linux-kernel - Re: [EXT] Re: [v2] rtc: pcf85363/pcf85263: fix error that failed to run hwclock -w

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Date:   Mon, 26 Aug 2019 12:06:50 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Biwen Li <biwen.li@....com>
Cc:     Nandor Han <nandor.han@...sala.com>, Leo Li <leoyang.li@....com>,
        Mark Brown <broonie@...nel.org>,
        "a.zummo@...ertech.it" <a.zummo@...ertech.it>,
        "linux-rtc@...r.kernel.org" <linux-rtc@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [EXT] Re: [v2] rtc: pcf85363/pcf85263: fix error that failed to
 run hwclock -w

Hi,

On 26/08/2019 09:49:49+0000, Biwen Li wrote:
> > 
> > On 8/26/19 7:29 AM, Biwen Li wrote:
> > >>
> > >> On 8/16/19 10:40 PM, Li Yang wrote:
> > >>> On Fri, Aug 16, 2019 at 11:30 AM Alexandre Belloni
> > >>> <alexandre.belloni@...tlin.com> wrote:
> > >>>>
> > >>>> On 16/08/2019 10:50:49-0500, Li Yang wrote:
> > >>>>> On Fri, Aug 16, 2019 at 3:05 AM Alexandre Belloni
> > >>>>> <alexandre.belloni@...tlin.com> wrote:
> > >>>>>>
> > >>>>>> On 16/08/2019 10:46:36+0800, Biwen Li wrote:
> > >>>>>>> Issue:
> > >>>>>>>       - # hwclock -w
> > >>>>>>>         hwclock: RTC_SET_TIME: Invalid argument
> > >>>>>>>
> > >>>>>>> Why:
> > >>>>>>>       - Relative patch:
> > >> https://lkml.org
> > >> %2Flkml%2F2019%2F4%2F3%2F55&amp;data=02%7C01%7Cbiwen.li%40n
> > xp.
> > >>
> > com%7Cff8cebc3f1034ae3fa9608d725ff9e5e%7C686ea1d3bc2b4c6fa92cd99
> > >>
> > c5c301635%7C0%7C0%7C637019652111923736&amp;sdata=spY6e22YOkOF
> > >> 3%2BF7crSM0M6xPmOhgULDqMZLQw%2BAmdI%3D&amp;reserved=0 ,
> > this patch
> > >>>>>>>         will always check for unwritable registers, it will compare reg
> > >>>>>>>         with max_register in regmap_writeable.
> > >>>>>>>
> > >>>>>>>       - In drivers/rtc/rtc-pcf85363.c, CTRL_STOP_EN is 0x2e, but
> > >> DT_100THS
> > >>>>>>>         is 0, max_regiter is 0x2f, then reg will be equal to 0x30,
> > >>>>>>>         '0x30 < 0x2f' is false,so regmap_writeable will return false.
> > >>>>>>>
> > >>>>>>>       - Root cause: the buf[] was written to a wrong place in the file
> > >>>>>>>         drivers/rtc/rtc-pcf85363.c
> > >>>>>>>
> > >>>>>>
> > >>>>>> This is not true, the RTC wraps the register accesses properly
> > >>>>>> and this
> > >>>>>
> > >>>>> This performance hack probably deserve some explanation in the
> > >>>>> code comment.  :)
> > >>>>>
> > >>>>>> is probably something that should be handled by regmap_writable.
> > >>>>>
> > >>>>> The address wrapping is specific to this RTC chip.  Is it also
> > >>>>> commonly used by other I2C devices?  I'm not sure if
> > >>>>> regmap_writable should handle the wrapping case if it is too special.
> > >>>>>
> > >>>>
> > >>>> Most of the i2c RTCs do address wrapping which is sometimes the
> > >>>> only way to properly set the time.
> > >>>
> > >>> Adding Mark and Nandor to the loop.
> > >>>
> > >>> Regards,
> > >>> Leo
> > >>>
> > >>
> > >> Hi,
> > >>     `regmap` provides couple of ways to validate the registers:
> > >> max_register, callback function and write table. All of these are
> > >> optional, so it gives you the freedom to customize it as needed.
> > >>
> > >> In this situation probably you could:
> > >>     1. Avoid using the wrapping feature of pcf85363 (you can just
> > >> provide separate calls for stop, reset and time confguration). In
> > >> this way the `max_register` validation method will work fine.
> > > Yes, I use this way. Path as follows:
> > > Stop and reset - > set time > stop
> > >
> > 
> > Some of the concerns regarding this method was that it might not be precise
> > enough. That because you need 2 I2C operations (one for stop and one for time
> > configuration). Not sure about your case if this is a problem or not.
> Ok, got it, thanks.

To be clear, for this RTC it is fine to separate both writes. Want I
want is a corrected commit message with a proper reference to
8b9f9d4dc511309918c4f6793bae7387c0c638af instead of a link to lkml.org
and a proper explanation.

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com