Message-ID: <CAF2d9jg0X_HKSZbiwPTEXdmrhY49D1zfT3Q4xzNAqv1z+TYXtA@mail.gmail.com>
Date: Mon, 15 Apr 2024 10:23:17 -0700
From: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com>
To: Sagi Maimon <maimon.sagi@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, richardcochran@...il.com, luto@...nel.org, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, arnd@...db.de, geert@...ux-m68k.org, peterz@...radead.org, 
	hannes@...xchg.org, sohil.mehta@...el.com, rick.p.edgecombe@...el.com, 
	nphamcs@...il.com, palmer@...ive.com, keescook@...omium.org, 
	legion@...nel.org, mark.rutland@....com, mszeredi@...hat.com, 
	casey@...aufler-ca.com, reibax@...il.com, davem@...emloft.net, 
	brauner@...nel.org, linux-kernel@...r.kernel.org, linux-api@...r.kernel.org, 
	linux-arch@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v7] posix-timers: add clock_compare system call

On Sun, Apr 14, 2024 at 5:22 AM Sagi Maimon <maimon.sagi@...il.com> wrote:
>
> On Thu, Apr 11, 2024 at 7:34 PM Mahesh Bandewar (महेश बंडेवार)
> <maheshb@...gle.com> wrote:
> >
> > On Thu, Apr 11, 2024 at 12:11 AM Sagi Maimon <maimon.sagi@...il.com> wrote:
> > >
> > > Hi Mahesh
> > > What is the status of your patch?
> > > if your patch is upstreamed , then it will have all I need.
> > > But, If not , I will upstream my patch.
> > > BR,
> > >
> > Hi Sagi,
> >
> > If you want to pursue the syscall option, then those are tangential
> > and please go ahead. (I cannot stop you!)
> > I'm interested in getting the "tight sandwich timestamps" that
> > gettimex64() ioctl offers and I would want enhancements to
> > gettimex64() done the way it was discussed in the later half of this
> > thread. If you want to sign-up for that please let me know.
> Hi Mahesh
> I do need to modify the  PTP_SYS_OFFSET_EXTENDED ioctl for cases which
> gettimex64
> not supported by the driver (look at Thomas suggestion), but I need
> your changes in ptp_read_system_prets.
> I like to add my changes above your changes, so we won't do duplicate work.
> please show me your latest patch and the status of it
> Once you have upstream yours , I will add my changes on the next patch.

OK, in that case let me post the patch since your changes would need
pieces from it.

thanks,
--mahesh..

