linux-kernel - Re: [PATCH v12 2/3] Documentation: driver-api: pps: Add Intel Timed I/O PPS generator

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Message-ID: <2024090304-viewing-lavish-c05e@gregkh>
Date: Tue, 3 Sep 2024 12:25:24 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Rodolfo Giometti <giometti@...eenne.com>
Cc: subramanian.mohan@...el.com, tglx@...utronix.de, corbet@....net,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	andriy.shevchenko@...ux.intel.com, eddie.dong@...el.com,
	christopher.s.hall@...el.com, pandith.n@...el.com,
	thejesh.reddy.t.r@...el.com, david.zage@...el.com,
	srinivasan.chinnadurai@...el.com
Subject: Re: [PATCH v12 2/3] Documentation: driver-api: pps: Add Intel Timed
 I/O PPS generator

On Tue, Aug 27, 2024 at 03:09:15PM +0200, Rodolfo Giometti wrote:
> On 24/08/24 04:21, Greg KH wrote:
> > On Fri, Aug 23, 2024 at 12:31:07PM +0530, subramanian.mohan@...el.com wrote:
> > > From: Subramanian Mohan <subramanian.mohan@...el.com>
> > > 
> > > Add Intel Timed I/O PPS usage instructions.
> > > 
> > > Co-developed-by: Pandith N <pandith.n@...el.com>
> > > Signed-off-by: Pandith N <pandith.n@...el.com>
> > > Signed-off-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@...el.com>
> > > Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > > Acked-by: Rodolfo Giometti <giometti@...eenne.com>
> > > Signed-off-by: Subramanian Mohan <subramanian.mohan@...el.com>
> > > ---
> > >   Documentation/driver-api/pps.rst | 24 ++++++++++++++++++++++++
> > >   1 file changed, 24 insertions(+)
> > > 
> > > diff --git a/Documentation/driver-api/pps.rst b/Documentation/driver-api/pps.rst
> > > index 78dded03e5d8..75f7b094f963 100644
> > > --- a/Documentation/driver-api/pps.rst
> > > +++ b/Documentation/driver-api/pps.rst
> > > @@ -246,3 +246,27 @@ delay between assert and clear edge as small as possible to reduce system
> > >   latencies. But if it is too small slave won't be able to capture clear edge
> > >   transition. The default of 30us should be good enough in most situations.
> > >   The delay can be selected using 'delay' pps_gen_parport module parameter.
> > > +
> > > +
> > > +Intel Timed I/O PPS signal generator
> > > +------------------------------------
> > > +
> > > +Intel Timed I/O is a high precision device, present on 2019 and newer Intel
> > > +CPUs, that can generate PPS signals.
> > > +
> > > +Timed I/O and system time are both driven by same hardware clock. The signal
> > > +is generated with a precision of ~20 nanoseconds. The generated PPS signal
> > > +is used to synchronize an external device with system clock. For example,
> > > +it can be used to share your clock with a device that receives PPS signal,
> > > +generated by Timed I/O device. There are dedicated Timed I/O pins to deliver
> > > +the PPS signal to an external device.
> > > +
> > > +Usage of Intel Timed I/O as PPS generator:
> > > +
> > > +Start generating PPS signal::
> > > +
> > > +        $echo 1 > /sys/devices/platform/INTCxxxx\:00/enable
> > > +
> > > +Stop generating PPS signal::
> > > +
> > > +        $echo 0 > /sys/devices/platform/INTCxxxx\:00/enable
> > 
> > As I mentioned on the sysfs documentation, why isn't this just a generic
> > pps class attribute instead?  Why did you make it
> > only-this-one-special-driver type of thing?
> 
> This is an Original Sin when PPS generators were introduced. :-(
> 
> In 2011 a patch from Alexander Gordeev <lasaine@....cs.msu.su> (which
> introduced the "parallel port PPS signal generator") was committed in the
> main kernel.
> 
> At the time it was something exotic and doing a PPS generator interface for
> it was not considered (since it actually has no controlling inputs), but now
> several Ethernet cards have such PPS generator functionalities, and they are
> enabled in a per-driver way or via the PTP API.
> 
> This code is a pure PPS generator and it cannot use any other way to enable
> such functionality than the one above since the PPS layer misses a proper
> implementation for PPS generator.
> 
> If you are willing to stop the inclusion due this fact maybe its time to add
> such PPS generators interface... on the other hand, if you agree for
> inclusion we can do this job as soon as the code has been included, in order
> to fix this anomalous status.

Please make a generic pps subsystem for this, it would make it simpler
for everyone.

thanks,

greg k-h