Allow Low EJERK with LA

Author: thisiskeithb

Marlin/Configuration_adv.h

@@ -1913,6 +1913,7 @@
   #define LIN_ADVANCE_K 0.22    // Unit: mm compression per 1mm/s extruder speed
   //#define LA_DEBUG            // If enabled, this will generate debug information output over USB.
   //#define EXPERIMENTAL_SCURVE // Enable this option to permit S-Curve Acceleration
+  //#define ALLOW_LOW_EJERK     // Allow a DEFAULT_EJERK value of <10. Enabled for direct drive hotends.
 #endif
 
 // @section leveling

Marlin/src/inc/SanityCheck.h

@@ -1242,7 +1242,7 @@ static_assert(Y_MAX_LENGTH >= Y_BED_SIZE, "Movement bounds (Y_MIN_POS, Y_MAX_POS
     #error "LIN_ADVANCE and S_CURVE_ACCELERATION may not play well together! Enable EXPERIMENTAL_SCURVE to continue."
   #elif ENABLED(DIRECT_STEPPING)
     #error "DIRECT_STEPPING is incompatible with LIN_ADVANCE. Enable in external planner if possible."
-  #elif !HAS_JUNCTION_DEVIATION && defined(DEFAULT_EJERK)
+  #elif !HAS_JUNCTION_DEVIATION && defined(DEFAULT_EJERK) && DISABLED(ALLOW_LOW_EJERK)
     static_assert(DEFAULT_EJERK >= 10, "It is strongly recommended to set DEFAULT_EJERK >= 10 when using LIN_ADVANCE.");
   #endif
 #endif