[PATCH v4 3/3] x86/tsc_msr: Make MSR derived TSC frequency more accurate

[Hans de Goede <hdegoede@redhat.com>]

The "Intel 64 and IA-32 Architectures Software Developer’s Manual
Volume 4: Model-Specific Registers" has the following table for the
values from freq_desc_byt:

   000B: 083.3 MHz
   001B: 100.0 MHz
   010B: 133.3 MHz
   011B: 116.7 MHz
   100B: 080.0 MHz

Notice how for e.g the 83.3 MHz value there are 3 significant digits,
which translates to an accuracy of a 1000 ppm, where as your typical
crystal oscillator is 20 - 100 ppm, so the accuracy of the frequency
format used in the Software Developer’s Manual is not really helpful.

As far as we know Bay Trail SoCs use a 25 MHz crystal and Cherry Trail
uses a 19.2 MHz crystal, the crystal is the source clk for a root PLL
which outputs 1600 and 100 MHz. It is unclear if the root PLL outputs are
used directly by the CPU clock PLL or if there is another PLL in between.

This does not matter though, we can model the chain of PLLs as a single
PLL with a quotient equal to the quotients of all PLLs in the chain
multiplied.

So we can create a simplified model of the CPU clock setup using a
reference clock of 100 MHz plus a quotient which gets us as close to the
frequency from the SDM as possible.

For the 83.3 MHz example from above this would give us 100 MHz * 5 / 6 =
83 and 1/3 MHz, which matches exactly what has been measured on actual hw.

This commit makes the tsc_msr.c code use a simplified PLL model with a
reference clock of 100 MHz for all Bay and Cherry Trail models.

This has been tested on the following models:

              CPU freq before:        CPU freq after this commit:
Intel N2840   2165.800 MHz            2166.667 MHz
Intel Z3736   1332.800 MHz            1333.333 MHz
Intel Z3775   1466.300 MHz            1466.667 MHz
Intel Z8350   1440.000 MHz            1440.000 MHz
Intel Z8750   1600.000 MHz            1600.000 MHz

This fixes the time drifting by about 1 second per hour (20 - 30 seconds
per day) on (some) devices which rely on the tsc_msr.c code to determine
the TSC frequency.

Cc: stable@vger.kernel.org
Reported-by: Vipul Kumar <vipulk0511@gmail.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
Changes in v4:
-Rework the code in cpu_khz_from_msr() which deals with the multiplier /
 divider pairs by adding a local md helper variable

Changes in v3:
-Some code style tweaks and variable renames suggested by Andy Shevchenko

Changes in v2:
-s/DSM/SDM/
-Do not refer to Merrifield / Moorefield as BYT / CHT, they only share the
 CPU core design and otherwise are significantly different
---
 arch/x86/kernel/tsc_msr.c | 97 ++++++++++++++++++++++++++++++++++-----
 1 file changed, 86 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/tsc_msr.c b/arch/x86/kernel/tsc_msr.c
index 95030895fffa..c65adaf81384 100644
--- a/arch/x86/kernel/tsc_msr.c
+++ b/arch/x86/kernel/tsc_msr.c
@@ -17,6 +17,28 @@
 
 #define MAX_NUM_FREQS	16 /* 4 bits to select the frequency */
 
+/*
+ * The frequency numbers in the SDM are e.g. 83.3 MHz, which does not contain a
+ * lot of accuracy which leads to clock drift. As far as we know Bay Trail SoCs
+ * use a 25 MHz crystal and Cherry Trail uses a 19.2 MHz crystal, the crystal
+ * is the source clk for a root PLL which outputs 1600 and 100 MHz. It is
+ * unclear if the root PLL outputs are used directly by the CPU clock PLL or
+ * if there is another PLL in between.
+ * This does not matter though, we can model the chain of PLLs as a single PLL
+ * with a quotient equal to the quotients of all PLLs in the chain multiplied.
+ * So we can create a simplified model of the CPU clock setup using a reference
+ * clock of 100 MHz plus a quotient which gets us as close to the frequency
+ * from the SDM as possible.
+ * For the 83.3 MHz example from above this would give us 100 MHz * 5 / 6 =
+ * 83 and 1/3 MHz, which matches exactly what has been measured on actual hw.
+ */
+#define TSC_REFERENCE_KHZ 100000
+
+struct muldiv {
+	u32 multiplier;
+	u32 divider;
+};
+
 /*
  * If MSR_PERF_STAT[31] is set, the maximum resolved bus ratio can be
  * read in MSR_PLATFORM_ID[12:8], otherwise in MSR_PERF_STAT[44:40].
@@ -26,6 +48,11 @@
  */
 struct freq_desc {
 	bool use_msr_plat;
+	struct muldiv muldiv[MAX_NUM_FREQS];
+	/*
+	 * Some CPU frequencies in the SDM do not map to known PLL freqs, in
+	 * that case the muldiv array is empty and the freqs array is used.
+	 */
 	u32 freqs[MAX_NUM_FREQS];
 	u32 mask;
 };
@@ -47,31 +74,66 @@ static const struct freq_desc freq_desc_clv = {
 	.mask = 0x07,
 };
 
