Date:   Wed, 19 Oct 2022 21:18:45 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     tollsimy <simonetollardo@...il.com>
Cc:     a.zummo@...ertech.it, linux-rtc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Microchip MCP795xx RTC driver patch

Hello,

On 16/10/2022 18:29:13+0200, tollsimy wrote:
> RTC Microchip mcp795 patch for 5.19-rc5
> 
> Here is the mcp795 patch for the 5.19-rc5 merge window.
> The main modification are the following:
>         - fixed wrong driver naming: RTC is a Microchip
>           mcp795 and not Maxim mcp795
>         - added date/time write sequences: added date/time      
>           write sequences accordingly to official silicon 
>           errata document to fix silicon bugs.
>         - added support to date/time permanent storage:
>           the driver was missing some register bits in order
>           to enable the storage of date/time whenever the       
>           device looses main power.
>         - added a check after starting oscillator in order
>           to be sure that it is properly working as specified
>           in the datasheet
>         - renamed OSCON bit to OSCRUN as in the datasheet
>         - add debug messages when setting and reading the
>           alarm
>         - removed wrong offset when retrieving the current
>           year from the device
> 
> The patch has been successfully tested in the latest
> linux-next releases, and the original problems that I found have
> all been resolved.
> 
> Signed-off-by: tollsimy <simonetollardo@...il.com>
> 
> ---
>  .../devicetree/bindings/rtc/maxim,mcp795.txt  |  11 -
>  .../bindings/rtc/microchip,mcp795.txt         |  11 +
>  drivers/rtc/rtc-mcp795.c                      | 359 ++++++++++++++----
>  3 files changed, 296 insertions(+), 85 deletions(-)
>  delete mode 100644 Documentation/devicetree/bindings/rtc/maxim,mcp795.txt
>  create mode 100644 Documentation/devicetree/bindings/rtc/microchip,mcp795.txt

This needs to be a separate change, also, please convert to yaml.

