netdev - Re: [PATCH net-next v2 7/9] ptp: ptp_clockmatrix: Add .getmaxphase ptp_clock

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Date: Thu, 25 May 2023 14:11:51 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Rahul Rameshbabu <rrameshbabu@...dia.com>, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>, Jacob Keller
	 <jacob.e.keller@...el.com>, Gal Pressman <gal@...dia.com>, Tariq Toukan
	 <tariqt@...dia.com>, Saeed Mahameed <saeed@...nel.org>, Richard Cochran
	 <richardcochran@...il.com>, Vincent Cheng <vincent.cheng.xh@...esas.com>
Subject: Re: [PATCH net-next v2 7/9] ptp: ptp_clockmatrix: Add .getmaxphase
 ptp_clock_info callback

On Thu, 2023-05-25 at 14:08 +0200, Paolo Abeni wrote:
> On Tue, 2023-05-23 at 13:54 -0700, Rahul Rameshbabu wrote:
> > Advertise the maximum offset the .adjphase callback is capable of
> > supporting in nanoseconds for IDT ClockMatrix devices.
> > 
> > Cc: Richard Cochran <richardcochran@...il.com>
> > Cc: Vincent Cheng <vincent.cheng.xh@...esas.com>
> > Signed-off-by: Rahul Rameshbabu <rrameshbabu@...dia.com>
> > ---
> >  drivers/ptp/ptp_clockmatrix.c | 36 +++++++++++++++++------------------
> >  drivers/ptp/ptp_clockmatrix.h |  2 +-
> >  2 files changed, 18 insertions(+), 20 deletions(-)
> > 
> > diff --git a/drivers/ptp/ptp_clockmatrix.c b/drivers/ptp/ptp_clockmatrix.c
> > index c9d451bf89e2..f6f9d4adce04 100644
> > --- a/drivers/ptp/ptp_clockmatrix.c
> > +++ b/drivers/ptp/ptp_clockmatrix.c
> > @@ -1692,14 +1692,23 @@ static int initialize_dco_operating_mode(struct idtcm_channel *channel)
> >  /* PTP Hardware Clock interface */
> >  
> >  /*
> > - * Maximum absolute value for write phase offset in picoseconds
> > - *
> > - * @channel:  channel
> > - * @delta_ns: delta in nanoseconds
> > + * Maximum absolute value for write phase offset in nanoseconds
> >   *
> >   * Destination signed register is 32-bit register in resolution of 50ps
> >   *
> > - * 0x7fffffff * 50 =  2147483647 * 50 = 107374182350
> > + * 0x7fffffff * 50 =  2147483647 * 50 = 107374182350 ps
> > + * Represent 107374182350 ps as 107374182 ns
> > + */
> > +static s32 idtcm_getmaxphase(struct ptp_clock_info *ptp __always_unused)
> > +{
> > +	return MAX_ABS_WRITE_PHASE_NANOSECONDS;
> > +}
> 
> This introduces a functional change WRT the current code. Prior to this
> patch ClockMatrix tries to adjust phase delta even above
> MAX_ABS_WRITE_PHASE_NANOSECONDS, limiting the delta to such value.
> After this patch it will error out.
> 
> Perhaps a more conservative approach would be keeping the existing
> logic in _idtcm_adjphase and let idtcm_getmaxphase return  
> S32_MAX?
> 
> Note that even that will error out for delta == S32_MIN so perhaps an
> API change to allow the driver specify unlimited delta would be useful
> (possibly regardless of the above).

What about allowing drivers with no getmaxphase() callback, meaning
such drivers allow adjusting unlimited phase delta? 

Thanks!

Paolo