Re: PPS functionality for Intel Timed I/O

[Rodolfo Giometti]

On 15/02/23 08:09, N, Pandith wrote:
> Hi Rodolfo,

Hello.

> > -----Original Message-----
> > From: Rodolfo Giometti <giometti@xxxxxxxxxxxx>
> > Sent: Monday, February 6, 2023 4:17 PM
> > To: N, Pandith <pandith.n@xxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx; Sangannavar, Mallikarjunappa
> > <mallikarjunappa.sangannavar@xxxxxxxxx>; D, Lakshmi Sowjanya
> > <lakshmi.sowjanya.d@xxxxxxxxx>; T R, Thejesh Reddy
> > <thejesh.reddy.t.r@xxxxxxxxx>; Hall, Christopher S
> > <christopher.s.hall@xxxxxxxxx>
> > Subject: Re: PPS functionality for Intel Timed I/O

[snip]

> > Mmm... I'm not sure this is correct since PPS generators should generate
> > their pulses according to system clock and not according to their internal
> > clocks even if they are synced with the system clock.
> Timed I/O and system time are both driven by the *same* hardware clock.
> Timed I/O is a high precision device (nanoseconds), able to output pulses.
> The driver gets the system time and schedules output
> at target_time in the future.

OK, in this case it would be OK. Please put an appropriate note within the 
generator's code and a detailed one within Documentation/driver-api/pps.rst.

Regarding Documentation/driver-api/pps.rst let me suggest you to prose a 
separate patch to rewrite the Generators section in such a way you easily can 
add your solution at the end. A possible example is attached but feel free to 
rewrite it according to your needs.

> struct timespec64 current_time;
> current_time = ktime_get_real_ts64(&current_time);
>
> target_time = current_time + offset_time;
> cycles = translate_system_time_to_clock_cycles(target_time);
> /* Translate function is developed as part of this driver development */
>
> /* Schedule pulse at target_time */
> write_timedio_reg(TIMEDIO_TRIGGER, cycles);
>
> The output always triggers with ~20 ns precision.
> It's not affected by system load or no need for drivers to disable interrupts for longer durations.
> With this driver proposal, Intel Timed I/O as PPS generator will be useful in IoT and server applications.

Great! Please also add these notes to the final Documentation.

Ciao,

Rodolfo

--
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diff --git a/Documentation/driver-api/pps.rst b/Documentation/driver-api/pps.rst
index 2d6b99766ee8..e25031845ec6 100644
--- a/Documentation/driver-api/pps.rst
+++ b/Documentation/driver-api/pps.rst
@@ -200,11 +200,17 @@ Generators
 
 Sometimes one needs to be able not only to catch PPS signals but to produce
 them also. For example, running a distributed simulation, which requires
-computers' clock to be synchronized very tightly. One way to do this is to
-invent some complicated hardware solutions but it may be neither necessary
-nor affordable. The cheap way is to load a PPS generator on one of the
-computers (master) and PPS clients on others (slaves), and use very simple
-cables to deliver signals using parallel ports, for example.
+computers' clock to be synchronized very tightly.
+
+
+Parallel port generator
+------------------------
+
+One way to do this is to invent some complicated hardware solutions
+but it may be neither necessary nor affordable. The cheap way is to
+load a PPS generator on one of the computers (master) and PPS clients
+on others (slaves), and use very simple cables to deliver signals
+using parallel ports, for example.
 
 Parallel port cable pinout::