x86_64: Update comments in pit.c

kernel/arch/x86_64/pit.c

@@ -3,27 +3,13 @@
  * @author K. Lange
  * @brief Legacy x86 Programmable Interrupt Timer
  *
- * Trusty old timer chip that still exists, and is still somehow
- * the only reliable to measure subsecond wallclock times.
- *
- * We continue to the use the PIT as the BSP timer interrupt source,
- * and we also use it as part of timer calibration for TSCs, which
- * is then used to calibrate LAPIC timers.
- *
- * Our main tick rate is 100Hz. We use periodic modes, so this
- * doesn't equate to 1/100s worth of CPU time per process before
- * it gets switched out, rather something less usually, but it
- * does mean we don't need to care about resetting timers or
- * even knowing which timer triggered a userspace pre-emption, since
- * APs use their LAPIC timers (which we also try to set to 100Hz).
- *
- * The actual time doesn't matter, as we don't use the PIT as a real
- * timing source after initialization of the TSC. 100Hz just feels nice?
+ * The PIT is used as a fallback preempt source if the LAPIC can
+ * not be configured. The preempt signal is 100Hz.
  *
  * @copyright
  * This file is part of ToaruOS and is released under the terms
  * of the NCSA / University of Illinois License - see LICENSE.md
- * Copyright (C) 2021 K. Lange
+ * Copyright (C) 2021-2024 K. Lange
  */
 #include <kernel/printf.h>
 #include <kernel/process.h>