linux-kernel - Re: PPS functionality for Intel Timed I/O

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Date:   Wed, 15 Feb 2023 08:46:25 +0100
From:   Rodolfo Giometti <giometti@...eenne.com>
To:     "N, Pandith" <pandith.n@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Sangannavar, Mallikarjunappa" 
        <mallikarjunappa.sangannavar@...el.com>,
        "D, Lakshmi Sowjanya" <lakshmi.sowjanya.d@...el.com>,
        "T R, Thejesh Reddy" <thejesh.reddy.t.r@...el.com>,
        "Hall, Christopher S" <christopher.s.hall@...el.com>,
        "Gross, Mark" <mark.gross@...el.com>
Subject: Re: PPS functionality for Intel Timed I/O

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

Hello.

>> -----Original Message-----
>> From: Rodolfo Giometti <giometti@...eenne.com>
>> Sent: Monday, February 6, 2023 4:17 PM
>> To: N, Pandith <pandith.n@...el.com>
>> Cc: linux-kernel@...r.kernel.org; Sangannavar, Mallikarjunappa
>> <mallikarjunappa.sangannavar@...el.com>; D, Lakshmi Sowjanya
>> <lakshmi.sowjanya.d@...el.com>; T R, Thejesh Reddy
>> <thejesh.reddy.t.r@...el.com>; Hall, Christopher S
>> <christopher.s.hall@...el.com>
>> 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

-- 
GNU/Linux Solutions                  e-mail: giometti@...eenne.com
Linux Device Driver                          giometti@...ux.it
Embedded Systems                     phone:  +39 349 2432127
UNIX programming                     skype:  rodolfo.giometti

View attachment "pps_rst_doc_proposal.patch" of type "text/x-patch" (1201 bytes)