> 
> diff --git a/Documentation/devicetree/bindings/rtc/maxim,mcp795.txt b/Documentation/devicetree/bindings/rtc/maxim,mcp795.txt
> deleted file mode 100644
> index a59fdd8c236d..000000000000
> --- a/Documentation/devicetree/bindings/rtc/maxim,mcp795.txt
> +++ /dev/null
> @@ -1,11 +0,0 @@
> -* Maxim MCP795		SPI Serial Real-Time Clock
> -
> -Required properties:
> -- compatible: Should contain "maxim,mcp795".
> -- reg: SPI address for chip
> -
> -Example:
> -	mcp795: rtc@0 {
> -		compatible = "maxim,mcp795";
> -		reg = <0>;
> -	};
> diff --git a/Documentation/devicetree/bindings/rtc/microchip,mcp795.txt b/Documentation/devicetree/bindings/rtc/microchip,mcp795.txt
> new file mode 100644
> index 000000000000..854364c3a173
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/rtc/microchip,mcp795.txt
> @@ -0,0 +1,11 @@
> +* Microchip MCP795		SPI Serial Real-Time Clock
> +
> +Required properties:
> +- compatible: Should contain "microchip,mcp795".
> +- reg: SPI address for chip
> +
> +Example:
> +	mcp795: rtc@0 {
> +		compatible = "microchip,mcp795";
> +		reg = <0>;
> +	};
> diff --git a/drivers/rtc/rtc-mcp795.c b/drivers/rtc/rtc-mcp795.c
> index 0d515b3df571..292a440b62dd 100644
> --- a/drivers/rtc/rtc-mcp795.c
> +++ b/drivers/rtc/rtc-mcp795.c
> @@ -3,11 +3,15 @@
>   * SPI Driver for Microchip MCP795 RTC
>   *
>   * Copyright (C) Josef Gajdusek <atx@....name>
> + * Copyright (C) 2022 Simone Tollardo <simone.tollardo@...il.com>
>   *
>   * based on other Linux RTC drivers
>   *
>   * Device datasheet:
> - * https://ww1.microchip.com/downloads/en/DeviceDoc/22280A.pdf
> + * https://ww1.microchip.com/downloads/en/DeviceDoc/MCP7951X-MCP7952X-Battery-Backed-SPI-RTCC-20002300C.pdf
> + *
> + * Device errata:
> + * http://ww1.microchip.com/downloads/en/DeviceDoc/MCP795XX-Family-Silicon-Errata-80000680D.pdf
>   */
>  
>  #include <linux/module.h>
> @@ -32,28 +36,148 @@
>  #define MCP795_UNLOCK	0x14
>  #define MCP795_IDWRITE	0x32
>  #define MCP795_IDREAD	0x33
> -#define MCP795_CLRWDT	0x44
>  #define MCP795_CLRRAM	0x54
>  
>  /* MCP795 RTCC registers, see datasheet table 4-1 */
> +#define MCP795_REG_TMSECONDS	0x00	/* Tenths of ms (=centi-seconds) */
>  #define MCP795_REG_SECONDS	0x01
> +#define MCP795_REG_MINUTES	0x02
> +#define MCP795_REG_HOURS	0x03
>  #define MCP795_REG_DAY		0x04
> +#define MCP795_REG_DATE		0x05
>  #define MCP795_REG_MONTH	0x06
> +#define MCP795_REG_YEAR		0x07
>  #define MCP795_REG_CONTROL	0x08
> +#define MCP795_REG_OSC_TRIM	0x09
> +/* Alarm 0 */
>  #define MCP795_REG_ALM0_SECONDS	0x0C
> +#define MCP795_REG_ALM0_MINUTES	0x0D
> +#define MCP795_REG_ALM0_HOURS	0x0E
>  #define MCP795_REG_ALM0_DAY	0x0F
> -
> -#define MCP795_ST_BIT		BIT(7)
> -#define MCP795_24_BIT		BIT(6)
> -#define MCP795_LP_BIT		BIT(5)
> -#define MCP795_EXTOSC_BIT	BIT(3)
> -#define MCP795_OSCON_BIT	BIT(5)
> -#define MCP795_ALM0_BIT		BIT(4)
> -#define MCP795_ALM1_BIT		BIT(5)
> -#define MCP795_ALM0IF_BIT	BIT(3)
> -#define MCP795_ALM0C0_BIT	BIT(4)
> -#define MCP795_ALM0C1_BIT	BIT(5)
> -#define MCP795_ALM0C2_BIT	BIT(6)
> +#define MCP795_REG_ALM0_DATE	0x10
> +#define MCP795_REG_ALM0_MONTH	0x11	/* Month only for alarm 0 */
> +/* Alarm 1 */
> +#define MCP795_REG_ALM1_TMSECONDS	0x12	/* Centi-seconds only for alarm 1 */
> +#define MCP795_REG_ALM1_SECONDS	0x13
> +#define MCP795_REG_ALM1_MINUTES	0x14
> +#define MCP795_REG_ALM1_HOURS	0x15
> +#define MCP795_REG_ALM1_DAY	0x16
> +#define MCP795_REG_ALM1_DATE	0x17
> +
> +/* Power-Fail Timestamp */
> +/* Power-Down Timetsamp */
> +#define MCP795_REG_PWD_MINUTES	0x18
> +#define MCP795_REG_PWD_HOURS	0x19
> +#define MCP795_REG_PWD_DATE	0x1A
> +#define MCP795_REG_PWD_MONTH	0x1B
> +/* Power-Up Timestamp */
> +#define MCP795_REG_PUP_MINUTES	0x1C
> +#define MCP795_REG_PUP_HOURS	0x1D
> +#define MCP795_REG_PUP_DATE	0x1E
> +#define MCP795_REG_PUP_MONTH	0x1F
> +
> +/* MCP795_REG_CONTROL bits */
> +
> +/* MISC */
> +#define MCP795_ST_BIT		BIT(7)	/* Status bit */
