linux-kernel - Re: [PATCH 1/4] time: add ktime_get

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Message-ID: <CAF2d9jg4Oxm3NwDuh21eeKC5-m7umZM3XLuxUKcFkchFjTgTtQ@mail.gmail.com>
Date:   Mon, 2 Oct 2023 17:12:36 -0700
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     John Stultz <jstultz@...gle.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Linux <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Don Hatchett <hatch@...gle.com>,
        Yuliang Li <yuliangli@...gle.com>,
        Mahesh Bandewar <mahesh@...dewar.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH 1/4] time: add ktime_get_cycles64() api

On Fri, Sep 29, 2023 at 12:07 AM John Stultz <jstultz@...gle.com> wrote:
>
> On Thu, Sep 28, 2023 at 11:56 PM John Stultz <jstultz@...gle.com> wrote:
> > On Thu, Sep 28, 2023 at 11:35 PM Mahesh Bandewar (महेश बंडेवार)
> > <maheshb@...gle.com> wrote:
> > > On Thu, Sep 28, 2023 at 10:15 PM John Stultz <jstultz@...gle.com> wrote:
> > > > 3) Nit: The interface is called ktime_get_cycles64 (timespec64
> > > > returning interfaces usually are postfixed with ts64).
> > > >
> > > Ah, thanks for the explanation. I can change to comply with the
> > > convention. Does ktime_get_cycles_ts64() make more sense?
> >
> > Maybe a little (it at least looks consistent), but not really if
> > you're sticking raw cycles in the timespec :)
> >
>
> Despite my concerns that it's a bad idea, If one was going to expose
> raw cycles from the timekeeping core, I'd suggest doing so directly as
> a u64 (`u64 ktime_get_cycles(void)`).
>
> That may mean widening (or maybe using a union in) your PTP ioctl data
> structure to have a explicit cycles field.
> Or introducing a separate ioctl that deals with cycles instead of timespec64s.
>
> Squeezing data into types that are canonically used for something else
> should always be avoided if possible (there are some cases where
> you're stuck with an existing interface, but that's not the case
> here).
>
> But I still think we should avoid exporting the raw cycle values
> unless there is some extremely strong argument for it (and if we can,
> they should be abstracted into some sort of cookie value to avoid
> userland using it as a raw clock).
>
Thanks for the input John. This change is basically to address the API
gap and allow it to give a user-given timebase for the sandwich time.
I will remove this RAW-CYCLES option for now. If it's deemed
necessary, we can always add it later into the same API.

> thanks
> -john