PX4IO not enough RAM left for VTOL models like convergence

[julianoes]

@sfuhrer found that sometimes RC did not work on the convergence and that the ACT LED on the Pixhawk 4 IO side would be always on instead of blinking.

We could also see a segfault using GDB via Dronecode Probe:

Program received signal SIGSEGV, Segmentation fault.
mm_free (heap=0x20000438 <g_mmheap>, mem=0x20001e38) at mm_heap/mm_free.c:130
130	  if ((prev->preceding & MM_ALLOC_BIT) == 0)
(gdb) bt
#0  mm_free (heap=0x20000438 <g_mmheap>, mem=0x20001e38)
    at mm_heap/mm_free.c:130
#1  0x08009132 in NullMixer::~NullMixer (this=0x20001e38, 
    __in_chrg=<optimized out>)
    at ../../../../../src/lib/mixer/NullMixer/NullMixer.hpp:47
#2  0x080077d4 in List<Mixer*>::clear (
    this=this@entry=0x20000e3c <mixer_group>)
    at ../../../../../src/include/containers/List.hpp:168
#3  0x08007872 in MixerGroup::reset (this=0x20000e3c <mixer_group>)
    at ../../../../../src/lib/mixer/MixerGroup.hpp:74
#4  mixer_handle_text (buffer=buffer@entry=0x20000c65 <dma_packet+4>, 
    length=62) at ../../../../../src/modules/px4iofirmware/mixer.cpp:601
#5  0x08005d0c in registers_set (page=<optimized out>, 
    offset=<optimized out>, values=values@entry=0x20000c65 <dma_packet+4>, 
    num_values=<optimized out>)
    at ../../../../../src/modules/px4iofirmware/registers.c:464
#6  0x08006446 in rx_handle_packet ()
    at ../../../../../src/modules/px4iofirmware/serial.c:176
#7  rx_dma_callback (handle=<optimized out>, status=status@entry=2 '\002', 
    arg=arg@entry=0x0)
    at ../../../../../src/modules/px4iofirmware/serial.c:234
#8  0x080065be in serial_interrupt (irq=<optimized out>, 
    context=<optimized out>, arg=<optimized out>)
    at ../../../../../src/modules/px4iofirmware/serial.c:325
#9  0x08002fe2 in irq_dispatch (irq=irq@entry=54, 
    context=context@entry=0x20001c5c) at irq/irq_dispatch.c:176
#10 0x08001d88 in up_doirq (irq=54, regs=0x20001c5c) at armv7-m/up_doirq.c:86
#11 0x08001a7a in exception_common () at armv7-m/gnu/up_exception.S:213
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb) 

According to git bisect the bytes free go from 568 to 88 (sometimes 24) anywhere here, presumably due to the NuttX upgrade:

The first bad commit could be any of:
58799dc
694ae56
e847698
36a75b7
645b6c3
0f2a7ad
522ed2d
78944a7
d965b92
69694ab

Steps to reproduce:

Set SYS_AUTOSTART to 13012 for convergence airframe.
Boot and do px4io status.
You should see:

88 bytes free

Or sometimes garbage values when it crashed.

FYI @bkueng @davids5.

[dagar]

For comparison.

PX4IO: starting
MEM: fre` 1160, Hargest 1160  <-- stable v1.10
MEM: free 622, largest 632    <-- master

[julianoes]

@dagar What is that comparing? And what airframe?

[dagar]

It's the SYSLOG output in px4io.c before the mixer is even loaded.

https://github.com/PX4/Firmware/blob/b399698306e5afa068903adda094adedf0423691/src/modules/px4iofirmware/px4io.c#L359

[davids5]

@julianoes - it was the last upgrade of NuttX, when the H7 came in. FD/streams became required. We could set it to 0 in the past and that saved memory. I adjusted the stack and free mem to boot. with out police lights, but Not all the mixers were tested. :(

[sfuhrer]

Hi guys, do you see an easy way to unblock people that are using this specific airframe?

[julianoes]

@sfuhrer #14128 might be just enough for now.

[sfuhrer]

@sfuhrer #14128 might be just enough for now.

True, the IO with the Convergence works again with this PR (with 40 bytes free).

[dagar]

This was fixed, although we're still slightly short for the most extreme case (dodecarotor coax).