> +#define MCP795_24_BIT		BIT(6)	/* 12/24 hour mode */
> +#define MCP795_LP_BIT		BIT(5)	/* Leap Year */
> +#define MCP795_OSCRUN_BIT	BIT(5)  /* Oscillator Run */
> +#define MCP795_PWRFAIL_BIT	BIT(4)  /* Power Fail */
> +#define MCP795_VBATEN_BIT	BIT(3)  /* VBAT Enable */
> +
> +/* RTCC 0x08 register bits */
> +#define MCP795_SQWEN_BIT	BIT(6)  /* Square Wave Enable (MFP pin) */
> +#define MCP795_ALM1_EN_BIT	BIT(5)  /* Alarm 1 Enable */
> +#define MCP795_ALM0_EN_BIT	BIT(4)  /* Alarm 0 Enable */
> +#define MCP795_EXTOSC_BIT	BIT(3)	/* External Oscillator */
> +#define MCP795_CRSTRIM_BIT	BIT(2)  /* Crystal Trim */
> +#define MCP795_SQWFS_BIT_H	BIT(1)  /* Square Wave Frequency Select H */
> +#define MCP795_SQWFS_BIT_L	BIT(0)  /* Square Wave Frequency Select L */
> +
> +/* Alarms (0 and 1) */
> +
> +/* Centi-seconds - ALARM 1 ONLY */
> +/* Alarms BCD Decimal Values of Centi-Second's Tens Digit (=deci-seconds) */
> +#define MCP795_ALM1_CSEC_T_C0_BIT	BIT(4)
> +#define MCP795_ALM1_CSEC_T_C1_BIT	BIT(5)
> +#define MCP795_ALM1_CSEC_T_C2_BIT	BIT(6)
> +#define MCP795_ALM1_CSEC_T_C3_BIT	BIT(6)
> +/* Alarms BCD Decimal Values of Centi-Second's Ones Digit (=centi-seconds) */
> +#define MCP795_ALM1_CSEC_O_C0_BIT	BIT(0)
> +#define MCP795_ALM1_CSEC_O_C1_BIT	BIT(1)
> +#define MCP795_ALM1_CSEC_O_C2_BIT	BIT(2)
> +#define MCP795_ALM1_CSEC_O_C3_BIT	BIT(3)
> +
> +/* Seconds */
> +/* Alarms BCD Decimal Values of Second's Tens Digit */
> +#define MCP795_ALM_SEC_T_C0_BIT	BIT(4)
> +#define MCP795_ALM_SEC_T_C1_BIT	BIT(5)
> +#define MCP795_ALM_SEC_T_C2_BIT	BIT(6)
> +/* Alarms BCD Decimal Values of Second's Ones Digit */
> +#define MCP795_ALM_SEC_O_C0_BIT	BIT(0)
> +#define MCP795_ALM_SEC_O_C1_BIT	BIT(1)
> +#define MCP795_ALM_SEC_O_C2_BIT	BIT(2)
> +#define MCP795_ALM_SEC_O_C3_BIT	BIT(3)
> +
> +/* Minutes */
> +/* Alarms BCD Decimal Values of Minute's Tens Digit */
> +#define MCP795_ALM_MIN_T_C0_BIT	BIT(4)
> +#define MCP795_ALM_MIN_T_C1_BIT	BIT(5)
> +#define MCP795_ALM_MIN_T_C2_BIT	BIT(6)
> +/* Alarms BCD Decimal Values of Minute's Ones Digit */
> +#define MCP795_ALM_MIN_O_C0_BIT	BIT(0)
> +#define MCP795_ALM_MIN_O_C1_BIT	BIT(1)
> +#define MCP795_ALM_MIN_O_C2_BIT	BIT(2)
> +#define MCP795_ALM_MIN_O_C3_BIT	BIT(3)
> +
> +/* Hours */
> +/* Alarms BCD Decimal Values of Hour's Tens Digit */
> +#define MCP795_ALM_HOUR_T_C0_BIT	BIT(4)
> +/* Alarms BCD Decimal Values of Hour's Ones Digit */
> +#define MCP795_ALM_HOUR_O_C0_BIT	BIT(0)
> +#define MCP795_ALM_HOUR_O_C1_BIT	BIT(1)
> +#define MCP795_ALM_HOUR_O_C2_BIT	BIT(2)
> +#define MCP795_ALM_HOUR_O_C3_BIT	BIT(3)
> +
> +/* Days */
> +/* Alarms Interrupt Flag */
> +#define MCP795_ALM_IF_BIT	BIT(3)
> +/* Alarms BCD Decimal Values of Day's Ones Digit */
> +#define MCP795_ALM_DAY_O_C0_BIT	BIT(0)
> +#define MCP795_ALM_DAY_O_C1_BIT	BIT(1)
> +#define MCP795_ALM_DAY_O_C2_BIT	BIT(2)
> +
> +/* Date */
> +/* Alarms BCD Decimal Values of Date's Tens Digit */
> +#define MCP795_ALM_DATE_T_C0_BIT	BIT(4)
> +#define MCP795_ALM_DATE_T_C1_BIT	BIT(5)
> +/* Alarms BCD Decimal Values of Date's Ones Digit */
> +#define MCP795_ALM_DATE_O_C0_BIT	BIT(0)
> +#define MCP795_ALM_DATE_O_C1_BIT	BIT(1)
> +#define MCP795_ALM_DATE_O_C2_BIT	BIT(2)
> +#define MCP795_ALM_DATE_O_C3_BIT	BIT(3)
> +
> +/* Months */
> +/* Alarms BCD Decimal Values of Month's Tens Digit */
> +#define MCP795_ALM0_MONTH_T_C0_BIT	BIT(3)
> +/* Alarms BCD Decimal Values of Month's Ones Digit */
> +#define MCP795_ALM0_MONTH_O_C0_BIT	BIT(0)
> +#define MCP795_ALM0_MONTH_O_C1_BIT	BIT(1)
> +#define MCP795_ALM0_MONTH_O_C2_BIT	BIT(2)
> +#define MCP795_ALM0_MONTH_O_C3_BIT	BIT(3)
> +
> +/* Alarms Masks */
> +#define MCP795_ALM_MASKS_BIT	BIT(4)
> +
> +/* BCD mask values for alarm matching - need BCD conversion */
> +#define MCP795_ALM_MASK_SEC	0x00
> +#define MCP795_ALM_MASK_MIN	0x01
> +#define MCP795_ALM_MASK_HOUR	0x02
> +#define MCP795_ALM_MASK_DAY	0x03
> +#define MCP795_ALM_MASK_DATE	0x04
> +#define MCP795_ALM_MASK_CSEC	0x05
> +#define MCP795_ALM_MASK_ALL	0x07

