Lin advance timing patch

[Sebastianv650]

Fixes #4114. Forgot to keep track of the step_loops value inside the estepper ISR..

Edit: No idea why it shows 3 Commits - I'm not smart enough for GitHub.

[thinkyhead]

I'm just following up in this manner…

git fetch sebastian    ;# already had this set up
git checkout sebastian/LIN_ADVANCE-timing-patch -b rc_lin_timing_patch
git rebase RCBugFix    ;# use my up-to-date local as new base
git rebase -i RCBugFix ;# squash commits (but none needed after rebase)
mfpr                   ;# script to make a new PR

Looks like all you need to do to bring this PR up to date is actually…

git fetch upstream
git rebase upstream/RCBugFix
git push -f

[thinkyhead]

I thought, for some reason, that step loops had been made obsolete by the variable stepper ISR timing. I guess not!

Q: This should also apply to regular ADVANCE extrusion, correct?

[Sebastianv650]

Yes, that's also true for ADVANCE.
At some point the timer resolution isn't sufficient, meaning eISRrate is approaching 0. I guess it will also block other things if an isr is fired at 20khz for example.

[thinkyhead]

We do have to be cautious about those ISRs. And always remember the maxim, "To make it go faster, make it do less."