feat: add fadt based lapic timer calibration

kernel/include/acpi/acpi.h

@@ -60,6 +60,7 @@ struct rsdp_t {
 static_assert(sizeof(rsdp_t) == 36);
 
 struct madt_t;
+struct fadt_t;
 
 struct Rsdp {
     explicit Rsdp(paddr_t rsdp_physical) : physical(rsdp_physical) {}
@@ -90,6 +91,7 @@ private:
 
         switch (static_cast<signature>(header->signature)) {
             ACPI_DISPATCH(madt, madt_t)
+            ACPI_DISPATCH(fadt, fadt_t)
         default:
             break;
         }

kernel/include/acpi/discovery.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "acpi/acpi.h"
+#include "acpi/fadt.h"
+#include "acpi/madt.h"
+#include "util/optional.h"
+
+namespace acpi {
+
+struct discovery_result_t {
+    optional<Rsdp> rsdp;
+    optional<Madt> madt;
+    // TODO(wiring): capture the FADT in setupAcpi and store it here
+    optional<Fadt> fadt;
+};
+
+} // namespace acpi

kernel/include/acpi/fadt.h

@@ -0,0 +1,98 @@
+#pragma once
+
+#include "acpi/acpi.h"
+#include "memory/pointer.h"
+#include "util/number.h"
+
+namespace acpi {
+
+struct fadt_flags_t {
+    uint32_t wbinvd : 1;
+    uint32_t wbinvd_flush : 1;
+    uint32_t processor_c1 : 1;
+    uint32_t p_level2_up : 1;
+    uint32_t power_button : 1;
+    uint32_t sleep_button : 1;
+    uint32_t fixed_rtc : 1;
+    uint32_t rtc_s4 : 1;
+    uint32_t timer_value_extended : 1;
+    uint32_t reserved : 23;
+};
+static_assert(sizeof(fadt_flags_t) == 4);
+
+// Prefix of the FADT through the flags field; real tables continue past this,
+// so no field after flags may be added without also handling table revisions.
+struct fadt_t {
+    sdt_header_t header;
+    uint32_t firmware_control;
+    uint32_t dsdt_address;
+    uint8_t reserved;
+    uint8_t preferred_pm_profile;
+    uint16_t sci_interrupt;
+    uint32_t smi_command_port;
+    uint8_t acpi_enable;
+    uint8_t acpi_disable;
+    uint8_t s4bios_request;
+    uint8_t pstate_control;
+    uint32_t pm1a_event_block;
+    uint32_t pm1b_event_block;
+    uint32_t pm1a_control_block;
+    uint32_t pm1b_control_block;
+    uint32_t pm2_control_block;
+    uint32_t pm_timer_block;
+    uint32_t gpe0_block;
+    uint32_t gpe1_block;
+    uint8_t pm1_event_length;
+    uint8_t pm1_control_length;
+    uint8_t pm2_control_length;
+    uint8_t pm_timer_length;
+    uint8_t gpe0_block_length;
+    uint8_t gpe1_block_length;
+    uint8_t gpe1_base;
+    uint8_t cstate_control;
+    uint16_t p_level2_latency;
+    uint16_t p_level3_latency;
+    uint16_t flush_size;
+    uint16_t flush_stride;
+    uint8_t duty_offset;
+    uint8_t duty_width;
+    uint8_t day_alarm;
+    uint8_t month_alarm;
+    uint8_t century;
+    uint16_t iapc_boot_architecture;
+    uint8_t reserved2;
+    fadt_flags_t flags;
+} __attribute__((packed));
+static_assert(sizeof(fadt_t) == 116);
+
+constexpr uint64_t pm_timer_frequency_hz = 3'579'545;
+
+struct pm_timer_duration_t {
+    uint64_t pm_ticks;
+};
+
+constexpr pm_timer_duration_t pm_timer_milliseconds(uint64_t count) {
+    return pm_timer_duration_t{count * pm_timer_frequency_hz / 1'000};
+}
+
+struct Fadt {
+    explicit Fadt(paddr_t fadt_physical) : table(fadt_physical.access<fadt_t>()) {}
+    explicit Fadt(const fadt_t* fadt_table) : table(fadt_table) {}
+
+    bool has_pm_timer() const {
+        return table->pm_timer_block != 0 && table->pm_timer_length == 4;
+    }
+
+    uint16_t pm_timer_port() const {
+        return static_cast<uint16_t>(table->pm_timer_block);
+    }
+
+    bool pm_timer_is_32bit() const {
+        return table->flags.timer_value_extended != 0;
+    }
+
+private:
+    const fadt_t* table;
+};
+
+} // namespace acpi

kernel/include/apic/timer.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "acpi/madt.h"
+#include "acpi/discovery.h"
 
 namespace apic {
 
@@ -12,6 +12,11 @@ struct timestamp_t {
     uint64_t timer_count;
 };
 
