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doc: add ARMv6 expected outputs to mpu_test README

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Add expected output for ARMv6 CPUs in the mpu_test README, as this differs
from that for other AArch32 CPUs due to differences in how ARMv6 handles
faults

Signed-off-by: Katherine Watson <katie@radicalsemiconductor.com>
This commit is contained in:
Katherine Watson 2023-04-20 19:10:01 -07:00 • committed by Carles Cufí
commit cfc4273bbb
1 changed files with 50 additions and 0 deletions
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50
samples/arch/mpu/mpu_test/README.rst
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@ -43,6 +43,9 @@ proper configs.
Sample Output
=============
When running the above on an ARMv7 or ARMv8 CPU, the output of each command may look
like the following.
.. code-block:: console
uart:~$ mpu read
@ -86,3 +89,50 @@ Sample Output
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x20000708 (shell_uart)
<err> os: Halting system
When running this test on an ARMv6 CPU (specifically on a Cortex-M0+), the output will
look different, as ARMv6 handles MPU faults as generic hard faults. Moreover, when
running the ``mpu run`` command, Zephyr's hard fault handler for AArch32 tries to
dereference the program counter from the exception stack frame, causing a second fault
and silently placing the processor into a lockup state.
To verify that the ``mpu run`` test in particular is running correctly, one can
* start the MPU test under gdb with ``west debug``
* execute ``mpu run`` over UART
* then interrupt gdb with Ctrl-C and show the program counter by running ``l`` in gdb.
The program counter should display as ``0xfffffffe``, indicating a lockup state.
.. code-block:: console
uart:~$ mpu read
<err> os: ***** HARD FAULT *****
<err> os: r0/a1: 0x0800a54c r1/a2: 0x00000008 r2/a3: 0x08003
<err> os: r3/a4: 0x24000000 r12/ip: 0x00000040 r14/lr: 0x0800d
<err> os: xpsr: 0x01000000
<err> os: Faulting instruction address (r15/pc): 0x08000486
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x200006a8 (shell_uart)
<err> os: Halting system
.. code-block:: console
uart:~$ mpu write
<err> os: ***** HARD FAULT *****
<err> os: r0/a1: 0x00000000 r1/a2: 0x0000000e r2/a3: 0x0000e
<err> os: r3/a4: 0x0badc0de r12/ip: 0x00000000 r14/lr: 0x08009
<err> os: xpsr: 0x61000000
<err> os: Faulting instruction address (r15/pc): 0x0800046a
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x200006a8 (shell_uart)
<err> os: Halting system
.. code-block:: console
uart~$ mpu run
<no output>