[PATCH v3 0/2] rust: use Delta and new Jiffies type instead of raw jiffies for timeouts and delays

From: FUJITA Tomonori <[email protected]>

CondVar::wait_interruptible_timeout() and Queue::enqueue_delayed() use
a raw jiffies count (a plain c_ulong alias with no type
safety). Callers have to know on their own that the value meant
jiffies and convert to/from it themselves, which is easy to get wrong
(e.g. passing a millisecond value where a jiffies value is expected).

Unlike the previous versions, v3 introduces a dedicated `Jiffies`
newtype instead. The APIs take `impl Into<Jiffies>`, so a caller can
pass either a `Delta` (the duration is converted once, rounding up) or
a `Jiffies` directly with no conversion. Jiffies returned by the C
side stay as `Jiffies`, so a caller that works in jiffies never
converts and the round trip is gone.

Why a `Jiffies` type, but no `Micros`/`Nanos`/... types?

`Delta` represents a span of time as a nanosecond count. Microseconds,
milliseconds and seconds are exact multiples of a nanosecond, so they
embed into `Delta` losslessly and round-trip through it losslessly; they
are already covered by Delta's constructors and accessors (from_millis(),
as_millis_ceil(), ...) and would gain nothing from their own types.

A jiffy is 1/HZ of a second, which is generally not an exact number of
nanoseconds (e.g. HZ=300). Converting between jiffies and Delta is
therefore inherently lossy in both directions, and that is exactly
what makes a jiffies count a distinct quantity -- a count of timer
ticks -- rather than just another way to spell a Delta. So it, and
only it, gets its own (unsigned) newtype.

The Delta -> Jiffies conversion reuses the C `__msecs_to_jiffies()`
helper -- the same routine the existing Rust `msecs_to_jiffies()`
wrapper already called. The arithmetic (and the range clamping) is
done on the C side so it works on 32-bit architectures.

v3:
- Add new Jiffies type and convert the APIs to take impl Into<Jiffies>
v2: https://lore.kernel.org/rust-for-linux/[email protected]/
- Fix potential overflow in from_jiffies()
- Fix inflating bug in as_jiffies_ceil()
- Add a patch to convert enqueue_delayed()
- Add a patch to remove Jiffies/Msecs aliases
v1: https://lore.kernel.org/rust-for-linux/[email protected]/

FUJITA Tomonori (2):
  rust: time: add Delta::as_millis_ceil()
  rust: use Delta and a Jiffies newtype for timeouts and delays

 drivers/android/binder/process.rs |  7 ++--
 rust/kernel/sync/condvar.rs       | 21 +++++++---
 rust/kernel/time.rs               | 64 ++++++++++++++++++++++++++-----
 rust/kernel/workqueue.rs          | 11 ++++--
 4 files changed, 82 insertions(+), 21 deletions(-)


base-commit: 7059bdf4f04a3e14f4fafb3ac35fdca913e3e21a
-- 
2.43.0