONTROL_FPCA_Msk
    msr CONTROL, r3
    isb
#endif

#if defined(CONFIG_MPU_STACK_GUARD) || defined(CONFIG_USERSPACE)
    /* Re-program dynamic memory map */
    push {r2,lr}
    mov r0, r2 /* _current thread */
    bl z_arm_configure_dynamic_mpu_regions
    pop {r2,lr}
#endif

#ifdef CONFIG_USERSPACE
    /* restore mode */
    ldr r0, [r2, #_thread_offset_to_mode]
    mrs r3, CONTROL
    bic r3, #1
    orr r3, r0
    msr CONTROL, r3

    /* ISB is not strictly necessary here (stack pointer is not being
     * touched), but it's recommended to avoid executing pre-fetched
     * instructions with the previous privilege.
     */
    isb

#endif

    /* load callee-saved + psp from thread */
    add r0, r2, #_thread_offset_to_callee_saved
    ldmia r0, {v1-v8, ip}
#elif defined(CONFIG_ARMV7_R) || defined(CONFIG_AARCH32_ARMV8_R) \
	|| defined(CONFIG_ARMV7_A)
_thread_irq_disabled:
    /* load _kernel into r1 and current k_thread into r2 */
    ldr r1, =_kernel
    ldr r2, [r1, #_kernel_offset_to_current]

    /* addr of callee-saved regs in thread in r0 */
    ldr r0, =_thread_offset_to_callee_saved
    add r0, r2

    /* restore r4-r11 and sp for incoming thread */
    cps #MODE_SYS
    ldm r0, {r4-r11, sp}
    cps #MODE_SVC

#if defined (CONFIG_ARM_MPU)
    /* r2 contains k_thread */
    mov r0, r2
    /* Re-program dynamic memory map */
    push {r2, lr}
    bl z_arm_configure_dynamic_mpu_regions
    pop {r2, lr}
#endif
#else
#error Unknown ARM architecture
#endif /* CONFIG_ARMV6_M_ARMV8_M_BASELINE */

#if defined(CONFIG_CPU_CORTEX_M)
    msr PSP, ip
#endif

#ifdef CONFIG_BUILTIN_STACK_GUARD
    /* r2 contains k_thread */
    add r0, r2, #0
    push {r2, lr}
    bl configure_builtin_stack_guard
    pop {r2, lr}
#endif /* CONFIG_BUILTIN_STACK_GUARD */

#ifdef CONFIG_INSTRUMENT_THREAD_SWITCHING
    /* Register the context switch */
    push {r0, lr}
    bl z_thread_mark_switched_in
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    pop {r0, r1}
    mov lr, r1
#else
    pop {r0, lr}
#endif
#endif /* CONFIG_INSTRUMENT_THREAD_SWITCHING */

    /*
     * Cortex-M: return from PendSV exception
     * Cortex-R: return to the caller (z_arm_{exc,int}_exit, or z_arm_svc)
     */
    bx lr

#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE) || \
  defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)

/**
 *
 * @brief Service call handler
 *
 * The service call (svc) is used in the following occasions:
 * - IRQ offloading
 * - Kernel run-time exceptions
 * - System Calls (User mode)
 *
 */
SECTION_FUNC(TEXT, z_arm_svc)
  /* Use EXC_RETURN state to find out if stack frame is on the
   * MSP or PSP
   */
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
  movs r0, #_EXC_RETURN_SPSEL_Msk
  mov r1, lr
  tst r1, r0
  beq _stack_frame_msp
  mrs r0, PSP
  bne _stack_frame_endif
_stack_frame_msp:
  mrs r0, MSP
_stack_frame_endif:
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    tst lr, #_EXC_RETURN_SPSEL_Msk /* did we come from thread mode ? */
    ite eq  /* if zero (equal), came from handler mode */
        mrseq r0, MSP   /* handler mode, stack frame is on MSP */
        mrsne r0, PSP   /* thread mode, stack frame is on PSP */
#endif


    /* Figure out what SVC call number was invoked */

    ldr r1, [r0, #24]   /* grab address of PC from stack frame */
    /* SVC is a two-byte instruction, point to it and read the
     * SVC number (lower byte of SCV instruction)
     */
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    subs r1, r1, #2
    ldrb r1, [r1]
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    ldrb r1, [r1, #-2]
#endif

   /*
    * grab service call number:
    * 0: Unused
    * 1: irq_offload (if configured)
    * 2: kernel panic or oops (software generated fatal exception)
    * 3: System call (if user mode supported)
    */
#if defined(CONFIG_USERSPACE)
    mrs r2, CONTROL

    cmp r1, #3
    beq _do_syscall

    /*
     * check that we are privileged before invoking other SVCs
     * oops if we are unprivileged
     */
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    movs r3, #0x1
    tst r2, r3
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    tst r2, #0x1
#endif
    bne _oops

