Marlin: M914 (TMC Bump Sensitivity) returns incorrect values when negative.

[marcio-ao]

Bug Report

• Description: M914 (TMC Bump Sensitivity) returns incorrect values when negative.
• Expected behaviour: It should print a negative value
• Actual behaviour: It prints a positive value
• Steps to reproduce:
  > M914 X-1
  < X driver homing sensitivity set to 127
  < Y driver homing sensitivity set to 4
  < E0 driver homing sensitivity set to 0

I've implemented a straightforward fix in our FW and am hoping it can be incorporated:

#define SIGN_EXTEND_SGT(sgt) int8_t(sgt | ((sgt << 1) & 0x80))

  template<typename TMC>
  static void tmc_get_sgt(TMC &st, const char name[]) {
    SERIAL_ECHO(name);
    SERIAL_ECHOPGM(" driver homing sensitivity set to ");
    MYSERIAL.println(SIGN_EXTEND_SGT(st.sgt()), DEC);
  }

[marcio-ao]

Hummm, actually, looking back on this code I wrote long ago, I have no idea what the bit-shift is doing. I think it does nothing and all that matters is the int8_t ...

UPDATE: Oh, I see. The SGT is a 7-bit signed quantity. This copies the 7th bit to the eighth position.

[teemuatlut]

This really is something that should be done at the library level. I can't have the sgt() reporting values that aren't correct and I believe I've already fixed this once already.
Either way, I'm getting a very different output and that led me to this oversight.

[marcio-ao]

@teemuatlut: I'm not sure why you would consider sign-extending a 7-bit value into an 8-bit value to be incorrect. I agree that this would be better done in the library, but no matter where you do it it is not incorrect.

[teemuatlut]

"I can't have it report values that aren't correct"
As in if the current implementation is bugged and reporting bad values. It was not a comment on your suggestion!

[marcio-ao]

@teemuatlut : Ah, gotcha :)

[teemuatlut]

After fixing the argument data type:

SENDING:M914 X-1
X driver homing sensitivity set to -1
>>> M914 X-60
SENDING:M914 X-60
X driver homing sensitivity set to -60
>>> M914
SENDING:M914
X driver homing sensitivity set to -60
>>> M914 X10
SENDING:M914 X10
X driver homing sensitivity set to 10

Edit: PRs https://github.com/MarlinFirmware/Marlin/pull/9650 https://github.com/MarlinFirmware/Marlin/pull/9651