punktfunk/clients/android/native/src/decode/vsync.rs

//! The vsync clock behind the timeline presenter: an `AChoreographer` thread publishing the
//! panel's vsync grid + upcoming frame timelines, and pulsing the decode loop's event channel so
//! a frame parked on a closed glass budget gets its retry tick.
//!
//! On API 33+ the thread rides `AChoreographer_postVsyncCallback`, whose callback payload carries
//! the platform's FRAME TIMELINES — for each upcoming refresh, when SurfaceFlinger expects to
//! present and the deadline by which a frame must be submitted to make it. That pair is exactly
//! what `AMediaCodec_releaseOutputBufferAtTime` wants as its target. On 31/32 the older
//! `postFrameCallback64` supplies only the vsync instant; the presenter then releases ASAP
//! (identical to the legacy path) and uses the measured period purely to predict the latch for
//! its glass budget.
//!
//! Every `AChoreographer_*` symbol is dlsym-resolved from `libandroid.so` (mirrors
//! [`super::setup::try_set_frame_rate`]): several sit above the crate's API floor, and one hard
//! import of a too-new symbol fails `System.loadLibrary` on every older device.
//!
//! Started LAZILY on the first decoded frame (the Apple deadline presenter's bootstrap lesson:
//! an eagerly started clock ticks uselessly for the whole connect window), stopped + joined via
//! [`VsyncClock`]'s `Drop`.

use std::ffi::c_void;
use std::sync::atomic::{AtomicBool, AtomicI64, Ordering};
use std::sync::{Arc, Mutex};
use std::time::Duration;

/// `CLOCK_MONOTONIC` now in nanoseconds — the clock AChoreographer stamps its timelines on and
/// the one `AMediaCodec_releaseOutputBufferAtTime` compares against (`System.nanoTime` basis).
/// Distinct from the stats path's `CLOCK_REALTIME`: presenter scheduling stays monotonic.
pub(super) fn now_monotonic_ns() -> i64 {
    let mut ts = libc::timespec {
        tv_sec: 0,
        tv_nsec: 0,
    };
    // SAFETY: `clock_gettime` with a valid out-pointer is an always-safe syscall.
    unsafe { libc::clock_gettime(libc::CLOCK_MONOTONIC, &mut ts) };
    // Explicit widening: timespec's fields are 32-bit on armv7 (time_t/c_long).
    ts.tv_sec as i64 * 1_000_000_000 + ts.tv_nsec as i64
}

/// One upcoming frame timeline (API 33+ payload): when SurfaceFlinger expects to present the
/// frame, and the last instant it can be submitted to make that present. Monotonic ns.
#[derive(Clone, Copy)]
pub(super) struct FrameTimeline {
    pub expected_present_ns: i64,
    pub deadline_ns: i64,
}

/// State the choreographer thread publishes and the decode loop reads. All monotonic ns.
pub(super) struct VsyncShared {
    stop: AtomicBool,
    /// The latest vsync callback's frame time (0 = no callback yet).
    last_vsync_ns: AtomicI64,
    /// Estimated vsync period (EMA over callback deltas / timeline spacing; 0 = unmeasured).
    ///
    /// ⚠ This is the APP's render rate, not necessarily the panel's: Android down-rates a
    /// process's vsync stream (frame-rate categories / per-uid overrides), so a quiet UI can be
    /// served 60 Hz callbacks while the panel scans at 120 (observed on-glass, A024). Pacing
    /// video to THIS rate would cap the stream — hence `panel_period_ns` + the subdivision in
    /// [`Self::next_target`].
    period_ns: AtomicI64,
    /// The panel's own refresh period (from the display mode Kotlin resolved at stream start;
    /// 0 = unknown). The grid SurfaceFlinger actually latches on.
    panel_period_ns: AtomicI64,
    /// Callback count, for the one-shot cadence diagnostic log.
    ticks: std::sync::atomic::AtomicU32,
    /// The latest callback's upcoming timelines, soonest first. Empty on the 31/32 fallback.
    timelines: Mutex<Vec<FrameTimeline>>,
}

impl VsyncShared {
    /// The measured vsync period, or 0 while unmeasured.
    pub(super) fn period_ns(&self) -> i64 {
        self.period_ns.load(Ordering::Relaxed)
    }

