cpu/esp: fix computation of coprocessor save area pointer in initial thread context

[gschorcht]

Contribution description

This PR fixes a problem that was figured out in issue #11443.

With commit 28d9599, the following change was introduced:

--- a/cpu/esp32/thread_arch.c
+++ b/cpu/esp32/thread_arch.c
@@ -186,1 +186,1 @@ char* thread_stack_init(thread_task_func_t task_func, void *arg, void *stack_sta
-    p = (uint32_t *)(((uint32_t)(top_of_stack + 1) - XT_CP_SIZE));
+    p = (uint32_t *)(((uint32_t)(top_of_stack + 1) - XT_CP_SIZE) & ~0xf);

top_of_stack isn't aligned down to the previous 16 byte aligned address. Furthermore, top_of_stack as well as XT_CP_SIZE are used unaligned in cpu/esp_common/vendor/xtensa/portasm.S in address computation for the coprocessor save area, .

Aligning pointer p down to the previous 16 byte aligned address results in a wrong address of the coprocessor save area during the initialization of the thread context. This leads to wrong values and wrong positions of these values in the coprocessor save area in inital thread context.

Since ESP8266 doesn't have a coprocessor, this bug affects only ESP32.

Testing procedure

The only known application suffering from this problem is examples/lorawan on one of the new LoRa32 board definitions in PR #11265 and #11418. Therefore, it is suggested to test this PR with one of these boards.

make BOARD=esp32-heltec-lora32-v2 -C examples/lorawan flash

Before the fix, the application hangs in function semtech_loramac_init and ends within a boot loop.

...
Starting RIOT kernel on PRO cpu
I (247) [main_trampoline]: main(): This is RIOT! (Version: 2019.07-devel-141-g203c-boards/esp32-heltec-lora32)
LoRaWAN Class A low-power application
=====================================
ets Jun  8 2016 00:22:57

rst:0x7 (TG0WDT_SYS_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
...

With the fix, function semtech_loramac_init works and the application prints the message "Starting join procedure".

...
Starting RIOT kernel on PRO cpu
I (247) [main_trampoline]: main(): This is RIOT! (Version: 2019.07-devel-141-g203c-boards/esp32-heltec-lora32)
LoRaWAN Class A low-power application
=====================================
Starting join procedure
...

Issues/PRs references

Fixes #11443

[yegorich]

Tested on TTGO T-Beam board with both examples/lorawan and a custom application. Bugfix confirmed.

[gschorcht]

@sming May I kindly ask you to review it since @yegorich isn't maintainer? Thanks.

[smlng]

Looks good to me, trust @yegorich testing

[fcgdam]

Can confirm that this fix also works and solves the issue for me.

[gschorcht]

@smlng Thanks for reviewing
@yegorich Thanks for figuring out the problem
@fcgdam Thanks for testing
😄

[yegorich]

@gschorcht thanks for a quick fix and let's start merging the boards :-)