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Subject: Re: [PATCH] rust: time: New module for timekeeping functions
From:   Gaelan Steele <gbs@canishe.com>
In-Reply-To: <87zg96n12k.ffs@tglx>
Date:   Wed, 22 Feb 2023 09:43:39 +0000
Cc:     Josh Stone <jistone@redhat.com>, Asahi Lina <lina@asahilina.net>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        John Stultz <jstultz@google.com>,
        Stephen Boyd <sboyd@kernel.org>, linux-kernel@vger.kernel.org,
        rust-for-linux@vger.kernel.org, asahi@lists.linux.dev
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Message-Id: <EE0C3422-B908-4263-B0E8-27B6DFFCF306@canishe.com>
References: <20230221-gpu-up-time-v1-1-bf8fe74b7f55@asahilina.net>
 <87v8jvnqq4.ffs@tglx> <65ab5cb1-2dbe-e853-48ea-d1df87706cf0@redhat.com>
 <87zg96n12k.ffs@tglx>
To:     Thomas Gleixner <tglx@linutronix.de>
Precedence: bulk


> On Feb 21, 2023, at 9:46 PM, Thomas Gleixner <tglx@linutronix.de> =
wrote:
>=20
> On Tue, Feb 21 2023 at 09:13, Josh Stone wrote:
>=20
>> On 2/21/23 4:32 AM, Thomas Gleixner wrote:
>>> Now the problem is that 'Instant' in it's specification is bound to
>>> CLOCK_MONOTONIC and there is no way to express CLOCK_BOOTTIME, but
>>> that's a shortcoming of the spec which ignores CLOCK_BOOTTIME
>>> completely. IOW, that's also a problem for user space.
>>=20
>> That's not exactly *specified* -- it's meant to be opaque time. It is
>> documented that this currently uses clock_gettime monotonic on unix
>> targets, but "Disclaimer: These system calls might change over time."
>> CLOCK_MONOTONIC isn't even consistent across unix targets whether =
that
>> counts suspended time. It's been debated if we should switch to
>> CLOCK_BOOTTIME on Linux, but for now we're sticking to monotonic:
>=20
> You'll need both when you want to implement substantial parts of the =
low
> level user space stack in Rust. Same for CLOCK_TAI.
>=20
> Thanks,
>=20
>        tglx

std isn=E2=80=99t really designed to provide full coverage of any =
particular OS
interface - it has to provide concepts that map cleanly onto Windows and
every flavor of Unix.[1] Low-level Unix Rust code typically uses the =
libc
crate, which just exports everything from libc as an unsafe function, or
one of several safe wrappers (nix is the most popular one, I=E2=80=99m =
partial to
rustix), alongside std.

[1]: std does in cases provide OS-specific functions - for example,
std::fs::Metadata (~=3D struct stat) has Unix-specific ways to get the
mode - but again, the goal here is to be broadly useful, not full
coverage.

Best wishes,
Gaelan