You have a bunch of unused defines, please remove them

>  
>  #define SEC_PER_DAY		(24 * 60 * 60)
>  
> @@ -69,7 +193,7 @@ static int mcp795_rtcc_read(struct device *dev, u8 addr, u8 *buf, u8 count)
>  
>  	if (ret)
>  		dev_err(dev, "Failed reading %d bytes from address %x.\n",
> -					count, addr);
> +			count, addr);

Unnecessary change

>  
>  	return ret;
>  }
> @@ -88,7 +212,7 @@ static int mcp795_rtcc_write(struct device *dev, u8 addr, u8 *data, u8 count)
>  
>  	if (ret)
>  		dev_err(dev, "Failed to write %d bytes to address %x.\n",
> -					count, addr);
> +			count, addr);
>  

Unnecessary change

>  	return ret;
>  }
> @@ -123,17 +247,16 @@ static int mcp795_stop_oscillator(struct device *dev, bool *extosc)
>  	if (ret)
>  		return ret;
>  	*extosc = !!(data & MCP795_EXTOSC_BIT);
> -	ret = mcp795_rtcc_set_bits(
> -				dev, MCP795_REG_CONTROL, MCP795_EXTOSC_BIT, 0);
> +	ret = mcp795_rtcc_set_bits(dev, MCP795_REG_CONTROL, MCP795_EXTOSC_BIT, 0);

ditto

>  	if (ret)
>  		return ret;
> -	/* wait for the OSCON bit to clear */
> +	/* wait for the OSCRUN bit to clear */
>  	do {
>  		usleep_range(700, 800);
>  		ret = mcp795_rtcc_read(dev, MCP795_REG_DAY, &data, 1);
>  		if (ret)
>  			break;
> -		if (!(data & MCP795_OSCON_BIT))
> +		if (!(data & MCP795_OSCRUN_BIT))

I'm not really in favour of changing a define name but do so in a
separate patch.

>  			break;
>  
>  	} while (--retries);
> @@ -143,17 +266,34 @@ static int mcp795_stop_oscillator(struct device *dev, bool *extosc)
>  
>  static int mcp795_start_oscillator(struct device *dev, bool *extosc)
>  {
> +	int ret;
> +	int retries = 5;
> +
>  	if (extosc) {
>  		u8 data = *extosc ? MCP795_EXTOSC_BIT : 0;
> -		int ret;
>  
> -		ret = mcp795_rtcc_set_bits(
> -			dev, MCP795_REG_CONTROL, MCP795_EXTOSC_BIT, data);
> +		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_CONTROL, MCP795_EXTOSC_BIT, data);

Unnecessary change

>  		if (ret)
>  			return ret;
>  	}
> -	return mcp795_rtcc_set_bits(
> -			dev, MCP795_REG_SECONDS, MCP795_ST_BIT, MCP795_ST_BIT);
> +	ret = mcp795_rtcc_set_bits(dev, MCP795_REG_SECONDS, MCP795_ST_BIT, MCP795_ST_BIT);
> +	if (ret)
> +		return ret;
> +
> +	/* wait for the OSCRUN bit to set */

This comment is not really useful

> +	do {
> +		u8 data = MCP795_OSCRUN_BIT;
> +
> +		usleep_range(700, 800);

but this needs to be documented


> +		ret = mcp795_rtcc_read(dev, MCP795_REG_DAY, &data, 1);
> +		if (ret)
> +			break;
> +		if ((data & MCP795_OSCRUN_BIT))
> +			break;
> +
> +	} while (--retries);

Then number of retires too. Maybe it would better to have a timeout
instead?

