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

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Message-ID: <20241003081015363ed024@mail.local>
Date: Thu, 3 Oct 2024 10:10:15 +0200
From: Alexandre Belloni <alexandre.belloni@...tlin.com>
To: Finn Thain <fthain@...ux-m68k.org>
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

Hello,

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?

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.

>  	tm->tm_wday	= bcd2bin(val & 0x07);
>  	tm->tm_hour	= bcd2bin(M48T59_READ(M48T59_HOUR) & 0x3F);
> @@ -108,10 +116,7 @@ static int m48t59_rtc_set_time(struct device *dev, struct rtc_time *tm)
>  	u8 val = 0;
>  	int year = tm->tm_year;
>  
> -#ifdef CONFIG_SPARC
> -	/* Sun SPARC machines count years since 1968 */
> -	year -= 68;
> -#endif
> +	year -= YEAR0;
>  
>  	dev_dbg(dev, "RTC set time %04d-%02d-%02d %02d/%02d/%02d\n",
>  		year + 1900, tm->tm_mon, tm->tm_mday,
> @@ -163,10 +168,7 @@ static int m48t59_rtc_readalarm(struct device *dev, struct rtc_wkalrm *alrm)
>  	M48T59_SET_BITS(M48T59_CNTL_READ, M48T59_CNTL);
>  
>  	tm->tm_year = bcd2bin(M48T59_READ(M48T59_YEAR));
> -#ifdef CONFIG_SPARC
> -	/* Sun SPARC machines count years since 1968 */
> -	tm->tm_year += 68;
> -#endif
> +	tm->tm_year += YEAR0;
>  	/* tm_mon is 0-11 */
>  	tm->tm_mon = bcd2bin(M48T59_READ(M48T59_MONTH)) - 1;
>  
> @@ -199,10 +201,7 @@ static int m48t59_rtc_setalarm(struct device *dev, struct rtc_wkalrm *alrm)
>  	unsigned long flags;
>  	int year = tm->tm_year;
>  
> -#ifdef CONFIG_SPARC
> -	/* Sun SPARC machines count years since 1968 */
> -	year -= 68;
> -#endif
> +	year -= YEAR0;
>  
>  	/* If no irq, we don't support ALARM */
>  	if (m48t59->irq == NO_IRQ)
> -- 
> 2.39.5
> 

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