> BR
> Sagi
>
> >
> > thanks,
> > --mahesh..
> >
> >
> > > On Thu, Apr 11, 2024 at 5:56 AM Mahesh Bandewar (महेश बंडेवार)
> > > <maheshb@...gle.com> wrote:
> > > >
> > > > On Wed, Apr 3, 2024 at 6:48 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> > > > >
> > > > > On Tue, Apr 02 2024 at 16:37, Mahesh Bandewar (महेश बंडेवार) wrote:
> > > > > > On Tue, Apr 2, 2024 at 3:37 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> > > > > > The modification that you have proposed (in a couple of posts back)
> > > > > > would work but it's still not ideal since the pre/post ts are not
> > > > > > close enough as they are currently  (properly implemented!)
> > > > > > gettimex64() would have. The only way to do that would be to have
> > > > > > another ioctl as I have proposed which is a superset of current
> > > > > > gettimex64 and pre-post collection is the closest possible.
> > > > >
> > > > > Errm. What I posted as sketch _is_ using gettimex64() with the extra
> > > > > twist of the flag vs. a clockid (which is an implementation detail) and
> > > > > the difference that I carry the information in ptp_system_timestamp
> > > > > instead of needing a new argument clockid to all existing callbacks
> > > > > because the modification to ptp_read_prets() and postts() will just be
> > > > > sufficient, no?
> > > > >
> > > > OK, that makes sense.
> > > >
> > > > > For the case where the driver does not provide gettimex64() then the
> > > > > extension of the original offset ioctl is still providing a better
> > > > > mechanism than the proposed syscall.
> > > > >
> > > > > I also clearly said that all drivers should be converted over to
> > > > > gettimex64().
> > > > >
> > > > I agree. Honestly that should have been mandatory and
> > > > ptp_register_clock() should fail otherwise! Probably should have been
> > > > part of gettimex64 implementation :(
> > > >
> > > > I don't think we can do anything other than just hoping all driver
> > > > implementations include gettimex64 implementation.
> > > >
> > > > > > Having said that, the 'flag' modification proposal is a good backup
> > > > > > for the drivers that don't have good implementation (close enough but
> > > > > > not ideal). Also, you don't need a new ioctl-op. So if we really want
> > > > > > precision, I believe, we need a new ioctl op (with supporting
> > > > > > implementation similar to the mlx4 code above). but we want to save
> > > > > > the new ioctl-op and have less precision then proposed modification
> > > > > > would work fine.
> > > > >
> > > > > I disagree. The existing gettimex64() is good enough if the driver
> > > > > implements it correctly today. If not then those drivers need to be
> > > > > fixed independent of this.
> > > > >
> > > > > So assumed that a driver does:
> > > > >
> > > > > gettimex64()
> > > > >    ptp_prets(sts);
> > > > >    read_clock();
> > > > >    ptp_postts(sts);
> > > > >
> > > > > today then having:
> > > > >
> > > > > static inline void ptp_read_system_prets(struct ptp_system_timestamp *sts)
> > > > > {
> > > > >         if (sts) {
> > > > >                 if (sts->flags & PTP_SYS_OFFSET_MONO_RAW)
> > > > >                         ktime_get_raw_ts64(&sts->pre_ts);
> > > > >                 else
> > > > >                         ktime_get_real_ts64(&sts->pre_ts);
> > > > >         }
> > > > > }
> > > > >
> > > > > static inline void ptp_read_system_postts(struct ptp_system_timestamp *sts)
> > > > > {
> > > > >         if (sts) {
> > > > >                 if (sts->flags & PTP_SYS_OFFSET_MONO_RAW)
> > > > >                         ktime_get_raw_ts64(&sts->post_ts);
> > > > >                 else
> > > > >                         ktime_get_real_ts64(&sts->post_ts);
> > > > >         }
> > > > > }
> > > > >
> > > > > or
> > > > >
> > > > > static inline void ptp_read_system_prets(struct ptp_system_timestamp *sts)
> > > > > {
> > > > >         if (sts) {
> > > > >                 switch (sts->clockid) {
> > > > >                 case CLOCK_MONOTONIC_RAW:
> > > > >                         time_get_raw_ts64(&sts->pre_ts);
> > > > >                         break;
> > > > >                 case CLOCK_REALTIME:
> > > > >                         ktime_get_real_ts64(&sts->pre_ts);
> > > > >                         break;
> > > > >                 }
> > > > >         }
> > > > > }
> > > > >
> > > > > static inline void ptp_read_system_postts(struct ptp_system_timestamp *sts)
> > > > > {
> > > > >         if (sts) {
> > > > >                 switch (sts->clockid) {
> > > > >                 case CLOCK_MONOTONIC_RAW:
> > > > >                         time_get_raw_ts64(&sts->post_ts);
> > > > >                         break;
> > > > >                 case CLOCK_REALTIME:
> > > > >                         ktime_get_real_ts64(&sts->post_ts);
> > > > >                         break;
> > > > >                 }
> > > > >         }
> > > > > }
> > > > >
> > > > > is doing the exact same thing as your proposal but without touching any
> > > > > driver which implements gettimex64() correctly at all.
> > > > >
> > > > I see. Yes, this makes sense.
> > > >
> > > > > While your proposal requires to touch every single driver for no reason,
> > > > > no?
> > > > >
> > > > > It is just an implementation detail whether you use a flag or a
> > > > > clockid. You can carry the clockid for the clocks which actually can be
> > > > > read in that context in a reserved field of PTP_SYS_OFFSET_EXTENDED:
> > > > >
> > > > > struct ptp_sys_offset_extended {
> > > > >         unsigned int    n_samples; /* Desired number of measurements. */
> > > > >         clockid_t       clockid;
> > > > >         unsigned int    rsv[2];    /* Reserved for future use. */
> > > > > };
> > > > >
> > > > > and in the IOCTL:
> > > > >
> > > > >         if (extoff->clockid != CLOCK_MONOTONIC_RAW)
> > > > >                 return -EINVAL;
> > > > >
> > > > >         sts.clockid = extoff->clockid;
> > > > >
> > > > > and it all just works, no?
> > > > >
> > > > Yes, this should work. However, I didn't check if struct
> > > > ptp_system_timestamp is used in some other context.
> > > >
> > > > > I have no problem to decide that PTP_SYS_OFFSET will not get this
> > > > > treatment and the drivers have to be converted over to
> > > > > PTP_SYS_OFFSET_EXTENDED.
> > > > >
> > > > > But adding yet another callback just to carry a clockid as argument is a
> > > > > more than pointless exercise as I demonstrated.
> > > > >
> > > > Agreed. As I said, I thought we cannot change the gettimex64() without
> > > > breaking the compatibility but the fact that CLOCK_REALTIME is "0"
> > > > works well for the backward compatibility case.
> > > >
> > > > I can spin up an updated patch/series that updates gettimex64
> > > > implementation instead of adding a new ioctl-op If you all agree.
> > > >
> > > > thanks,
> > > > --mahesh..
> > > >
> > > > > Thanks,
> > > > >
> > > > >         tglx

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