[PATCH 1/3] x86/tsc: Future-proof native_calibrate_tsc()

From: Len Brown <lenb@kernel.org>
To: x86@kernel.org
Cc: peterz@infradead.org, linux-kernel@vger.kernel.org,
	Len Brown <len.brown@intel.com>, Bin Gao <bin.gao@intel.com>,
	stable@vger.kernel.org
Subject: [PATCH 1/3] x86/tsc: Future-proof native_calibrate_tsc()
Date: Fri, 22 Dec 2017 00:27:54 -0500	[thread overview]
Message-ID: <0fe2503aa7d7fc69137141fc705541a78101d2b9.1513920414.git.len.brown@intel.com> (raw)

From: Len Brown <len.brown@intel.com>

If native_calibrate_tsc() can not discover the TSC frequency,
via CPUID or via built-in table, it must return without
setting X86_FEATURE_TSC_KNOWN_FREQ.  Otherwise, X86_FEATURE_TSC_KNOWN_FREQ
will prevent TSC refined calibration.

This patch allows Linux to correctly support future Intel hardware,
that has (cpu_khz != tsc_khz), and support for CPUID.15
without support for CPUID.15.crystal_khz.

This patch is needed since X86_FEATURE_TSC_KNOWN_FREQ was added in Linux-4.10:

	commit 4ca4df0b7eb0
	("x86/tsc: Mark TSC frequency determined by CPUID as known")

If not applied, such systems will run with tsc_khz = cpu_khz,
which may result in under-stated TSC rate, and time-of-day drift.

Signed-off-by: Len Brown <len.brown@intel.com>
Cc: Bin Gao <bin.gao@intel.com>
Cc: <stable@vger.kernel.org> # v4.10+
---
 arch/x86/kernel/tsc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 8ea117f8142e..ce4b71119c36 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -612,6 +612,8 @@ unsigned long native_calibrate_tsc(void)
 		}
 	}
 
+	if (crystal_khz == 0)
+		return 0;
 	/*
 	 * TSC frequency determined by CPUID is a "hardware reported"
 	 * frequency and is the most accurate one so far we have. This
-- 
2.14.0-rc0
