linux-kernel - Re: [PATCH v8 12/14] rust: hrtimer: add `HrTimerMode`

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Message-ID: <c750f04e-1b0b-4818-8554-c30fc535d07d@proton.me>
Date: Sat, 22 Feb 2025 09:34:25 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Danilo Krummrich <dakr@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Lyude Paul <lyude@...hat.com>, Guangbo Cui <2407018371@...com>, Dirk Behme <dirk.behme@...il.com>, Daniel Almeida <daniel.almeida@...labora.com>, Tamir Duberstein <tamird@...il.com>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, "Timothy G." <c6qchwbke@...ay.firefox.com>
Subject: Re: [PATCH v8 12/14] rust: hrtimer: add `HrTimerMode`

On 21.02.25 12:39, Andreas Hindborg wrote:
> "Benno Lossin" <benno.lossin@...ton.me> writes:
> 
>> On 21.02.25 10:17, Andreas Hindborg wrote:
>>> "Benno Lossin" <benno.lossin@...ton.me> writes:
>>>
>>>> On 18.02.25 14:27, Andreas Hindborg wrote:
>>>>> +/// Operational mode of [`HrTimer`].
>>>>> +#[derive(Clone, Copy)]
>>>>> +pub enum HrTimerMode {
>>>>> +    /// Timer expires at the given expiration time.
>>>>> +    Absolute,
>>>>> +    /// Timer expires after the given expiration time interpreted as a duration from now.
>>>>> +    Relative,
>>>>> +    /// Timer does not move between CPU cores.
>>>>> +    Pinned,
>>>>> +    /// Timer handler is executed in soft irq context.
>>>>> +    Soft,
>>>>> +    /// Timer handler is executed in hard irq context.
>>>>> +    Hard,
>>>>> +    /// Timer expires at the given expiration time.
>>>>> +    /// Timer does not move between CPU cores.
>>>>> +    AbsolutePinned,
>>>>> +    /// Timer expires after the given expiration time interpreted as a duration from now.
>>>>> +    /// Timer does not move between CPU cores.
>>>>> +    RelativePinned,
>>>>> +    /// Timer expires at the given expiration time.
>>>>> +    /// Timer handler is executed in soft irq context.
>>>>> +    AbsoluteSoft,
>>>>> +    /// Timer expires after the given expiration time interpreted as a duration from now.
>>>>> +    /// Timer handler is executed in soft irq context.
>>>>> +    RelativeSoft,
>>>>> +    /// Timer expires at the given expiration time.
>>>>> +    /// Timer does not move between CPU cores.
>>>>> +    /// Timer handler is executed in soft irq context.
>>>>> +    AbsolutePinnedSoft,
>>>>> +    /// Timer expires after the given expiration time interpreted as a duration from now.
>>>>> +    /// Timer does not move between CPU cores.
>>>>> +    /// Timer handler is executed in soft irq context.
>>>>> +    RelativePinnedSoft,
>>>>> +    /// Timer expires at the given expiration time.
>>>>> +    /// Timer handler is executed in hard irq context.
>>>>> +    AbsoluteHard,
>>>>> +    /// Timer expires after the given expiration time interpreted as a duration from now.
>>>>> +    /// Timer handler is executed in hard irq context.
>>>>> +    RelativeHard,
>>>>> +    /// Timer expires at the given expiration time.
>>>>> +    /// Timer does not move between CPU cores.
>>>>> +    /// Timer handler is executed in hard irq context.
>>>>> +    AbsolutePinnedHard,
>>>>> +    /// Timer expires after the given expiration time interpreted as a duration from now.
>>>>> +    /// Timer does not move between CPU cores.
>>>>> +    /// Timer handler is executed in hard irq context.
>>>>> +    RelativePinnedHard,
>>>>> +}
>>>>
>>>> At some point we probably want to move this to bitfields, or do you
>>>> think it's better to keep it like this?
