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From: Andreas Hindborg <a.hindborg@kernel.org>
To: "FUJITA Tomonori" <fujita.tomonori@gmail.com>
Cc: <linux-kernel@vger.kernel.org>,  <daniel.almeida@collabora.com>,
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Subject: Re: [PATCH v11 4/8] rust: time: Introduce Instant type
In-Reply-To: <20250403.134038.2188356790179825602.fujita.tomonori@gmail.com>
	(FUJITA Tomonori's message of "Thu, 03 Apr 2025 13:40:38 +0900")
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Date: Thu, 03 Apr 2025 12:41:52 +0200
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"FUJITA Tomonori" <fujita.tomonori@gmail.com> writes:

> On Sat, 22 Mar 2025 14:58:16 +0100
> Andreas Hindborg <a.hindborg@kernel.org> wrote:
>
>> FUJITA Tomonori <fujita.tomonori@gmail.com> writes:
>>
>>> Introduce a type representing a specific point in time. We could use
>>> the Ktime type but C's ktime_t is used for both timestamp and
>>> timedelta. To avoid confusion, introduce a new Instant type for
>>> timestamp.
>>>
>>> Rename Ktime to Instant and modify their methods for timestamp.
>>>
>>> Implement the subtraction operator for Instant:
>>>
>>> Delta = Instant A - Instant B
>>>
>>> Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
>>> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
>>> Reviewed-by: Gary Guo <gary@garyguo.net>
>>> Reviewed-by: Fiona Behrens <me@kloenk.dev>
>>> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
>>
>>
>> Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
>>
>>
>> As Boqun mentioned, we should make this generic over `ClockId` when the
>> hrtimer patches land.
>
> Seems that I overlooked his mail. Can you give me a pointer?
>
> I assume that you want the Instance type to vary depending on the
> clock source.

Yea, basically it is only okay to subtract instants if they are derived
from the same clock source. Boqun suggested here [1] before hrtimer
patches landed I think.

At any rate, now we have `kernel::time::ClockId`. It is an enum though,
so I am not sure how to go about it in practice. But we would want
`Instant<RealTime> - Instant<BootTime>` to give a compiler error.


Best regards,
Andreas Hindborg


[1] https://lore.kernel.org/all/ZxwFyl0xIje5gv7J@Boquns-Mac-mini.local