Message-ID: <65ab5cb1-2dbe-e853-48ea-d1df87706cf0@redhat.com>
Date:   Tue, 21 Feb 2023 09:13:24 -0800
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Subject: Re: [PATCH] rust: time: New module for timekeeping functions
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To:     Thomas Gleixner <tglx@linutronix.de>,
        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>
Cc:     linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org,
        asahi@lists.linux.dev
References: <20230221-gpu-up-time-v1-1-bf8fe74b7f55@asahilina.net>
 <87v8jvnqq4.ffs@tglx>
From:   Josh Stone <jistone@redhat.com>
In-Reply-To: <87v8jvnqq4.ffs@tglx>
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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.

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:

https://github.com/rust-lang/rust/pull/88714

But as others mentioned, this is in the std crate which isn't used by
the kernel. You could absolutely define your own with more specificity,
like kernel::time::BootTime, and copy an API similar to Instant's.