    /// The panel's own refresh period (0 = unknown) — for the pf-present line's decomposition.
    pub(super) fn panel_period_ns(&self) -> i64 {
        self.panel_period_ns.load(Ordering::Relaxed)
    }

    /// The release target for a frame submitted at `now`: the earliest stored timeline whose
    /// EXPECTED PRESENT is still `margin` away, extrapolated forward by whole periods once the
    /// stored set has aged out (timelines refresh once per vsync callback; a frame can decode
    /// anywhere inside that window). `None` on the 31/32 fallback — the caller releases ASAP.
    ///
    /// Gated on `expected_present`, NOT the timeline's `deadline`, on purpose: the deadline
    /// budgets for GPU rendering the app has yet to submit (`presDeadline` — 11.3 ms on the
    /// A024, more than a full 120 Hz period), but a video buffer is already fully rendered —
    /// the only real constraint is SurfaceFlinger's own latch lead, which is what the caller's
    /// `margin` represents. Targeting by deadline cost every frame an extra refresh of waiting
    /// (measured: latch p50 ~21 ms vs the ~2-interval floor); a mis-gamble here just means the
    /// frame presents one vsync later — exactly what the conservative gate always paid.
    ///
    /// The picked target is then SUBDIVIDED onto the panel grid: the platform reports timelines
    /// at the app's assigned render rate, but the panel latches at its own — when the app is
    /// down-rated (60 Hz callbacks on a 120 Hz panel) the reported timelines are a whole panel
    /// period apart or more, and pacing to them would cap the video. Pulling the target earlier
    /// by whole panel periods (while its present still clears the margin) restores the true
    /// grid; when callbacks run at the panel rate the pull condition is never true and this is
    /// a no-op.
    pub(super) fn next_target(&self, now_ns: i64, margin_ns: i64) -> Option<FrameTimeline> {
        let mut t = {
            let g = self
                .timelines
                .lock()
                .unwrap_or_else(std::sync::PoisonError::into_inner);
            let found = g
                .iter()
                .find(|t| t.expected_present_ns > now_ns + margin_ns)
                .copied();
            match found {
                Some(t) => t,
                None => {
                    let last = g.last().copied()?;
                    let period = self.period_ns();
                    if period <= 0 {
                        return None;
                    }
                    // All stored timelines have passed — step the last one forward whole
                    // periods until its present clears `now + margin` again.
                    let behind = (now_ns + margin_ns).saturating_sub(last.expected_present_ns);
                    let k = behind / period + 1;
                    FrameTimeline {
                        expected_present_ns: last.expected_present_ns + k * period,
                        deadline_ns: last.deadline_ns + k * period,
                    }
                }
            }
        };
        let panel = self.panel_period_ns.load(Ordering::Relaxed);
        if panel > 0 {
            while t.expected_present_ns - panel > now_ns + margin_ns {
                t.deadline_ns -= panel;
                t.expected_present_ns -= panel;
            }
        }
        Some(t)
    }
}

// ---- dlsym'd AChoreographer surface ----

type PostFrameCallback64 =
    unsafe extern "C" fn(*mut c_void, unsafe extern "C" fn(i64, *mut c_void), *mut c_void);
type PostVsyncCallback = unsafe extern "C" fn(
    *mut c_void,
    unsafe extern "C" fn(*const c_void, *mut c_void),
    *mut c_void,
);

struct ChoreoApi {
    get_instance: unsafe extern "C" fn() -> *mut c_void,
    /// API 33: vsync callback with frame-timeline payload. Preferred.
    post_vsync: Option<PostVsyncCallback>,
    /// API 29 fallback: frame callback with only the vsync instant.
    post_frame64: Option<PostFrameCallback64>,
    // AChoreographerFrameCallbackData accessors (API 33; present iff `post_vsync` is).
    fcd_frame_time: Option<unsafe extern "C" fn(*const c_void) -> i64>,
    fcd_timelines_len: Option<unsafe extern "C" fn(*const c_void) -> usize>,
    fcd_preferred_index: Option<unsafe extern "C" fn(*const c_void) -> usize>,
    fcd_expected_present: Option<unsafe extern "C" fn(*const c_void, usize) -> i64>,
    fcd_deadline: Option<unsafe extern "C" fn(*const c_void, usize) -> i64>,
}

