GM: no speed limit to resume

[vntarasov]

Remove "18mph to engage" limit, through catching a PCM fault and showing the same "Speed too low" error instead.

Pros:

• "resume" re-engagement is possible at much lower speeds. For 2017 Volt:
  • resume works at any speed, if brake pedal was not pressed since disengagement. Heh, I was able to resume from rolling backwards against an uphill (resume from negative speed?).
  • resume works above ~7 mph if brake pedal was pressed since disengagement.
• 2018 has even lower limit to engage for both "set" and "res". With this change, we have a precise speed limit for engagement, no need for overshooting the limit, no need to dig for PCM states for various years.

Cons:

• If PCM fault happens due to an unrelated bug, "Speed too low" would be a wrong message to show.
• Different user experience compared to "wrong gear" and other known state errors: instead of showing "Speed too low" right away, PCM fault catching & message will happen split second after openpilot engages.

[vntarasov]

Resuming from near-zero speeds:

[rbiasini]

@vntarasov , have you seen the AccState becoming FAULTED for other reasons other than the low speed lockout?

[rbiasini]

Actually, I think I understand what you mean: in this case the AccState will become FAULTED only immediately after the engagement. Does it stay FAULTED or it clears after you stop commanding gas?

[vntarasov]

Good point, let me find someone willing to test it on 2018 Volt. On my 2017 Volt, speed-related FAULTED clears out as soon as openpilot disengages, but I haven't tested it on 2018 models. It would be rather inconvenient if PCM stays faulted.

Re: other ways to get to FAULTED: There was a bug with "isActive" vs "isEnabled", where PCM would go to FAULTED (for a few seconds on 2017 Volts, and until ACC is reset on 2018), fixed in #352. I don't know of other ways to get it to FAULTED other than stop sending keepalive messages (same GasRegenCmd, with "enabled" = False).

[vntarasov]

Recovery from PCM fault due to attempt to engage at low speed is almost instant on 2017 Volt:
(It's also interesing that PCM goes to "1 - Success" at first, and then faults out)

[rbiasini]

behavior makes sense. Probably would not fault if you were commanding brakes, so it waits for a certain number of Gas commands before faulting.
So a weird behavior would be: you engage when Op wants to brake, and ACC would not fault until positive gas is commanded. Correct?

[vntarasov]

@rbiasini, there doesn't seem to be a trivial way to trick PCM to accelerate from below previously known limits :(

I tried various combinations of

• "zero gas (no regen)", no friction braking
• "zero gas (no regen)", 1 unit of friction braking
• "little regen", no friction braking
• "max ACC regen", zero friction braking
• "max ACC regen", 80 units of friction braking

Everything would fault out PCM the same way, at same speed limits: going to "1" for a few messages, then to "3 - faulted".

Obviously there must be a way for FCB to kick in somehow, but I don't have a cabana logs for stock FCB, and chances are commanding gas would still not work from there.

[vntarasov]

@rbiasini PCM fault in question clears on 2018 Volt as well, as soon as OP disengages.
The most interesting is that 2018 Volt has a much lower speed limit for the initial "set"-engagement (~11 kph), and it can always be resumed at any speed, regardless of brake pedal.

[rbiasini]

@vntarasov , I'm ok merging this. Any more tests to be done you think?

[vntarasov]

@rbiasini, the only quirk is that the first time user presses "res" instead of "set", the message is "speed too low" instead of 'first engage must be through "set" '

[rbiasini]

@vntarasov mmm... but that's unrelated to this change, right?

[vntarasov]

The solution is to not make the resume button an ENABLE event if there is no set prior set speed. Will reopen when done.