+struct seconds_nanoseconds_t {
+    uint64_t seconds;
+    uint64_t nanoseconds;
+};
+
 constexpr duration_t operator+(duration_t left, duration_t right) {
     return duration_t{left.timer_count + right.timer_count};
 }
@@ -44,13 +49,17 @@ constexpr duration_t operator-(timestamp_t left, timestamp_t right) {
     return duration_t{left.timer_count - right.timer_count};
 }
 
-bool calibrateTimer(const acpi::Madt& madt);
+bool calibrateTimer(const acpi::discovery_result_t& discovery);
 
 duration_t seconds(uint64_t count);
 duration_t milliseconds(uint64_t count);
 duration_t microseconds(uint64_t count);
 duration_t nanoseconds(uint64_t count);
 
+seconds_nanoseconds_t toSecondsAndNanoseconds(duration_t duration);
+
+void busyWait(duration_t duration);
+
 timestamp_t now();
 
 // TODO: when dynamic allocation and lists are available

kernel/include/util/port_io.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include "util/number.h"
+
+inline uint8_t port_read_8(uint16_t port) {
+    uint8_t value = 0;
+    __asm__ volatile("inb %1, %0" : "=a"(value) : "Nd"(port));
+    return value;
+}
+
+inline uint16_t port_read_16(uint16_t port) {
+    uint16_t value = 0;
+    __asm__ volatile("inw %1, %0" : "=a"(value) : "Nd"(port));
+    return value;
+}
+
+inline uint32_t port_read_32(uint16_t port) {
+    uint32_t value = 0;
+    __asm__ volatile("inl %1, %0" : "=a"(value) : "Nd"(port));
+    return value;
+}
+
+inline void port_write_8(uint16_t port, uint8_t value) {
+    __asm__ volatile("outb %0, %1" : : "a"(value), "Nd"(port));
+}
+
+inline void port_write_16(uint16_t port, uint16_t value) {
+    __asm__ volatile("outw %0, %1" : : "a"(value), "Nd"(port));
+}
+
+inline void port_write_32(uint16_t port, uint32_t value) {
+    __asm__ volatile("outl %0, %1" : : "a"(value), "Nd"(port));
+}

kernel/src/apic/timer.cpp

@@ -1,6 +1,7 @@
 #include "apic/timer.h"
 
 #include "util/cpuid.h"
+#include "util/port_io.h"
 
 namespace {
 
@@ -46,24 +47,59 @@ optional<uint64_t> tscFrequencyFromProcessorBase() {
     });
 }
 
-optional<uint64_t> tscFrequency() {
-    return tscFrequencyFromCrystalRatio().or_else(tscFrequencyFromProcessorBase);
+// The counter is 24 or 32 bit wide (fadt_flags_t::timer_value_extended); masking
+// deltas to 24 bits is correct for both as long as polling outruns a wrap (~4.7 s).
+constexpr uint32_t pm_timer_counter_mask = 0xFFFFFF;
+
+uint64_t readTscSerialized() {
+    uint32_t low = 0;
+    uint32_t high = 0;
+    __asm__ volatile("lfence; rdtsc" : "=a"(low), "=d"(high));
+    return (static_cast<uint64_t>(high) << 32) | low;
+}
+
+[[maybe_unused]] optional<uint64_t>
+tscFrequencyFromPmTimer(const acpi::Fadt& fadt, acpi::pm_timer_duration_t sample_window) {
+    if (!fadt.has_pm_timer() || sample_window.pm_ticks == 0) {
+        return nullopt;
+    }
+    uint16_t timer_port = fadt.pm_timer_port();
+    uint32_t previous_count = port_read_32(timer_port) & pm_timer_counter_mask;
+    uint64_t tsc_start = readTscSerialized();
+    uint64_t elapsed_pm_ticks = 0;
+    while (elapsed_pm_ticks < sample_window.pm_ticks) {
+        uint32_t current_count = port_read_32(timer_port) & pm_timer_counter_mask;
+        elapsed_pm_ticks += (current_count - previous_count) & pm_timer_counter_mask;
+        previous_count = current_count;
+    }
+    uint64_t tsc_end = readTscSerialized();
+    return (tsc_end - tsc_start) * acpi::pm_timer_frequency_hz / elapsed_pm_ticks;
 }
 
 apic::duration_t durationFromUnit(uint64_t count, uint64_t units_per_second) {
     uint64_t whole_seconds = count / units_per_second;
     uint64_t remainder_units = count % units_per_second;
-    return apic::duration_t{whole_seconds * timer_calibration.tsc_frequency_hz +
-                            remainder_units * timer_calibration.tsc_frequency_hz / units_per_second};
+    return apic::duration_t{
+        whole_seconds * timer_calibration.tsc_frequency_hz +
+        remainder_units * timer_calibration.tsc_frequency_hz / units_per_second
+    };
+}
+
+optional<uint64_t> tscFrequency(const acpi::discovery_result_t& discovery) {
+    return tscFrequencyFromCrystalRatio().or_else(tscFrequencyFromProcessorBase).or_else([&discovery] {
+        return discovery.fadt.bind([](const acpi::Fadt& fadt) {
+            return tscFrequencyFromPmTimer(fadt, acpi::pm_timer_milliseconds(50));
+        });
+    });
 }
 
