cpufreq: Add Rust-based cpufreq-dt driver

Introduce a Rust-based implementation of the cpufreq-dt driver, covering
most of the functionality provided by the existing C version. Some
features, such as retrieving platform data from `cpufreq-dt-platdev.c`,
are still pending.

The driver has been tested with QEMU, and frequency scaling works as
expected.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/Kconfig

@@ -217,6 +217,18 @@ config CPUFREQ_DT
 
 	  If in doubt, say N.
 
+config CPUFREQ_DT_RUST
+	tristate "Rust based Generic DT based cpufreq driver"
+	depends on HAVE_CLK && OF && RUST
+	select CPUFREQ_DT_PLATDEV
+	select PM_OPP
+	help
+	  This adds a Rust based generic DT based cpufreq driver for frequency
+	  management.  It supports both uniprocessor (UP) and symmetric
+	  multiprocessor (SMP) systems.
+
+	  If in doubt, say N.
+
 config CPUFREQ_VIRT
 	tristate "Virtual cpufreq driver"
 	depends on GENERIC_ARCH_TOPOLOGY

drivers/cpufreq/Makefile

@@ -15,6 +15,7 @@ obj-$(CONFIG_CPU_FREQ_GOV_COMMON)		+= cpufreq_governor.o
 obj-$(CONFIG_CPU_FREQ_GOV_ATTR_SET)	+= cpufreq_governor_attr_set.o
 
 obj-$(CONFIG_CPUFREQ_DT)		+= cpufreq-dt.o
+obj-$(CONFIG_CPUFREQ_DT_RUST)		+= rcpufreq_dt.o
 obj-$(CONFIG_CPUFREQ_DT_PLATDEV)	+= cpufreq-dt-platdev.o
 obj-$(CONFIG_CPUFREQ_VIRT)		+= virtual-cpufreq.o
 