impl ChoreoApi {
    /// Resolve from `libandroid.so`. `None` when even the baseline symbols are missing.
    fn resolve() -> Option<ChoreoApi> {
        // SAFETY: dlopen of the always-mapped libandroid.so (refcount bump, never closed); each
        // dlsym is null-checked before the transmute to its fn-pointer type.
        unsafe {
            let lib = libc::dlopen(c"libandroid.so".as_ptr(), libc::RTLD_NOW);
            if lib.is_null() {
                return None;
            }
            let sym = |name: &std::ffi::CStr| {
                let p = libc::dlsym(lib, name.as_ptr());
                (!p.is_null()).then_some(p)
            };
            let get_instance = sym(c"AChoreographer_getInstance")?;
            let post_vsync = sym(c"AChoreographer_postVsyncCallback");
            let post_frame64 = sym(c"AChoreographer_postFrameCallback64");
            post_vsync.or(post_frame64)?; // neither post entry point — no clock on this device
            Some(ChoreoApi {
                get_instance: std::mem::transmute::<
                    *mut c_void,
                    unsafe extern "C" fn() -> *mut c_void,
                >(get_instance),
                post_vsync: post_vsync.map(|p| std::mem::transmute::<*mut c_void, PostVsyncCallback>(p)),
                post_frame64: post_frame64
                    .map(|p| std::mem::transmute::<*mut c_void, PostFrameCallback64>(p)),
                fcd_frame_time: sym(c"AChoreographerFrameCallbackData_getFrameTimeNanos").map(|p| {
                    std::mem::transmute::<*mut c_void, unsafe extern "C" fn(*const c_void) -> i64>(p)
                }),
                fcd_timelines_len: sym(c"AChoreographerFrameCallbackData_getFrameTimelinesLength")
                    .map(|p| {
                        std::mem::transmute::<*mut c_void, unsafe extern "C" fn(*const c_void) -> usize>(
                            p,
                        )
                    }),
                fcd_preferred_index: sym(
                    c"AChoreographerFrameCallbackData_getPreferredFrameTimelineIndex",
                )
                .map(|p| {
                    std::mem::transmute::<*mut c_void, unsafe extern "C" fn(*const c_void) -> usize>(p)
                }),
                fcd_expected_present: sym(
                    c"AChoreographerFrameCallbackData_getFrameTimelineExpectedPresentationTimeNanos",
                )
                .map(|p| {
                    std::mem::transmute::<
                        *mut c_void,
                        unsafe extern "C" fn(*const c_void, usize) -> i64,
                    >(p)
                }),
                fcd_deadline: sym(c"AChoreographerFrameCallbackData_getFrameTimelineDeadlineNanos")
                    .map(|p| {
                        std::mem::transmute::<
                            *mut c_void,
                            unsafe extern "C" fn(*const c_void, usize) -> i64,
                        >(p)
                    }),
            })
        }
    }
}

/// Everything a callback invocation needs. Owned by the choreographer thread's stack; callbacks
/// only ever fire inside that thread's looper poll, so the borrow can't outlive the thread.
struct CallbackCtx {
    api: ChoreoApi,
    choreographer: *mut c_void,
    shared: Arc<VsyncShared>,
    on_tick: Box<dyn Fn() + Send>,
}