> +
> +	return !retries ? -EIO : ret;
>  }
>  
>  /* Enable or disable Alarm 0 in RTC */
> @@ -164,18 +304,24 @@ static int mcp795_update_alarm(struct device *dev, bool enable)
>  	dev_dbg(dev, "%s alarm\n", enable ? "Enable" : "Disable");
>  
>  	if (enable) {
> +		/* Disable Square Wave Function to enable Alarm output */
> +		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_CONTROL,
> +					   MCP795_SQWEN_BIT, 0);

ret is not checked here?

Also, the SQW may be useful to calibrate the oscillator, this
unconditionally breaks that feature

>  		/* clear ALM0IF (Alarm 0 Interrupt Flag) bit */
>  		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_ALM0_DAY,
> -					MCP795_ALM0IF_BIT, 0);
> +					   MCP795_ALM_IF_BIT, 0);

This renaming also seems unnecessary and makes reading the patch more
complicated

>  		if (ret)
>  			return ret;
>  		/* enable alarm 0 */
>  		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_CONTROL,
> -					MCP795_ALM0_BIT, MCP795_ALM0_BIT);
> +					   MCP795_ALM0_EN_BIT, MCP795_ALM0_EN_BIT);
>  	} else {
>  		/* disable alarm 0 and alarm 1 */
>  		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_CONTROL,
> -					MCP795_ALM0_BIT | MCP795_ALM1_BIT, 0);
> +					   MCP795_ALM0_EN_BIT | MCP795_ALM1_EN_BIT, 0);
> +		/* Disable Alarm output to let enable Square Wave Function */
> +		ret = mcp795_rtcc_set_bits(dev, MCP795_REG_CONTROL,
> +					   MCP795_SQWEN_BIT, 0);

The code doesn't do what the comment says and so the comment is useless

>  	}
>  	return ret;
>  }
> @@ -183,7 +329,7 @@ static int mcp795_update_alarm(struct device *dev, bool enable)
>  static int mcp795_set_time(struct device *dev, struct rtc_time *tim)
>  {
>  	int ret;
> -	u8 data[7];
> +	u8 data[8];
>  	bool extosc;
>  
>  	/* Stop RTC and store current value of EXTOSC bit */
> @@ -192,32 +338,42 @@ static int mcp795_set_time(struct device *dev, struct rtc_time *tim)
>  		return ret;
>  
>  	/* Read first, so we can leave config bits untouched */
> -	ret = mcp795_rtcc_read(dev, MCP795_REG_SECONDS, data, sizeof(data));
> +	ret = mcp795_rtcc_read(dev, MCP795_REG_TMSECONDS, data, sizeof(data));

Why do you read centiseconds here?

>  
>  	if (ret)
>  		return ret;
>  
> -	data[0] = (data[0] & 0x80) | bin2bcd(tim->tm_sec);
> -	data[1] = (data[1] & 0x80) | bin2bcd(tim->tm_min);
> -	data[2] = bin2bcd(tim->tm_hour);
> -	data[3] = (data[3] & 0xF8) | bin2bcd(tim->tm_wday + 1);
> -	data[4] = bin2bcd(tim->tm_mday);
> -	data[5] = (data[5] & MCP795_LP_BIT) | bin2bcd(tim->tm_mon + 1);
> -
> -	if (tim->tm_year > 100)
> -		tim->tm_year -= 100;
> -
> -	data[6] = bin2bcd(tim->tm_year);
> -
> -	/* Always write the date and month using a separate Write command.
> -	 * This is a workaround for a know silicon issue that some combinations
> -	 * of date and month values may result in the date being reset to 1.
> -	 */
> -	ret = mcp795_rtcc_write(dev, MCP795_REG_SECONDS, data, 5);
> +	data[0] = 0x00;	//centiseconds not implemented
> +	data[1] = (data[0] & 0x80) | bin2bcd(tim->tm_sec);
I think one of the index is not correct
> +	data[2] = (data[1] & 0x80) | bin2bcd(tim->tm_min);
ditto
> +	data[3] = (data[3] & 0xC0) | bin2bcd(tim->tm_hour);
> +	/* Add one beacause Linux weekday is from 0 to 6 */

