[tip:,x86/timers] x86/tsc_msr: Make MSR derived TSC frequency more accurate

diff --git a/arch/x86/kernel/tsc_msr.c b/arch/x86/kernel/tsc_msr.c
index 9503089..c65adaf 100644
--- a/arch/x86/kernel/tsc_msr.c
+++ b/arch/x86/kernel/tsc_msr.c
@@ -18,6 +18,28 @@ 
 #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].
  * Unfortunately some Intel Atom SoCs aren't quite compliant to this,
@@ -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);