1// SPDX-License-Identifier: GPL-2.0
2/*
3 * TSC frequency enumeration via MSR
4 *
5 * Copyright (C) 2013, 2018 Intel Corporation
6 * Author: Bin Gao <bin.gao@intel.com>
7 */
8
9#include <linux/kernel.h>
10#include <linux/thread_info.h>
11
12#include <asm/apic.h>
13#include <asm/cpu_device_id.h>
14#include <asm/intel-family.h>
15#include <asm/msr.h>
16#include <asm/param.h>
17#include <asm/tsc.h>
18
19#define MAX_NUM_FREQS 16 /* 4 bits to select the frequency */
20
21/*
22 * The frequency numbers in the SDM are e.g. 83.3 MHz, which does not contain a
23 * lot of accuracy which leads to clock drift. As far as we know Bay Trail SoCs
24 * use a 25 MHz crystal and Cherry Trail uses a 19.2 MHz crystal, the crystal
25 * is the source clk for a root PLL which outputs 1600 and 100 MHz. It is
26 * unclear if the root PLL outputs are used directly by the CPU clock PLL or
27 * if there is another PLL in between.
28 * This does not matter though, we can model the chain of PLLs as a single PLL
29 * with a quotient equal to the quotients of all PLLs in the chain multiplied.
30 * So we can create a simplified model of the CPU clock setup using a reference
31 * clock of 100 MHz plus a quotient which gets us as close to the frequency
32 * from the SDM as possible.
33 * For the 83.3 MHz example from above this would give us 100 MHz * 5 / 6 =
34 * 83 and 1/3 MHz, which matches exactly what has been measured on actual hw.
35 */
36#define TSC_REFERENCE_KHZ 100000
37
38struct muldiv {
39 u32 multiplier;
40 u32 divider;
41};
42
43/*
44 * If MSR_PERF_STAT[31] is set, the maximum resolved bus ratio can be
45 * read in MSR_PLATFORM_ID[12:8], otherwise in MSR_PERF_STAT[44:40].
46 * Unfortunately some Intel Atom SoCs aren't quite compliant to this,
47 * so we need manually differentiate SoC families. This is what the
48 * field use_msr_plat does.
49 */
50struct freq_desc {
51 bool use_msr_plat;
52 struct muldiv muldiv[MAX_NUM_FREQS];
53 /*
54 * Some CPU frequencies in the SDM do not map to known PLL freqs, in
55 * that case the muldiv array is empty and the freqs array is used.
56 */
57 u32 freqs[MAX_NUM_FREQS];
58 u32 mask;
59};
60
61/*
62 * Penwell and Clovertrail use spread spectrum clock,
63 * so the freq number is not exactly the same as reported
64 * by MSR based on SDM.
65 */
66static const struct freq_desc freq_desc_pnw = {
67 .use_msr_plat = false,
68 .freqs = { 0, 0, 0, 0, 0, 99840, 0, 83200 },
69 .mask = 0x07,
70};
71
72static const struct freq_desc freq_desc_clv = {
73 .use_msr_plat = false,
74 .freqs = { 0, 133200, 0, 0, 0, 99840, 0, 83200 },
75 .mask = 0x07,
76};
77
78/*
79 * Bay Trail SDM MSR_FSB_FREQ frequencies simplified PLL model:
80 * 000: 100 * 5 / 6 = 83.3333 MHz
81 * 001: 100 * 1 / 1 = 100.0000 MHz
82 * 010: 100 * 4 / 3 = 133.3333 MHz
83 * 011: 100 * 7 / 6 = 116.6667 MHz
84 * 100: 100 * 4 / 5 = 80.0000 MHz
85 */
86static const struct freq_desc freq_desc_byt = {
87 .use_msr_plat = true,
88 .muldiv = { { 5, 6 }, { 1, 1 }, { 4, 3 }, { 7, 6 },
89 { 4, 5 } },
90 .mask = 0x07,
91};
92
93/*
94 * Cherry Trail SDM MSR_FSB_FREQ frequencies simplified PLL model:
95 * 0000: 100 * 5 / 6 = 83.3333 MHz
96 * 0001: 100 * 1 / 1 = 100.0000 MHz
97 * 0010: 100 * 4 / 3 = 133.3333 MHz
98 * 0011: 100 * 7 / 6 = 116.6667 MHz
99 * 0100: 100 * 4 / 5 = 80.0000 MHz
100 * 0101: 100 * 14 / 15 = 93.3333 MHz
101 * 0110: 100 * 9 / 10 = 90.0000 MHz
102 * 0111: 100 * 8 / 9 = 88.8889 MHz
103 * 1000: 100 * 7 / 8 = 87.5000 MHz
104 */
105static const struct freq_desc freq_desc_cht = {
106 .use_msr_plat = true,
107 .muldiv = { { 5, 6 }, { 1, 1 }, { 4, 3 }, { 7, 6 },
108 { 4, 5 }, { 14, 15 }, { 9, 10 }, { 8, 9 },
109 { 7, 8 } },
110 .mask = 0x0f,
111};
112
113/*
114 * Merriefield SDM MSR_FSB_FREQ frequencies simplified PLL model:
115 * 0001: 100 * 1 / 1 = 100.0000 MHz
116 * 0010: 100 * 4 / 3 = 133.3333 MHz
117 */
118static const struct freq_desc freq_desc_tng = {
119 .use_msr_plat = true,
120 .muldiv = { { 0, 0 }, { 1, 1 }, { 4, 3 } },
121 .mask = 0x07,
122};
123
124/*
125 * Moorefield SDM MSR_FSB_FREQ frequencies simplified PLL model:
126 * 0000: 100 * 5 / 6 = 83.3333 MHz
127 * 0001: 100 * 1 / 1 = 100.0000 MHz
128 * 0010: 100 * 4 / 3 = 133.3333 MHz
129 * 0011: 100 * 1 / 1 = 100.0000 MHz
130 */
131static const struct freq_desc freq_desc_ann = {
132 .use_msr_plat = true,
133 .muldiv = { { 5, 6 }, { 1, 1 }, { 4, 3 }, { 1, 1 } },
134 .mask = 0x0f,
135};
136
137/*
138 * 24 MHz crystal? : 24 * 13 / 4 = 78 MHz
139 * Frequency step for Lightning Mountain SoC is fixed to 78 MHz,
140 * so all the frequency entries are 78000.