#endif /* CONFIG_USERSPACE */

    cmp r1, #2
    beq _oops

#if defined(CONFIG_IRQ_OFFLOAD)
    push {r0, lr}
    bl z_irq_do_offload  /* call C routine which executes the offload */
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    pop {r0, r3}
    mov lr, r3
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    pop {r0, lr}
#endif

    /* exception return is done in z_arm_int_exit() */
    b z_arm_int_exit
#endif

_oops:
    push {r0, lr}
    bl z_do_kernel_oops
    /* return from SVC exception is done here */
    pop {r0, pc}

#if defined(CONFIG_USERSPACE)
    /*
     * System call will setup a jump to the z_arm_do_syscall() function
     * when the SVC returns via the bx lr.
     *
     * There is some trickery involved here because we have to preserve
     * the original PC value so that we can return back to the caller of
     * the SVC.
     *
     * On SVC exeption, the stack looks like the following:
     * r0 - r1 - r2 - r3 - r12 - LR - PC - PSR
     *
     * Registers look like:
     * r0 - arg1
     * r1 - arg2
     * r2 - arg3
     * r3 - arg4
     * r4 - arg5
     * r5 - arg6
     * r6 - call_id
     * r8 - saved link register
     */
_do_syscall:
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    movs r3, #24
    ldr r1, [r0, r3]   /* grab address of PC from stack frame */
    mov r8, r1
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    ldr r8, [r0, #24]   /* grab address of PC from stack frame */
#endif
    ldr r1, =z_arm_do_syscall
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    str r1, [r0, r3]   /* overwrite the PC to point to z_arm_do_syscall */
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    str r1, [r0, #24]   /* overwrite the PC to point to z_arm_do_syscall */
#endif

#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    ldr r3, =K_SYSCALL_LIMIT
    cmp r6, r3
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    /* validate syscall limit */
    ldr ip, =K_SYSCALL_LIMIT
    cmp r6, ip
#endif
    /* The supplied syscall_id must be lower than the limit
     * (Requires unsigned integer comparison)
     */
    blo valid_syscall_id

    /* bad syscall id.  Set arg1 to bad id and set call_id to SYSCALL_BAD */
    str r6, [r0]
    ldr r6, =K_SYSCALL_BAD

    /* Bad syscalls treated as valid syscalls with ID K_SYSCALL_BAD. */

valid_syscall_id:
    ldr r0, =_kernel
    ldr r0, [r0, #_kernel_offset_to_current]
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
    mov ip, r2
    ldr r1, =_thread_offset_to_mode
    ldr r3, [r0, r1]
    movs r2, #1
    bics r3, r2
    /* Store (privileged) mode in thread's mode state variable */
    str r3, [r0, r1]
    mov r2, ip
    dsb
    /* set mode to privileged, r2 still contains value from CONTROL */
    movs r3, #1
    bics r2, r3
#elif defined(CONFIG_ARMV7_M_ARMV8_M_MAINLINE)
    ldr r1, [r0, #_thread_offset_to_mode]
    bic r1, #1
    /* Store (privileged) mode in thread's mode state variable */
    str r1, [r0, #_thread_offset_to_mode]
    dsb
    /* set mode to privileged, r2 still contains value from CONTROL */
    bic r2, #1
#endif
    msr CONTROL, r2

    /* ISB is not strictly necessary here (stack pointer is not being
     * touched), but it's recommended to avoid executing pre-fetched
     * instructions with the previous privilege.
     */
    isb

#if defined(CONFIG_BUILTIN_STACK_GUARD)
    /* Thread is now in privileged mode; after returning from SCVall it
     * will use the default (user) stack before switching to the privileged
     * stack to execute the system call. We need to protect the user stack
     * against stack overflows until this stack transition.
     */
    ldr r1, [r0, #_thread_offset_to_stack_info_start]    /* stack_info.start */
    msr PSPLIM, r1
#endif /* CONFIG_BUILTIN_STACK_GUARD */

    /* return from SVC to the modified LR - z_arm_do_syscall */
    bx lr
#endif /* CONFIG_USERSPACE */

#elif defined(CONFIG_ARMV7_R) || defined(CONFIG_AARCH32_ARMV8_R) \
	|| defined(CONFIG_ARMV7_A)

/**
 *
 * @brief Service call handler
 *
 * The service call (svc) is used in the following occasions:
 * - Cooperative context switching
 * - IRQ offloading
 * - Kernel run-time exceptions
 *
 */
SECTION_FUNC(TEXT, z_arm_svc)
#if defined(CONFIG_USERSPACE)
    /* Determine if incoming thread was in user context */
    push {r0}
    mrs r0, spsr
    and r0, #MODE_MASK
    cmp r0, #MODE_USR
    bne svc_system_thread

    ldr r0, =_kernel
    ldr r0, [r0, #_kernel_offset_to_current]

    /* Save away user stack pointer */
    cps #MODE_SYS
    str sp, [r0, #_thread_offset_to_sp_usr] /* sp_usr */

