linux-kernel - Re: [PATCH 1/2] rtc: m48t59: Accommodate chips that lack a century bit

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Message-ID: <63298bb6-2921-b57d-729d-21e83a945944@linux-m68k.org>
Date: Fri, 4 Oct 2024 08:31:10 +1000 (AEST)
From: Finn Thain <fthain@...ux-m68k.org>
To: Alexandre Belloni <alexandre.belloni@...tlin.com>
cc: Geert Uytterhoeven <geert@...ux-m68k.org>, Daniel Palmer <daniel@...f.com>, 
    Michael Pavone <pavone@...rodev.com>, linux-m68k@...ts.linux-m68k.org, 
    linux-rtc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rtc: m48t59: Accommodate chips that lack a century
 bit


On Thu, 3 Oct 2024, Alexandre Belloni wrote:

> On 03/10/2024 13:23:22+1000, Finn Thain wrote:
> > The m48t59 driver is needed by both SPARC and MVME systems. Linux on
> > MVME uses 1970 as "year zero" rather than 1968 that's used on SPARC.
> > Add support for the MVME convention. Otherwise, the RTC on non-SPARC
> > systems can only read and write dates between 1900 and 1999.
> > 
> > Tested-by: Daniel Palmer <daniel@...f.com>
> > Signed-off-by: Finn Thain <fthain@...ux-m68k.org>
> > ---
> >  drivers/rtc/rtc-m48t59.c | 31 +++++++++++++++----------------
> >  1 file changed, 15 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/rtc/rtc-m48t59.c b/drivers/rtc/rtc-m48t59.c
> > index f0f6b9b6daec..e2d882ea5c2f 100644
> > --- a/drivers/rtc/rtc-m48t59.c
> > +++ b/drivers/rtc/rtc-m48t59.c
> > @@ -57,6 +57,17 @@ m48t59_mem_readb(struct device *dev, u32 ofs)
> >  	return readb(m48t59->ioaddr+ofs);
> >  }
> >  
> > +/*
> > + * Sun SPARC machines count years since 1968. MVME machines running Linux
> > + * count years since 1970.
> > + */
> > +
> > +#ifdef CONFIG_SPARC
> > +#define YEAR0 68
> > +#else
> > +#define YEAR0 70
> > +#endif
> > +
> >  /*
> >   * NOTE: M48T59 only uses BCD mode
> >   */
> > @@ -82,10 +93,7 @@ static int m48t59_rtc_read_time(struct device *dev, struct rtc_time *tm)
> >  		dev_dbg(dev, "Century bit is enabled\n");
> >  		tm->tm_year += 100;	/* one century */
> >  	}
> > -#ifdef CONFIG_SPARC
> > -	/* Sun SPARC machines count years since 1968 */
> > -	tm->tm_year += 68;
> > -#endif
> > +	tm->tm_year += YEAR0;
> >  
> 
> I'm super happy to see someone working on this, while you are it, can 
> you use m48t59->rtc->start_secs and m48t59->rtc->set_start_time in probe 
> instead of offsetting tm_year in read_time/set_time so we can later use 
> device tree or any other mechanism to extend the range?
> 

Sure, I will look into it.

> It is super funny because I was telling Geert that I wanted to get rid 
> of this #ifdef CONFIG_SPARC two weeks ago at LPC. That could indeed then 
> come from platform data.
> 

I can't test any patches on SPARC, unless there's some way to do so using 
QEMU that would satisfy maintainers. (I rely on Daniel to test my patches 
on an MVME147 system.)