>>>
>>> Yes, eventually the would transition. The main difficulty is that not
>>> all flag combinations are legal, and the zero value is also a flag.
>>> There was some promising work being shared on Zulip for this [1], but I
>>> don't think it is completed yet. Added Timothy to CC.
>>>
>>> [1] https://rust-for-linux.zulipchat.com/#narrow/channel/291565-Help/topic/Best.20way.20to.20handle.20enum.2Fflags.20situation
>>
>> Ah yeah I remember that. And also the complication about certain
>> combinations not being allowed.
>>
>>>>> +
>>>>> +impl From<HrTimerMode> for bindings::hrtimer_mode {
>>>>> +    fn from(value: HrTimerMode) -> Self {
>>>>> +        use bindings::*;
>>>>> +        match value {
>>>>> +            HrTimerMode::Absolute => hrtimer_mode_HRTIMER_MODE_ABS,
>>>>> +            HrTimerMode::Relative => hrtimer_mode_HRTIMER_MODE_REL,
>>>>> +            HrTimerMode::Pinned => hrtimer_mode_HRTIMER_MODE_PINNED,
>>>>> +            HrTimerMode::Soft => hrtimer_mode_HRTIMER_MODE_SOFT,
>>>>> +            HrTimerMode::Hard => hrtimer_mode_HRTIMER_MODE_HARD,
>>>>> +            HrTimerMode::AbsolutePinned => hrtimer_mode_HRTIMER_MODE_ABS_PINNED,
>>>>> +            HrTimerMode::RelativePinned => hrtimer_mode_HRTIMER_MODE_REL_PINNED,
>>>>> +            HrTimerMode::AbsoluteSoft => hrtimer_mode_HRTIMER_MODE_ABS_SOFT,
>>>>> +            HrTimerMode::RelativeSoft => hrtimer_mode_HRTIMER_MODE_REL_SOFT,
>>>>> +            HrTimerMode::AbsolutePinnedSoft => hrtimer_mode_HRTIMER_MODE_ABS_PINNED_SOFT,
>>>>> +            HrTimerMode::RelativePinnedSoft => hrtimer_mode_HRTIMER_MODE_REL_PINNED_SOFT,
>>>>> +            HrTimerMode::AbsoluteHard => hrtimer_mode_HRTIMER_MODE_ABS_HARD,
>>>>> +            HrTimerMode::RelativeHard => hrtimer_mode_HRTIMER_MODE_REL_HARD,
>>>>> +            HrTimerMode::AbsolutePinnedHard => hrtimer_mode_HRTIMER_MODE_ABS_PINNED_HARD,
>>>>> +            HrTimerMode::RelativePinnedHard => hrtimer_mode_HRTIMER_MODE_REL_PINNED_HARD,
>>>>> +        }
>>>>> +    }
>>>>> +}
>>>>> +
>>>>> +impl From<HrTimerMode> for u64 {
>>>>> +    fn from(value: HrTimerMode) -> Self {
>>>>> +        Into::<bindings::hrtimer_mode>::into(value) as u64
>>>>> +    }
>>>>> +}
>>>>
>>>> Hmm do drivers really need these impls? If not, then you could also just
>>>> have a private function that does the conversion and use it only in the
>>>> abstraction layer.
>>>
>>> Similar to the other impls you commented on, I can move them private. I
>>> would prefer using `From` rather than some other function.
>>
>> What's the reason for you preferring `From`? I don't think it's
>> important to forbid access from the drivers, but if it's unnecessary,
>> why would we give them access in the first place?
> 
> TIL trait implementations cannot be hidden.

Yeah trait impls are always public.

> I like `From` because every single rust developer seeing a `From`
> implementation will immediately know what it does. It is more
> idiomatic in that way than having another conversion function. I think
> using existing traits with well defined semantics is preferable to a
> local function.

Well yes, but that only holds if other people need to use it.

> But since driver implementer do not need it, I'm undecided. If you want
> them converted to a private function I can do that, for all the ones you
> called out. But I am also OK with keeping as is. You decide.

I think it's better to keep these enums somewhat opaque.

---
Cheers,
Benno