drivers/cpufreq/rcpufreq_dt.rs

@@ -0,0 +1,226 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust based implementation of the cpufreq-dt driver.
+
+use kernel::{
+    c_str,
+    clk::Clk,
+    cpu, cpufreq,
+    cpumask::CpumaskVar,
+    device::{Core, Device},
+    error::code::*,
+    fmt,
+    macros::vtable,
+    module_platform_driver, of, opp, platform,
+    prelude::*,
+    str::CString,
+    sync::Arc,
+};
+
+/// Finds exact supply name from the OF node.
+fn find_supply_name_exact(dev: &Device, name: &str) -> Option<CString> {
+    let prop_name = CString::try_from_fmt(fmt!("{}-supply", name)).ok()?;
+    dev.property_present(&prop_name)
+        .then(|| CString::try_from_fmt(fmt!("{name}")).ok())
+        .flatten()
+}
+
+/// Finds supply name for the CPU from DT.
+fn find_supply_names(dev: &Device, cpu: u32) -> Option<KVec<CString>> {
+    // Try "cpu0" for older DTs, fallback to "cpu".
+    let name = (cpu == 0)
+        .then(|| find_supply_name_exact(dev, "cpu0"))
+        .flatten()
+        .or_else(|| find_supply_name_exact(dev, "cpu"))?;
+
+    let mut list = KVec::with_capacity(1, GFP_KERNEL).ok()?;
+    list.push(name, GFP_KERNEL).ok()?;
+
+    Some(list)
+}
+
+/// Represents the cpufreq dt device.
+struct CPUFreqDTDevice {
+    opp_table: opp::Table,
+    freq_table: opp::FreqTable,
+    _mask: CpumaskVar,
+    _token: Option<opp::ConfigToken>,
+    _clk: Clk,
+}
+
+#[derive(Default)]
+struct CPUFreqDTDriver;
+
+#[vtable]
+impl opp::ConfigOps for CPUFreqDTDriver {}
+
+#[vtable]
+impl cpufreq::Driver for CPUFreqDTDriver {
+    const NAME: &'static CStr = c_str!("cpufreq-dt");
+    const FLAGS: u16 = cpufreq::flags::NEED_INITIAL_FREQ_CHECK | cpufreq::flags::IS_COOLING_DEV;
+    const BOOST_ENABLED: bool = true;
+
+    type PData = Arc<CPUFreqDTDevice>;
+
+    fn init(policy: &mut cpufreq::Policy) -> Result<Self::PData> {
+        let cpu = policy.cpu();
+        // SAFETY: The CPU device is only used during init; it won't get hot-unplugged. The cpufreq
+        // core  registers with CPU notifiers and the cpufreq core/driver won't use the CPU device,
+        // once the CPU is hot-unplugged.
+        let dev = unsafe { cpu::from_cpu(cpu)? };
+        let mut mask = CpumaskVar::new_zero(GFP_KERNEL)?;
+
+        mask.set(cpu);
+
+        let token = find_supply_names(dev, cpu)
+            .map(|names| {
+                opp::Config::<Self>::new()
+                    .set_regulator_names(names)?
+                    .set(dev)
+            })
+            .transpose()?;
+
+        // Get OPP-sharing information from "operating-points-v2" bindings.
+        let fallback = match opp::Table::of_sharing_cpus(dev, &mut mask) {
+            Ok(()) => false,
+            Err(e) if e == ENOENT => {
+                // "operating-points-v2" not supported. If the platform hasn't
+                // set sharing CPUs, fallback to all CPUs share the `Policy`
+                // for backward compatibility.
+                opp::Table::sharing_cpus(dev, &mut mask).is_err()
+            }
+            Err(e) => return Err(e),
+        };
+
+        // Initialize OPP tables for all policy cpus.
+        //
+        // For platforms not using "operating-points-v2" bindings, we do this
+        // before updating policy cpus. Otherwise, we will end up creating
+        // duplicate OPPs for the CPUs.
+        //
+        // OPPs might be populated at runtime, don't fail for error here unless
+        // it is -EPROBE_DEFER.
+        let mut opp_table = match opp::Table::from_of_cpumask(dev, &mut mask) {
+            Ok(table) => table,
+            Err(e) => {
+                if e == EPROBE_DEFER {
+                    return Err(e);
+                }
+
+                // The table is added dynamically ?
+                opp::Table::from_dev(dev)?
+            }
+        };
+
+        // The OPP table must be initialized, statically or dynamically, by this point.
+        opp_table.opp_count()?;
+
+        // Set sharing cpus for fallback scenario.
+        if fallback {
+            mask.setall();
+            opp_table.set_sharing_cpus(&mut mask)?;
+        }
+
+        let mut transition_latency = opp_table.max_transition_latency_ns() as u32;
+        if transition_latency == 0 {
+            transition_latency = cpufreq::ETERNAL_LATENCY_NS;
+        }
+
+        policy
+            .set_dvfs_possible_from_any_cpu(true)
+            .set_suspend_freq(opp_table.suspend_freq())
+            .set_transition_latency_ns(transition_latency);
+
+        let freq_table = opp_table.cpufreq_table()?;
+        // SAFETY: The `freq_table` is not dropped while it is getting used by the C code.
+        unsafe { policy.set_freq_table(&freq_table) };
+
+        // SAFETY: The returned `clk` is not dropped while it is getting used by the C code.
+        let clk = unsafe { policy.set_clk(dev, None)? };
+
+        mask.copy(policy.cpus());
+
+        Ok(Arc::new(
+            CPUFreqDTDevice {
+                opp_table,
+                freq_table,
+                _mask: mask,
+                _token: token,
+                _clk: clk,
+            },
+            GFP_KERNEL,
+        )?)
+    }
+
+    fn exit(_policy: &mut cpufreq::Policy, _data: Option<Self::PData>) -> Result {
+        Ok(())
+    }
+
+    fn online(_policy: &mut cpufreq::Policy) -> Result {
+        // We did light-weight tear down earlier, nothing to do here.
+        Ok(())
+    }
+
+    fn offline(_policy: &mut cpufreq::Policy) -> Result {
+        // Preserve policy->data and don't free resources on light-weight
+        // tear down.
+        Ok(())
+    }
+
+    fn suspend(policy: &mut cpufreq::Policy) -> Result {
+        policy.generic_suspend()
+    }
+
+    fn verify(data: &mut cpufreq::PolicyData) -> Result {
+        data.generic_verify()
+    }
+
+    fn target_index(policy: &mut cpufreq::Policy, index: cpufreq::TableIndex) -> Result {
+        let Some(data) = policy.data::<Self::PData>() else {
+            return Err(ENOENT);
+        };
+
+        let freq = data.freq_table.freq(index)?;
+        data.opp_table.set_rate(freq)
+    }
+
+    fn get(policy: &mut cpufreq::Policy) -> Result<u32> {
+        policy.generic_get()
+    }
+
+    fn set_boost(_policy: &mut cpufreq::Policy, _state: i32) -> Result {
+        Ok(())
+    }
+
+    fn register_em(policy: &mut cpufreq::Policy) {
+        policy.register_em_opp()
+    }
+}
+
+kernel::of_device_table!(
+    OF_TABLE,
+    MODULE_OF_TABLE,
+    <CPUFreqDTDriver as platform::Driver>::IdInfo,
+    [(of::DeviceId::new(c_str!("operating-points-v2")), ())]
+);
+
+impl platform::Driver for CPUFreqDTDriver {
+    type IdInfo = ();
+    const OF_ID_TABLE: Option<of::IdTable<Self::IdInfo>> = Some(&OF_TABLE);
+
+    fn probe(
+        pdev: &platform::Device<Core>,
+        _id_info: Option<&Self::IdInfo>,
+    ) -> Result<Pin<KBox<Self>>> {
+        cpufreq::Registration::<CPUFreqDTDriver>::new_foreign_owned(pdev.as_ref())?;
+        Ok(KBox::new(Self {}, GFP_KERNEL)?.into())
+    }
+}
+
+module_platform_driver! {
+    type: CPUFreqDTDriver,
+    name: "cpufreq-dt",
+    author: "Viresh Kumar <viresh.kumar@linaro.org>",
+    description: "Generic CPUFreq DT driver",
+    license: "GPL v2",
+}