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arch: arm: Add Cortex-R exception handling documentation.

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Add in-line documentation describing the process of register
preservation and exception handling on Cortex-R.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
This commit is contained in:
Stephanos Ioannidis 2019-09-27 09:44:35 +09:00 • committed by Maureen Helm
commit fe85c2e2e0
2 changed files with 33 additions and 7 deletions
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15
arch/arm/core/exc_exit.S
View file

@ -72,6 +72,7 @@ SECTION_SUBSEC_FUNC(TEXT, _HandlerModeExit, z_arm_int_exit)
SECTION_SUBSEC_FUNC(TEXT, _HandlerModeExit, z_arm_exc_exit)
#if defined(CONFIG_CPU_CORTEX_R)
/* r0 contains the caller mode */
push {r0, lr}
#endif
@ -116,11 +117,17 @@ _EXIT_EXC:
#if defined(CONFIG_CPU_CORTEX_M)
bx lr
#elif defined(CONFIG_CPU_CORTEX_R)
/*
* r0-r3 are either the values from the thread before it was switched out
* or they are the args to _new_thread for a new thread
*/
/* Restore the caller mode to r0 */
pop {r0, lr}
/*
* Restore r0-r3, r12 and lr stored into the process stack by the mode
* entry function. These registers are saved by _isr_wrapper for IRQ mode
* and z_arm_svc for SVC mode.
*
* r0-r3 are either the values from the thread before it was switched out
* or they are the args to _new_thread for a new thread.
*/
push {r4, r5}
cmp r0, #RET_FROM_SVC

25
arch/arm/core/swap_helper.S
View file

@ -39,6 +39,10 @@ GDATA(_kernel)
* 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-R, PendSV exception is not supported by the architecture and this
* function is directly called either by _IntExit in case of preemption, or
* z_arm_svc in case of cooperative switching.
*/
SECTION_FUNC(TEXT, z_arm_pendsv)
@ -328,7 +332,10 @@ _thread_irq_disabled:
#endif
#endif /* CONFIG_TRACING */
/* exc return */
/*
* Cortex-M: return from PendSV exception
* Cortex-R: return to the caller (_IntExit or z_arm_svc)
*/
bx lr
#if defined(CONFIG_ARMV6_M_ARMV8_M_BASELINE)
@ -514,13 +521,25 @@ valid_syscall_id:
/* return from SVC to the modified LR - z_arm_do_syscall */
bx lr
#endif
#endif /* CONFIG_USERSPACE */
#elif defined(CONFIG_ARMV7_R)
/**
*
* @brief Service call handler
*
* The service call (svc) is used in the following occasions:
* - Cooperative context switching
* - IRQ offloading
* - Kernel run-time exceptions
*
* @return N/A
*/
SECTION_FUNC(TEXT, z_arm_svc)
/*
* Switch to system mode to store r0-r3 to the process stack pointer.
* Save r12 and the lr as we will be swapping in another process and
* Save r12 and the lr as we could be swapping in another process and
* returning to a different location.
*/
push {r4, r5}