Update src/callbacks_step/glm_speed.jl

Co-authored-by: Hendrik Ranocha <[email protected]>

src/callbacks_step/glm_speed.jl

@@ -83,6 +83,8 @@ function update_cleaning_speed!(semi, glm_speed_callback, dt)
     c_h_deltat = calc_dt_for_cleaning_speed(cfl, mesh, equations, solver, cache)
 
     # c_h is proportional to its own time step divided by the complete MHD time step
+    # We use @reset here since the equations are immutable (to work on GPUs etc.).
+    # Thus, we need to modify the equations field of the semidiscretization.
     @reset equations.c_h = glm_scale * c_h_deltat / dt
     semi.equations = equations