linux-kernel - Re: [PATCH v3] posix-timers: add multi_clock

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Date: Mon, 15 Jan 2024 15:41:45 -0800
From: Richard Cochran <richardcochran@...il.com>
To: Sagi Maimon <maimon.sagi@...il.com>
Cc: Arnd Bergmann <arnd@...db.de>, Andy Lutomirski <luto@...nel.org>,
	tglx@...utronix.de, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Sohil Mehta <sohil.mehta@...el.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Nhat Pham <nphamcs@...il.com>, Palmer Dabbelt <palmer@...ive.com>,
	Kees Cook <keescook@...omium.org>,
	Alexey Gladkov <legion@...nel.org>,
	Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v3] posix-timers: add multi_clock_gettime system call

On Mon, Jan 15, 2024 at 05:49:32PM +0200, Sagi Maimon wrote:

> Thanks for your notes, all of them will be done on the next patch (it
> will take some time due to work overload).

No hurry, glad you are keeping this going...

> The only question that I have is: why not implement it as an IOCTL?
> It makes more sense to me since it is close to another IOCTL, the
> "PTP_SYS_OFFSET" family.

I've often needed other clock offsets, like CLOCK_REALTIME - CLOCK_MONOTONIC.

Those don't have a character device, and so there is no way to call
ioctl() on them.  That is why I'd like to have a system call that
handles any two clock_t instances, using the most accurate back end
based on the kinds of the two clocks.

Thanks,
Richard