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

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Date:   Fri, 16 Aug 2019 10:50:49 -0500
From:   Li Yang <leoyang.li@....com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     Biwen Li <biwen.li@....com>, a.zummo@...ertech.it,
        linux-rtc@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [v2] rtc: pcf85363/pcf85263: fix error that failed to run hwclock -w

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/lkml/2019/4/3/55 , 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.

Regards,
Leo