linux-kernel - Re: [PATCH v2 0/8] rtc: enable adjustment of clock offset

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Message-ID: <20160126160031.GA25597@piout.net>
Date:	Tue, 26 Jan 2016 17:00:31 +0100
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Joshua Clayton <stillcompiling@...il.com>
Cc:	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/8] rtc: enable adjustment of clock offset

On 26/01/2016 at 06:56:40 -0800, Joshua Clayton wrote :
> Ping?
> 
> On Monday, January 04, 2016 10:31:18 AM Joshua Clayton wrote:
> > Alexandre,
> > this is the second version of apatch set to be able to adjust the
> > clock on a pcf2123, but with a big addition. I am also proposing
> > a couple of new rtc api functions and a new sysfs file.
> > 
> > Patches 1-5 are very much v2 patches that address All of the
> > concerns you had with v1. It is my hope to get them merged without
> > too much more trouble.
> > 
> The rtc _needs_ patches 1-5 in order to use the clock offset
> adjustment at all (current upstream code resets it to zero during
> device probe, so it doesn't survive a reboot)
> Should I split out patches 1-5 to make them easier to review?
> 

The merge widows just closed so I've started to collect a few patches
since monday. This is a big series and I didn't have time to review
everything yet.

> > Patches 6-8 are attemping to add a new function to the rtc class
> > to adjust the clock rate. I hope I am going about this the right way,
> > But here goes.
> > 
> I am hoping to have some discussion about 6, 7 and 8.
> Particularly whether parts per billion is the right unit (I think it is)
> and whether "offset" is the best name for the attribute (not as sure).
> 

Yeah, My plan was to take 1 to 5 and leave out 6-8 for discussion, I'm
not yet sure what the correct api should look like.

> > A number of rtc devices, such as the NXP pcf2123 include a facility
> > to adjust the clock in order to compensate for temperature or a
> > crystal, capacitor, etc, that results in the rtc clock not running
> > at exactly 32.768 kHz.
> > 
> > This patchset adds kernel and sysfs hooks to access that ability.
> > 
> > One datasheet suggests it might be adjusted based on input from 
> > a temperature sensor. I could also potentially see it being set
> > as part of ntp calibration.
> > 
> > Data sheets I have seen refer to this as a clock offset, and measure it
> > in parts per million (ppm), however they often reference ppm to 2 digits
> > of precision, which makes integer ppm less than ideal. I use parts per
> > billion, which more than covers the precision needed and works nicely
> > within 32 bits
> > 
> > The name "offset" came from the pcf-2123 datasheet and is used by at
> > least some other data sheets. I would be happy to use a different term
> > if someone else comes up with something more concise.
> >     
> > Changes since v1:
> > - Use the BIT() macro for all register bits
> > - Remove unneeded range checks from read/write functions
> > - Merge patch 3 (replace magic numbers with defines) into patch 2
> > - Add a proper commit message to patch 5, now patch 4
> > - Fix a function alignment bug.
> > - Move OSC_HAS_STOPPED check into pcf2123_rtc_read_time, and get rid of
> >   pcf2123_time_valid()
> > - Drop patches refactoring pcf2123 sysfs.
> > - Add rtc interface and rtc sysfs file for clock offset
> > 
> > Joshua Clayton (8):
> >   rtc-pcf2123: define registers and bit macros
> >   rtc-pcf2123: clean up reads from the chip
> >   rtc-pcf2123: clean up writes to the rtc chip
> >   rtc-pcf2123: refactor chip reset into a function
> >   rtc-pcf2123: avoid resetting the clock if possible
> >   rtc: Add functions to set and read clock offset
> >   rtc: implement a sysfs interface for clock offset
> >   rtc-pcf2123: implement read_offset and set_offset
> > 
> >  drivers/rtc/interface.c   |  57 ++++++++++
> >  drivers/rtc/rtc-pcf2123.c | 271 +++++++++++++++++++++++++++++++++-------------
> >  drivers/rtc/rtc-sysfs.c   |  29 +++++
> >  include/linux/rtc.h       |   4 +
> >  4 files changed, 284 insertions(+), 77 deletions(-)
> > 
> > 
> Thanks again,
> Joshua Clayton

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com