impl CallbackCtx {
    /// Common tail of both callback flavours: update the grid estimate, publish, pulse, re-arm.
    fn tick(&self, frame_time_ns: i64, timelines: Vec<FrameTimeline>) {
        let prev = self
            .shared
            .last_vsync_ns
            .swap(frame_time_ns, Ordering::Relaxed);
        // Panel-grid learner: timeline spacing is SurfaceFlinger's own grid, and the finest
        // spacing ever observed is the panel's true period — trustworthy where the configured
        // value is not (under a per-uid frame-rate override, `Display.getRefreshRate` REPORTS
        // THE OVERRIDE, observed on-glass: a 120 Hz panel read back as 60 while early timelines
        // ran at 8.28 ms). Corrects DOWNWARD only: subdividing onto a finer real grid is always
        // valid, widening on a later down-rated window never is.
        if timelines.len() >= 2 {
            let spacing = timelines[1].expected_present_ns - timelines[0].expected_present_ns;
            if (2_000_000..=42_000_000).contains(&spacing) {
                let cur = self.shared.panel_period_ns.load(Ordering::Relaxed);
                if cur == 0 || spacing < cur - 200_000 {
                    self.shared
                        .panel_period_ns
                        .store(spacing, Ordering::Relaxed);
                }
            }
        }
        // One-shot cadence diagnostic (3rd tick, once deltas exist): the callback cadence vs the
        // panel period is exactly the down-rating question, and this line answers it on-glass.
        if self.shared.ticks.fetch_add(1, Ordering::Relaxed) == 2 {
            let spacing = if timelines.len() >= 2 {
                timelines[1].expected_present_ns - timelines[0].expected_present_ns
            } else {
                0
            };
            log::info!(
                "vsync: cadence Δ={:.2}ms timelines={} spacing={:.2}ms panel={:.2}ms",
                if prev > 0 {
                    (frame_time_ns - prev) as f64 / 1e6
                } else {
                    0.0
                },
                timelines.len(),
                spacing as f64 / 1e6,
                self.shared.panel_period_ns.load(Ordering::Relaxed) as f64 / 1e6,
            );
        }
        // Period: prefer timeline spacing (exact, straight from the platform), else the delta of
        // successive callbacks (jittery — EMA'd), clamped to sane panel rates (24..500 Hz).
        let mut period = 0i64;
        if timelines.len() >= 2 {
            period = timelines[1].expected_present_ns - timelines[0].expected_present_ns;
        } else if prev > 0 {
            period = frame_time_ns - prev;
        }
        if (2_000_000..=42_000_000).contains(&period) {
            let old = self.shared.period_ns.load(Ordering::Relaxed);
            let smoothed = if old > 0 {
                (old * 7 + period) / 8
            } else {
                period
            };
            self.shared.period_ns.store(smoothed, Ordering::Relaxed);
        }
        if !timelines.is_empty() {
            let mut g = self
                .shared
                .timelines
                .lock()
                .unwrap_or_else(std::sync::PoisonError::into_inner);
            *g = timelines;
        }
        (self.on_tick)();
        if !self.shared.stop.load(Ordering::Relaxed) {
            self.repost();
        }
    }

    fn repost(&self) {
        // SAFETY: `choreographer` is this thread's instance; the ctx pointer stays valid for the
        // thread's life and callbacks only fire on this thread (see the struct doc).
        unsafe {
            let ud = self as *const CallbackCtx as *mut c_void;
            if let Some(post) = self.api.post_vsync {
                post(self.choreographer, on_vsync, ud);
            } else if let Some(post) = self.api.post_frame64 {
                post(self.choreographer, on_frame64, ud);
            }
        }
    }
}

/// API 33+ trampoline: harvest the frame timelines, then the common tick. Panic-free (an unwind
/// out of an `extern "C"` fn aborts).
unsafe extern "C" fn on_vsync(data: *const c_void, ud: *mut c_void) {
    // SAFETY: `ud` is the thread's `CallbackCtx`, alive for the whole poll loop (see struct doc).
    let ctx = unsafe { &*(ud as *const CallbackCtx) };
    let api = &ctx.api;
    let (mut frame_time, mut timelines) = (now_monotonic_ns(), Vec::new());
    // SAFETY: `data` is the platform's callback payload, valid for this invocation; the accessors
    // were resolved together with `post_vsync` (same API level) and are only called when present.
    unsafe {
        if let Some(f) = api.fcd_frame_time {
            frame_time = f(data);
        }
        if let (Some(len_f), Some(pref_f), Some(exp_f), Some(dl_f)) = (
            api.fcd_timelines_len,
            api.fcd_preferred_index,
            api.fcd_expected_present,
            api.fcd_deadline,
        ) {
            let len = len_f(data).min(8);
            // From the PREFERRED index on: earlier timelines are ones the platform already
            // considers missed for a frame starting now.
            let start = pref_f(data).min(len);
            timelines = (start..len)
                .map(|i| FrameTimeline {
                    expected_present_ns: exp_f(data, i),
                    deadline_ns: dl_f(data, i),
                })
                .collect();
        }
    }
    ctx.tick(frame_time, timelines);
}

