cpu/{atmega_common,atxmega}: increase idle thread stack size

[maribu]

Contribution description

Our AVR port doesn't make use of an ISR stack and just victimizes the
stack of whatever thread happens to be running, which in most cases is
the idle thread. Hence, the idle stack has to be large enough to
support the ztimer ISR.

Testing procedure

Run program given in #18245 (comment) - with master you'll get something like

2022-06-27 12:49:44,725 # main(): This is RIOT! (Version: 2022.07-devel-898-gd9fc08)
2022-06-27 12:49:45,770 # scheduler(): stack overflow detected, pid=1
2022-06-27 12:49:46,815 # scheduler(): stack overflow detected, pid=1
2022-06-27 12:49:47,859 # scheduler(): stack overflow detected, pid=1

but with this PR the stack overflow should no longer be detected. (Test successfully with both Alpine's AVR toolchain in BUILD_IN_DOCKER=1.)

Issues/PRs references

Reported in #18245 (comment)

[kaspar030]

same, 192 -> 128

[maribu]

Copy-pasted error :/ Good catch!

[benpicco]

Why does ATXMega need more stack @nandojve?

[maribu]

To some degree this can be explained with larger AVRs the RAMPZ, RAMPY, RAMPX, RAMPD and the 3 byte program counter, so that 5 bytes more are pushed onto the stack in case of context switching or IRQs. But the diff is quite a bit more than 5 bytes.

(Note that these registers are also not unique to ATXMegas, the better ATMegas can also have them.)

[nandojve]

@benpicco , @maribu is correct! It is important mention that call on large devices have a 24-bit PC. So, depending how code is generated it may be good have some additional stack space. This is not restricted to ATXmega it is about devices that have EIND.

(Rare) models with >128 KiB of ROM have a 3-byte program counter. Subroutine calls and returns use an additional byte of stack space, there is a new EIND register to provide additional high bits for indirect jumps and calls, and there are new extended instructions EIJMP and EICALL which use EIND:Z as the destination address. (The previous IJMP and ICALL instructions use zero-extended Z.)

[maribu]

Yes, of course, the add additional byte for $pc is not only needed in the context switch but for every function call :)

[benpicco]

Still does this require to double the required stack size?

[maribu]

I can later run a test to measure what exact size it needs. I also intend to merge this to a common header in avr8_common, which takes __AVR_3_BYTE_PC__, __AVR_HAVE_RAMPZ__, __AVR_HAVE_RAMPY__, __AVR_HAVE_RAMPX__, and __AVR_HAVE_RAMPD__ into account for the stack size, as some ATmega boards also have large stack requirements as well.

[maribu]

Hmpf, the unrelated hash mismatches are still a thing