+/*
+ * Bay Trail SDM MSR_FSB_FREQ frequencies simplified PLL model:
+ *  000:   100 *  5 /  6  =  83.3333 MHz
+ *  001:   100 *  1 /  1  = 100.0000 MHz
+ *  010:   100 *  4 /  3  = 133.3333 MHz
+ *  011:   100 *  7 /  6  = 116.6667 MHz
+ *  100:   100 *  4 /  5  =  80.0000 MHz
+ */
 static const struct freq_desc freq_desc_byt = {
 	.use_msr_plat = true,
-	.freqs = { 83300, 100000, 133300, 116700, 80000, 0, 0, 0 },
+	.muldiv = { { 5, 6 }, { 1, 1 }, { 4, 3 }, { 7, 6 },
+		    { 4, 5 } },
 	.mask = 0x07,
 };
 
+/*
+ * Cherry Trail SDM MSR_FSB_FREQ frequencies simplified PLL model:
+ * 0000:   100 *  5 /  6  =  83.3333 MHz
+ * 0001:   100 *  1 /  1  = 100.0000 MHz
+ * 0010:   100 *  4 /  3  = 133.3333 MHz
+ * 0011:   100 *  7 /  6  = 116.6667 MHz
+ * 0100:   100 *  4 /  5  =  80.0000 MHz
+ * 0101:   100 * 14 / 15  =  93.3333 MHz
+ * 0110:   100 *  9 / 10  =  90.0000 MHz
+ * 0111:   100 *  8 /  9  =  88.8889 MHz
+ * 1000:   100 *  7 /  8  =  87.5000 MHz
+ */
 static const struct freq_desc freq_desc_cht = {
 	.use_msr_plat = true,
-	.freqs = { 83300, 100000, 133300, 116700, 80000, 93300, 90000,
-		   88900, 87500 },
+	.muldiv = { { 5, 6 }, {  1,  1 }, { 4,  3 }, { 7, 6 },
+		    { 4, 5 }, { 14, 15 }, { 9, 10 }, { 8, 9 },
+		    { 7, 8 } },
 	.mask = 0x0f,
 };
 
+/*
+ * Merriefield SDM MSR_FSB_FREQ frequencies simplified PLL model:
+ * 0001:   100 *  1 /  1  = 100.0000 MHz
+ * 0010:   100 *  4 /  3  = 133.3333 MHz
+ */
 static const struct freq_desc freq_desc_tng = {
 	.use_msr_plat = true,
-	.freqs = { 0, 100000, 133300, 0, 0, 0, 0, 0 },
+	.muldiv = { { 0, 0 }, { 1, 1 }, { 4, 3 } },
 	.mask = 0x07,
 };
 
+/*
+ * Moorefield SDM MSR_FSB_FREQ frequencies simplified PLL model:
+ * 0000:   100 *  5 /  6  =  83.3333 MHz
+ * 0001:   100 *  1 /  1  = 100.0000 MHz
+ * 0010:   100 *  4 /  3  = 133.3333 MHz
+ * 0011:   100 *  1 /  1  = 100.0000 MHz
+ */
 static const struct freq_desc freq_desc_ann = {
 	.use_msr_plat = true,
-	.freqs = { 83300, 100000, 133300, 100000, 0, 0, 0, 0 },
+	.muldiv = { { 5, 6 }, { 1, 1 }, { 4, 3 }, { 1, 1 } },
 	.mask = 0x0f,
 };
 
+/* 24 MHz crystal? : 24 * 13 / 4 = 78 MHz */
 static const struct freq_desc freq_desc_lgm = {
 	.use_msr_plat = true,
 	.freqs = { 78000, 78000, 78000, 78000, 78000, 78000, 78000, 78000 },
@@ -97,9 +159,10 @@ static const struct x86_cpu_id tsc_msr_cpu_ids[] = {
  */
 unsigned long cpu_khz_from_msr(void)
 {
-	u32 lo, hi, ratio, freq;
+	u32 lo, hi, ratio, freq, tscref;
 	const struct freq_desc *freq_desc;
 	const struct x86_cpu_id *id;
+	const struct muldiv *md;
 	unsigned long res;
 	int index;
 
@@ -119,12 +182,24 @@ unsigned long cpu_khz_from_msr(void)
 	/* Get FSB FREQ ID */
 	rdmsr(MSR_FSB_FREQ, lo, hi);
 	index = lo & freq_desc->mask;
+	md = &freq_desc->muldiv[index];
 
-	/* Map CPU reference clock freq ID(0-7) to CPU reference clock freq(KHz) */
-	freq = freq_desc->freqs[index];
-
-	/* TSC frequency = maximum resolved freq * maximum resolved bus ratio */
-	res = freq * ratio;
+	/*
+	 * Note this also catches cases where the index points to an unpopulated
+	 * part of muldiv, in that case the else will set freq and res to 0.
+	 */
+	if (md->divider) {
+		tscref = TSC_REFERENCE_KHZ * md->multiplier;
+		freq = DIV_ROUND_CLOSEST(tscref, md->divider);
+		/*
+		 * Multiplying by ratio before the division has better
+		 * accuracy than just calculating freq * ratio.
+		 */
+		res = DIV_ROUND_CLOSEST(tscref * ratio, md->divider);
+	} else {
+		freq = freq_desc->freqs[index];
+		res = freq * ratio;
+	}
 
 	if (freq == 0)
 		pr_err("Error MSR_FSB_FREQ index %d is unknown\n", index);
-- 
2.25.0