/// API 29 fallback trampoline: vsync instant only.
unsafe extern "C" fn on_frame64(frame_time_ns: i64, ud: *mut c_void) {
    // SAFETY: `ud` is the thread's `CallbackCtx` (see `on_vsync`).
    let ctx = unsafe { &*(ud as *const CallbackCtx) };
    ctx.tick(frame_time_ns, Vec::new());
}

/// The clock: a dedicated looper thread the choreographer calls back on. Dropping stops + joins.
pub(super) struct VsyncClock {
    shared: Arc<VsyncShared>,
    join: Option<std::thread::JoinHandle<()>>,
}

impl VsyncClock {
    /// Spawn the choreographer thread. `on_tick` fires once per vsync ON THAT THREAD — it must
    /// only do something cheap and `Send` (the decode loop passes an event-channel send).
    /// `panel_hz` is the display mode's own refresh rate (0 = unknown), the latch grid that
    /// [`VsyncShared::next_target`] subdivides onto. `None` when the platform surface is missing
    /// (very old device) — the presenter then runs clock-less (ASAP targets, predicted-latch
    /// budget).
    pub(super) fn start(panel_hz: i32, on_tick: Box<dyn Fn() + Send>) -> Option<VsyncClock> {
        let api = ChoreoApi::resolve()?;
        let timelines_live = api.post_vsync.is_some();
        let shared = Arc::new(VsyncShared {
            stop: AtomicBool::new(false),
            last_vsync_ns: AtomicI64::new(0),
            period_ns: AtomicI64::new(0),
            panel_period_ns: AtomicI64::new(if panel_hz > 0 {
                1_000_000_000 / panel_hz as i64
            } else {
                0
            }),
            ticks: std::sync::atomic::AtomicU32::new(0),
            timelines: Mutex::new(Vec::new()),
        });
        let thread_shared = shared.clone();
        let join = std::thread::Builder::new()
            .name("pf-vsync".into())
            .spawn(move || {
                let looper = ndk::looper::ThreadLooper::prepare();
                // SAFETY: getInstance on a thread with a prepared looper returns this thread's
                // choreographer (never null once a looper exists).
                let choreographer = unsafe { (api.get_instance)() };
                if choreographer.is_null() {
                    log::warn!("vsync: AChoreographer_getInstance returned null — no clock");
                    return;
                }
                let ctx = CallbackCtx {
                    api,
                    choreographer,
                    shared: thread_shared,
                    on_tick,
                };
                ctx.repost();
                // The bounded poll doubles as the stop check: no cross-thread wake needed, worst
                // case teardown waits one timeout out. Callbacks fire inside poll_once_timeout.
                while !ctx.shared.stop.load(Ordering::Relaxed) {
                    let _ = looper.poll_once_timeout(Duration::from_millis(250));
                }
                // `ctx` drops here — after the loop, so no queued callback can outlive it (they
                // only ever fire inside this thread's poll).
            })
            .ok()?;
        log::info!(
            "vsync: choreographer clock started ({})",
            if timelines_live {
                "frame timelines"
            } else {
                "frame callback fallback"
            }
        );
        Some(VsyncClock {
            shared,
            join: Some(join),
        })
    }

    pub(super) fn shared(&self) -> &Arc<VsyncShared> {
        &self.shared
    }
}

impl Drop for VsyncClock {
    fn drop(&mut self) {
        self.shared.stop.store(true, Ordering::Relaxed);
        if let Some(j) = self.join.take() {
            let _ = j.join();
        }
    }
}