[tip:x86/platform] x86/platform/intel/baytrail: Add comments about why we disabled HPET on Baytrail

[tip-bot for Feng Tang <tipbot@zytor.com>]

Commit-ID:  b58d930750135d6c5b8e5aa084c0e9303c78c286
Gitweb:     http://git.kernel.org/tip/b58d930750135d6c5b8e5aa084c0e9303c78c286
Author:     Feng Tang <feng.tang@intel.com>
AuthorDate: Mon, 15 Jun 2015 17:40:01 +0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 18 Jun 2015 10:57:38 +0200

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 | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/early-quirks.c b/arch/x86/kernel/early-quirks.c
index fe9f0b7..5cb9a4d 100644
--- a/arch/x86/kernel/early-quirks.c
+++ b/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},