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

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Message-ID: <CANDhNCp_BvN5GMGjLnZTKQBXwVAn72CuMkCwK9LhhjNGTmujzQ@mail.gmail.com>
Date:   Mon, 2 Oct 2023 19:32:10 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...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 Mon, Oct 2, 2023 at 5:13 PM Mahesh Bandewar (महेश बंडेवार)
<maheshb@...gle.com> wrote:
>
> 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.

Sounds reasonable to me.

thanks
-john