Kernel/riscv64: Add a Timer class for RISC-V

This is a basic Timer class based on the aarch64 RPi Timer.
It uses the hart-local timer, as defined by the privileged ISA.

Kernel/Arch/riscv64/Timer.cpp

@@ -0,0 +1,89 @@
+/*
+ * Copyright (c) 2023, Sönke Holz <sholz8530@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Format.h>
+#include <AK/NeverDestroyed.h>
+#include <Kernel/Arch/riscv64/SBI.h>
+#include <Kernel/Arch/riscv64/Timer.h>
+
+namespace Kernel::RISCV64 {
+
+Timer::Timer()
+    : HardwareTimer(to_underlying(CSR::SCAUSE::SupervisorTimerInterrupt) & ~CSR::SCAUSE_INTERRUPT_MASK)
+{
+    // FIXME: Actually query the frequency of the timer from the device tree.
+    // Based on the "/cpus/timebase-frequency" device tree node for the QEMU virt machine
+    m_frequency = 10'000'000; // in Hz
+
+    set_interrupt_interval_usec(m_frequency / OPTIMAL_TICKS_PER_SECOND_RATE);
+    enable_interrupt_mode();
+}
+
+Timer::~Timer() = default;
+
+NonnullLockRefPtr<Timer> Timer::initialize()
+{
+    return adopt_lock_ref(*new Timer);
+}
+
+u64 Timer::microseconds_since_boot()
+{
+    return RISCV64::CSR::read(RISCV64::CSR::Address::TIME);
+}
+
+bool Timer::handle_irq(RegisterState const& regs)
+{
+    auto result = HardwareTimer::handle_irq(regs);
+
+    set_compare(microseconds_since_boot() + m_interrupt_interval);
+
+    return result;
+}
+
+u64 Timer::update_time(u64& seconds_since_boot, u32& ticks_this_second, bool query_only)
+{
+    // Should only be called by the time keeper interrupt handler!
+    u64 current_value = microseconds_since_boot();
+    u64 delta_ticks = m_main_counter_drift;
+    if (current_value >= m_main_counter_last_read) {
+        delta_ticks += current_value - m_main_counter_last_read;
+    } else {
+        // the counter wrapped around
+        delta_ticks += (NumericLimits<u64>::max() - m_main_counter_last_read + 1) + current_value;
+    }
+
+    u64 ticks_since_last_second = (u64)ticks_this_second + delta_ticks;
+    auto ticks_per_second = frequency();
+    seconds_since_boot += ticks_since_last_second / ticks_per_second;
+    ticks_this_second = ticks_since_last_second % ticks_per_second;
+
+    if (!query_only) {
+        m_main_counter_drift = 0;
+        m_main_counter_last_read = current_value;
+    }
+
+    // Return the time passed (in ns) since last time update_time was called
+    return (delta_ticks * 1000000000ull) / ticks_per_second;
+}
+
+void Timer::enable_interrupt_mode()
+{
+    set_compare(microseconds_since_boot() + m_interrupt_interval);
+    enable_irq();
+}
+
+void Timer::set_interrupt_interval_usec(u32 interrupt_interval)
+{
+    m_interrupt_interval = interrupt_interval;
+}
+
+void Timer::set_compare(u64 compare)
+{
+    if (SBI::Timer::set_timer(compare).is_error())
+        MUST(SBI::Legacy::set_timer(compare));
+}
+
+}