git: 08161fd3b207 - stable/13 - x86: Skip late calibration if our reference timer has low quality

The branch stable/13 has been updated by markj:

URL: https://cgit.FreeBSD.org/src/commit/?id=08161fd3b207105847bdd1dd8729ea4cbda6e537

commit 08161fd3b207105847bdd1dd8729ea4cbda6e537
Author:     Mark Johnston <markj@FreeBSD.org>
AuthorDate: 2022-01-03 15:14:41 +0000
Commit:     Mark Johnston <markj@FreeBSD.org>
CommitDate: 2022-01-06 13:46:33 +0000

    x86: Skip late calibration if our reference timer has low quality
    
    Some AMD Geode-based systems end up using the 8254 PIT to calibrate the
    TSC during late calibration, which doesn't work because that
    timecounter's mask (65535) is much smaller than its frequency (1193182).
    Moreover, early calibration is done against the 8254 timer anyway.
    
    Work around the problem by simply using early calibration results if no
    high-quality timecounters exist.
    
    PR:             260868
    Fixes:          22875f88799e ("x86: Implement deferred TSC calibration")
    Reported and tested by: mike@sentex.net, Stefan Hegnauer <stefan.hegnauer@gmx.ch>
    Reviewed by:    imp, kib
    Sponsored by:   The FreeBSD Foundation
    
    (cherry picked from commit 0e494a9e3fd86ef54899dcbe0268866629096c1e)
---
 sys/x86/x86/tsc.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sys/x86/x86/tsc.c b/sys/x86/x86/tsc.c
index 6debe3ecca6d..6f0ef697cb2f 100644
--- a/sys/x86/x86/tsc.c
+++ b/sys/x86/x86/tsc.c
@@ -708,7 +708,14 @@ tsc_calibrate(void)
 	if (tsc_disabled)
 		return;
 
+	/*
+	 * Avoid using a low-quality timecounter to re-calibrate.  In
+	 * particular, old 32-bit platforms might only have the 8254 timer to
+	 * calibrate against.
+	 */
 	tc = atomic_load_ptr(&timecounter);
+	if (tc->tc_quality <= 0)
+		goto calibrated;
 
 	flags = intr_disable();
 	cpu = curcpu;