Re: [PATCH v1] rust: serdev: use Delta<Jiffy> for timeouts

[FUJITA Tomonori <tomo@flapping.org>]

On Thu, 01 Oct 2026 03:27:39 +0100
"Gary Guo" <gary@garyguo.net> wrote:

> On Thu Oct 1, 2026 at 2:28 AM BST, FUJITA Tomonori wrote:

[...]

>>> Alternatively, just use `MAX_SCHEDULE_TIMEOUT` (i.e.
>>> `Delta::from_jiffies(isize::MAX)`). We can perhaps add a `Delta::MAX` for this.
>>
>> I don't think Delta::<Jiffy>::MAX is a good idea. Whether the maximum
>> value means an indefinite wait depends on the function. For example,
>> it doesn't for Queue::enqueue_delayed().
> 
> Whether it means infinite or not depends on the function, but that's still mean
> "MAX", no?

If I understand you correctly, a Rust function that takes Delta<Jiffy>
turns Delta::<Jiffy>::MAX into the maximum value of the C function it
calls, and documents what MAX means.

isize::MAX is MAX_SCHEDULE_TIMEOUT, so schedule_timeout() and the
functions built on it need nothing. Other functions might need a
clamp, because C avoids LONG_MAX as a jiffies span:

| /*
|  * Change timeval to jiffies, trying to avoid the
|  * most obvious overflows..
|  *
|  * And some not so obvious.
|  *
|  * Note that we don't want to return LONG_MAX, because
|  * for various timeout reasons we often end up having
|  * to wait "jiffies+1" in order to guarantee that we wait
|  * at _least_ "jiffies" - so "jiffies+1" had better still
|  * be positive.
|  */
| #define MAX_JIFFY_OFFSET ((LONG_MAX >> 1)-1)

With that, I'm fine with Delta::<Jiffy>::MAX.