 } // namespace
 
-bool apic::calibrateTimer(const acpi::Madt&) {
+bool apic::calibrateTimer(const acpi::discovery_result_t& discovery) {
     if (!isTscSupported()) {
         return false;
     }
-    return tscFrequency()
+    return tscFrequency(discovery)
         .map([](uint64_t tsc_frequency_hz) {
             timer_calibration.tsc_frequency_hz = tsc_frequency_hz;
             return true;
@@ -87,6 +123,23 @@ apic::duration_t apic::nanoseconds(uint64_t count) {
     return durationFromUnit(count, 1'000'000'000);
 }
 
+apic::seconds_nanoseconds_t apic::toSecondsAndNanoseconds(duration_t duration) {
+    uint64_t frequency_hz = timer_calibration.tsc_frequency_hz;
+    if (frequency_hz == 0) {
+        return seconds_nanoseconds_t{};
+    }
+    uint64_t whole_seconds = duration.timer_count / frequency_hz;
+    uint64_t remainder_ticks = duration.timer_count % frequency_hz;
+    return seconds_nanoseconds_t{whole_seconds, remainder_ticks * 1'000'000'000 / frequency_hz};
+}
+
+void apic::busyWait(duration_t duration) {
+    timestamp_t start = now();
+    while ((now() - start).timer_count < duration.timer_count) {
+        __asm__ volatile("pause");
+    }
+}
+
 apic::timestamp_t apic::now() {
     uint32_t low = 0;
     uint32_t high = 0;

kernel/src/init/init.cpp

@@ -1,5 +1,7 @@
 #include "init/init.h"
 #include "acpi/acpi.h"
+#include "acpi/discovery.h"
+#include "acpi/fadt.h"
 #include "acpi/madt.h"
 #include "apic/timer.h"
 #include "init/multiboot2.h"
@@ -187,38 +189,53 @@ static acpi::Rsdp findRsdp() {
     return acpi::Rsdp(*rsdp);
 }
 
-struct AcpiDiscoveryResult {
-    optional<acpi::Rsdp> rsdp;
-    optional<acpi::Madt> madt;
-};
-
-static AcpiDiscoveryResult setupAcpi() {
-    AcpiDiscoveryResult res{};
+static acpi::discovery_result_t setupAcpi() {
+    acpi::discovery_result_t res{};
     res.rsdp = findRsdp();
-    res.rsdp->for_each_table(overloaded{[&](const acpi::madt_t* header) {
-        res.madt = acpi::Madt{header};
-    }});
+    res.rsdp->for_each_table(
+        overloaded{
+            [&](const acpi::madt_t* header) {
+                res.madt = acpi::Madt{header};
+            },
+            [&](const acpi::fadt_t* header) {
+                res.fadt = acpi::Fadt{header};
+            }
+        }
+    );
     return res;
 }
 
-static void calibrateTimer(optional<acpi::Madt> madt) {
-    auto success = madt.map(apic::calibrateTimer).value_or(false);
-    if (!success) {
+static void calibrateTimer(acpi::discovery_result_t acpi) {
+    if (!apic::calibrateTimer(acpi)) {
         panic("Could not calibrate timer");
     }
 }
 
+static void demonstateTimer() {
+    auto time1 = apic::now();
+    print("Time 1: ");
+    print_hex(time1.timer_count);
+    print("\nTime 2: ");
+    auto time2 = apic::now();
+    print_hex(time2.timer_count);
+    print("\n");
+    auto duration = time2 - time1;
+    auto readableDuration = apic::toSecondsAndNanoseconds(duration);
+    print_dec(readableDuration.seconds);
+    print("s ");
+    print_dec(readableDuration.nanoseconds);
+    print("ns\n");
+    print("Wait for 5s!\n");
+    apic::busyWait(apic::seconds(5));
+    print("Waited for 5s!\n");
+}
+
 [[maybe_unused]] [[noreturn]] __attribute__((used)) void init() {
     initFromLow();
     print("Reached init\n");
     step("memory", setupMemory);
     auto acpi = step("acpi", setupAcpi);
-    step("timer", partial(calibrateTimer, acpi.madt));
-    auto now = apic::now();
-    print_hex(now.timer_count);
-    now = apic::now();
-    print_hex(now.timer_count);
-    now = apic::now();
-    print_hex(now.timer_count);
+    step("timer", partial(calibrateTimer, acpi));
+    demonstateTimer();
     halt();
 }