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Ensure interrupts are enabled at first task start (#214)

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Critical sections in FreeRTOS are implemented using the following two
functions:

void vPortEnterCritical( void )
{
    portDISABLE_INTERRUPTS();
    uxCriticalNesting++;
}

void vPortExitCritical( void )
{
    uxCriticalNesting--;

    if( uxCriticalNesting == 0 )
    {
        portENABLE_INTERRUPTS();
    }
}

uxCriticalNesting is initialized to a large value at the start and set
to zero when the scheduler is started (xPortStartScheduler). As a
result, before the scheduler is started, a pair of enter/exit critical
section will leave the interrupts disabled because uxCriticalNesting
will not reach zero in the vPortExitCritical function. This is done to
ensure that the interrupts remain disabled from the time first FreeRTOS
API is called to the time when the scheduler is started. The scheduler
starting code is expected to enure that interrupts are enabled before
the first task starts executing.

Cortex-M33 ports were not enabling interrupts before starting the first
task and as a result, the first task was started with interrupts
disabled. This PR fixes the issue by ensuring that interrupts are
enabled before the first task is started.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
This commit is contained in:
Gaurav-Aggarwal-AWS 2020-11-05 09:26:56 -08:00 • committed by GitHub
parent 1431b65110
commit ebbe2cf854
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GPG key ID: 4AEE18F83AFDEB23
16 changed files with 59 additions and 33 deletions
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4
portable/IAR/ARM_CM33/non_secure/portasm.s
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@ -128,6 +128,8 @@ vRestoreContextOfFirstTask:
adds r0, #32 /* Discard everything up to r0. */
msr psp, r0 /* This is now the new top of stack to use in the task. */
isb
mov r0, #0
msr basepri, r0 /* Ensure that interrupts are enabled when the first task starts. */
bx r4 /* Finally, branch to EXC_RETURN. */
#else /* configENABLE_MPU */
ldm r0!, {r1-r3} /* Read from stack - r1 = xSecureContext, r2 = PSPLIM and r3 = EXC_RETURN. */
@ -139,6 +141,8 @@ vRestoreContextOfFirstTask:
adds r0, #32 /* Discard everything up to r0. */
msr psp, r0 /* This is now the new top of stack to use in the task. */
isb
mov r0, #0
msr basepri, r0 /* Ensure that interrupts are enabled when the first task starts. */
bx r3 /* Finally, branch to EXC_RETURN. */
#endif /* configENABLE_MPU */
/*-----------------------------------------------------------*/