141 */
142static const struct freq_desc freq_desc_lgm = {
143 .use_msr_plat = true,
144 .freqs = { 78000, 78000, 78000, 78000, 78000, 78000, 78000, 78000,
145 78000, 78000, 78000, 78000, 78000, 78000, 78000, 78000 },
146 .mask = 0x0f,
147};
148
149static const struct x86_cpu_id tsc_msr_cpu_ids[] = {
150 X86_MATCH_VFM(INTEL_ATOM_SALTWELL_MID, &freq_desc_pnw),
151 X86_MATCH_VFM(INTEL_ATOM_SALTWELL_TABLET, &freq_desc_clv),
152 X86_MATCH_VFM(INTEL_ATOM_SILVERMONT, &freq_desc_byt),
153 X86_MATCH_VFM(INTEL_ATOM_SILVERMONT_MID, &freq_desc_tng),
154 X86_MATCH_VFM(INTEL_ATOM_AIRMONT, &freq_desc_cht),
155 X86_MATCH_VFM(INTEL_ATOM_SILVERMONT_MID2, &freq_desc_ann),
156 X86_MATCH_VFM(INTEL_ATOM_AIRMONT_NP, &freq_desc_lgm),
157 {}
158};
159
160/*
161 * MSR-based CPU/TSC frequency discovery for certain CPUs.
162 *
163 * Set global "lapic_timer_period" to bus_clock_cycles/jiffy
164 * Return processor base frequency in KHz, or 0 on failure.
165 */
166unsigned long cpu_khz_from_msr(void)
167{
168 u32 lo, hi, ratio, freq, tscref;
169 const struct freq_desc *freq_desc;
170 const struct x86_cpu_id *id;
171 const struct muldiv *md;
172 unsigned long res;
173 int index;
174
175 id = x86_match_cpu(match: tsc_msr_cpu_ids);
176 if (!id)
177 return 0;
178
179 freq_desc = (struct freq_desc *)id->driver_data;
180 if (freq_desc->use_msr_plat) {
181 rdmsr(MSR_PLATFORM_INFO, lo, hi);
182 ratio = (lo >> 8) & 0xff;
183 } else {
184 rdmsr(MSR_IA32_PERF_STATUS, lo, hi);
185 ratio = (hi >> 8) & 0x1f;
186 }
187
188 /* Get FSB FREQ ID */
189 rdmsr(MSR_FSB_FREQ, lo, hi);
190 index = lo & freq_desc->mask;
191 md = &freq_desc->muldiv[index];
192
193 /*
194 * Note this also catches cases where the index points to an unpopulated
195 * part of muldiv, in that case the else will set freq and res to 0.
196 */
197 if (md->divider) {
198 tscref = TSC_REFERENCE_KHZ * md->multiplier;
199 freq = DIV_ROUND_CLOSEST(tscref, md->divider);
200 /*
201 * Multiplying by ratio before the division has better
202 * accuracy than just calculating freq * ratio.
203 */
204 res = DIV_ROUND_CLOSEST(tscref * ratio, md->divider);
205 } else {
206 freq = freq_desc->freqs[index];
207 res = freq * ratio;
208 }
209
210 if (freq == 0)
211 pr_err("Error MSR_FSB_FREQ index %d is unknown\n", index);
212
213#ifdef CONFIG_X86_LOCAL_APIC
214 lapic_timer_period = (freq * 1000) / HZ;
215#endif
216
217 /*
218 * TSC frequency determined by MSR is always considered "known"
219 * because it is reported by HW.
220 * Another fact is that on MSR capable platforms, PIT/HPET is
221 * generally not available so calibration won't work at all.
222 */
223 setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ);
224
225 /*
226 * Unfortunately there is no way for hardware to tell whether the
227 * TSC is reliable. We were told by silicon design team that TSC
228 * on Atom SoCs are always "reliable". TSC is also the only
229 * reliable clocksource on these SoCs (HPET is either not present
230 * or not functional) so mark TSC reliable which removes the
231 * requirement for a watchdog clocksource.
232 */
233 setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
234
235 return res;
236}
237