    /* Switch to privileged stack */
    ldr sp, [r0, #_thread_offset_to_priv_stack_end] /* priv stack end */
    cps #MODE_SVC

svc_system_thread:
    pop {r0}
#endif

    /*
     * Switch to system mode to store r0-r3 to the process stack pointer.
     * Save r12 and the lr as we could be swapping in another process and
     * returning to a different location.
     */
    srsdb #MODE_SYS!
    cps #MODE_SYS
    push {r0-r3, r12, lr}
    mov ip, sp
    cps #MODE_SVC

    /*
     * Store lr_svc to the SVC mode stack. This value will be restored prior to
     * exiting the SVC call in z_arm_int_exit.
     */
    push {lr}

    /* Align stack at double-word boundary */
    and r3, sp, #4
    sub sp, sp, r3
    push {r2, r3}

    /* Increment interrupt nesting count */
    ldr r2, =_kernel
    ldr r0, [r2, #_kernel_offset_to_nested]
    add r0, r0, #1
    str r0, [r2, #_kernel_offset_to_nested]

    /* Get SVC number */
    mrs r0, spsr
    tst r0, #0x20

    ldreq r1, [lr, #-4]
    biceq r1, #0xff000000
    beq demux

    ldr r1, [lr, #-2]
    and r1, #0xff

   /*
    * grab service call number:
    * 0: context switch
    * 1: irq_offload (if configured)
    * 2: kernel panic or oops (software generated fatal exception)
    * 3: system calls for memory protection
    */
demux:

#if defined(CONFIG_USERSPACE)
    cmp r1, #_SVC_CALL_SYSTEM_CALL
    beq _do_syscall
#endif

    cmp r1, #_SVC_CALL_CONTEXT_SWITCH
    beq _context_switch

    cmp r1, #_SVC_CALL_RUNTIME_EXCEPT
    beq _oops

#if CONFIG_IRQ_OFFLOAD
    blx z_irq_do_offload  /* call C routine which executes the offload */

    /* exception return is done in z_arm_int_exit() */
    b z_arm_int_exit
#endif

_context_switch:
    /* handler mode exit, to PendSV */
    bl z_arm_pendsv

    b z_arm_int_exit

_oops:
    /*
     * Pass the exception frame to z_do_kernel_oops.  r0 contains the
     * exception reason.
     */
    cps #MODE_SYS
    mov r0, sp
    cps #MODE_SVC
    bl z_do_kernel_oops
    b z_arm_int_exit

#if defined(CONFIG_USERSPACE)
    /*
     * System call will setup a jump to the _do_arm_syscall function
     * running in system mode when returning from the exception.
     *
     * There is some trickery involved here because we have to preserve
     * the original PC value so that we can return back to the caller of
     * the SVC.
     *
     * On SVC exception, the USER/SYSTEM stack looks like the following:
     * r0 - r1 - r2 - r3 - r12 - LR - { possible FPU space } - PC - SPSR
     *
     * Registers look like:
     * r0 - arg1
     * r1 - arg2
     * r2 - arg3
     * r3 - arg4
     * r4 - arg5
     * r5 - arg6
     * r6 - call_id
     * r8 - saved link register
     */
_do_syscall:
    ldr r8, [ip, #24]   /* grab address of LR from stack frame */

    /* Make the exception return to system state */
    ldr r1, [ip, #28]

    /* If leaving thumb mode, set the return address to thumb mode */
    tst r1, #T_BIT
    orrne r8, #1

    bic r1, #(MODE_MASK | T_BIT)
    orr r1, r1, #MODE_SYS
    str r1, [ip, #28]

    /*
     * Store the address of z_arm_do_syscall for the exit so the exception
     * return goes there in system state.
     */
    ldr r1, =z_arm_do_syscall
    str r1, [ip, #24]   /* overwrite the LR to point to z_arm_do_syscall */

    /* validate syscall limit, only set priv mode if valid */
    ldr ip, =K_SYSCALL_LIMIT
    cmp r6, ip
    blo valid_syscall_id

    /* bad syscall id.  Set arg0 to bad id and set call_id to SYSCALL_BAD */
    cps #MODE_SYS
    str r6, [sp]
    cps #MODE_SVC
    ldr r6, =K_SYSCALL_BAD

valid_syscall_id:
    ldr r0, =_kernel
    ldr r0, [r0, #_kernel_offset_to_current]
    ldr r1, [r0, #_thread_offset_to_mode]
    bic r1, #1
    /* Store (privileged) mode in thread's mode state variable */
    str r1, [r0, #_thread_offset_to_mode]
    dsb

    /* ISB is not strictly necessary here (stack pointer is not being
     * touched), but it's recommended to avoid executing pre-fetched
     * instructions with the previous privilege.
     */
    isb

    /* Return to _arm_do_syscall in system state. */
    b z_arm_int_exit
#endif

GTEXT(z_arm_cortex_r_svc)
SECTION_FUNC(TEXT, z_arm_cortex_r_svc)
    svc #_SVC_CALL_CONTEXT_SWITCH
    bx lr

#else
#error Unknown ARM architecture
#endif /* CONFIG_ARMV6_M_ARMV8_M_BASELINE */