linux-kernel - Re: [RFC PATCH v2] rtc: pcf2127: Add PPS capability through Seconds Interrupt

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Date: Wed, 12 Jun 2024 09:50:26 +0200
From: Miroslav Lichvar <mlichvar@...hat.com>
To: Richard Cochran <richardcochran@...il.com>
Cc: Csókás, Bence <csokas.bence@...lan.hu>,
	linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Szentendrei, Tamás <szentendrei.tamas@...lan.hu>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [RFC PATCH v2] rtc: pcf2127: Add PPS capability through Seconds
 Interrupt

On Tue, Jun 11, 2024 at 10:06:39PM -0700, Richard Cochran wrote:
> On Tue, Jun 11, 2024 at 05:04:57PM +0200, Csókás, Bence wrote:
> 
> > PCF2127/29/31 is capable of generating an interrupt on every
> > second (SI) or minute (MI) change. It signals this through
> > the Minute/Second Flag (MSF) as well, which needs to be cleared.
> 
> This is a RFC, and my comment is that a PPS from an RTC is not useful
> to the Linux kernel.

I think a TCXO-based RTC can be useful to user space to improve
holdover performance with NTP/PTP. There already is the RTC_UIE_ON
ioctl to enable interrupts and receive them in user space.

The advantage of the PPS device over the ioctl would be more accurate
timestamping (kernel vs user-space). Should PPS be supported, it would
be nice if it worked generally with all drivers that support RTC_UIE_ON.

-- 
Miroslav Lichvar