x86/platform/intel/baytrail: Add comments about why we disabled HPET …

…on Baytrail

This question has been asked many times, and finally I found the
official document which explains the problem of HPET on Baytrail,
that it will halt in deep idle states.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john.stultz@linaro.org
Cc: len.brown@intel.com
Cc: matthew.lee@intel.com
Link: http://lkml.kernel.org/r/1434361201-31743-1-git-send-email-feng.tang@intel.com
[ Prettified things a bit. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/early-quirks.c

@@ -627,8 +627,12 @@ static struct chipset early_qrk[] __initdata = {
 	{ PCI_VENDOR_ID_INTEL, PCI_ANY_ID, PCI_CLASS_DISPLAY_VGA, PCI_ANY_ID,
 	  QFLAG_APPLY_ONCE, intel_graphics_stolen },
 	/*
-	 * HPET on current version of Baytrail platform has accuracy
-	 * problems, disable it for now:
+	 * HPET on the current version of the Baytrail platform has accuracy
+	 * problems: it will halt in deep idle state - so we disable it.
+	 *
+	 * More details can be found in section 18.10.1.3 of the datasheet:
+	 *
+	 *    http://www.intel.com/content/dam/www/public/us/en/documents/datasheets/atom-z8000-datasheet-vol-1.pdf
 	 */
 	{ PCI_VENDOR_ID_INTEL, 0x0f00,
 		PCI_CLASS_BRIDGE_HOST, PCI_ANY_ID, 0, force_disable_hpet},