I don't think this comment is useful

> +	data[4] = (data[3] & 0xF8) | bin2bcd(tim->tm_wday + 1);
> +	data[5] = (data[5] & 0xC0) | bin2bcd(tim->tm_mday);
> +	/* Add one because Linux month is from 0 to 11 */

ditto

> +	data[6] = (data[6] & 0xC0) | ((data[6] & MCP795_LP_BIT)
> +				| bin2bcd(tim->tm_mon + 1));
> +	data[7] = bin2bcd(tim->tm_year);
> +
> +	/* Set time sequence needed to avoid common device problems, see errata */
> +	/* Write month */
> +	ret = mcp795_rtcc_write(dev, MCP795_REG_MONTH, &data[6], 1);
>  	if (ret)
>  		return ret;
> -
> -	ret = mcp795_rtcc_write(dev, MCP795_REG_MONTH, &data[5], 2);
> +	/* Write data (month day) */
> +	ret = mcp795_rtcc_write(dev, MCP795_REG_DATE, &data[5], 1);
> +	if (ret)
> +		return ret;
> +	/* Write month again */
> +	ret = mcp795_rtcc_write(dev, MCP795_REG_MONTH, &data[6], 1);
> +	if (ret)
> +		return ret;
> +	/* Send the rest apart from year and dayweek */
> +	ret = mcp795_rtcc_write(dev, MCP795_REG_TMSECONDS, data, 4);
> +	if (ret)
> +		return ret;
> +	/* Send the year */
> +	ret = mcp795_rtcc_write(dev, MCP795_REG_YEAR, &data[7], 1);
>  	if (ret)
>  		return ret;
>  
> @@ -229,6 +385,14 @@ static int mcp795_set_time(struct device *dev, struct rtc_time *tim)
>  	if (ret)
>  		return ret;
>  
> +	/* Always set the weekday using a separate Write command and when
> +	 * oscillator is running.
> +	 * This is a workaround for a know silicon issue.
> +	 */
> +	ret = mcp795_rtcc_write(dev, MCP795_REG_DAY, &data[4], 1);
> +	if (ret)
> +		return ret;
> +
>  	dev_dbg(dev, "Set mcp795: %ptR\n", tim);
>  
>  	return 0;
> @@ -237,20 +401,23 @@ static int mcp795_set_time(struct device *dev, struct rtc_time *tim)
>  static int mcp795_read_time(struct device *dev, struct rtc_time *tim)
>  {
>  	int ret;
> -	u8 data[7];
> +	u8 data[8];
> +	int tm_c_sec;
>  
> -	ret = mcp795_rtcc_read(dev, MCP795_REG_SECONDS, data, sizeof(data));
> +	ret = mcp795_rtcc_read(dev, MCP795_REG_TMSECONDS, data, sizeof(data));
>  
>  	if (ret)
>  		return ret;
>  
> -	tim->tm_sec	= bcd2bin(data[0] & 0x7F);
> -	tim->tm_min	= bcd2bin(data[1] & 0x7F);
> -	tim->tm_hour	= bcd2bin(data[2] & 0x3F);
> -	tim->tm_wday	= bcd2bin(data[3] & 0x07) - 1;
> -	tim->tm_mday	= bcd2bin(data[4] & 0x3F);
> -	tim->tm_mon	= bcd2bin(data[5] & 0x1F) - 1;
> -	tim->tm_year	= bcd2bin(data[6]) + 100; /* Assume we are in 20xx */
> +	tm_c_sec	= bcd2bin(data[0] & 0xFF);
> +	tim->tm_sec	= bcd2bin(data[1] & 0x7F);
> +	tim->tm_min	= bcd2bin(data[2] & 0x7F);
> +	/* This is only for 24h mode, see datasheet for 12h mode */
> +	tim->tm_hour	= bcd2bin(data[3] & 0x3F);
> +	tim->tm_wday	= bcd2bin(data[4] & 0x07) - 1;
> +	tim->tm_mday	= bcd2bin(data[5] & 0x3F);
> +	tim->tm_mon	= bcd2bin(data[6] & 0x1F) - 1;
> +	tim->tm_year	= bcd2bin(data[7]);
>  
>  	dev_dbg(dev, "Read from mcp795: %ptR\n", tim);
>  
> @@ -265,6 +432,17 @@ static int mcp795_set_alarm(struct device *dev, struct rtc_wkalrm *alm)
>  	u8 tmp[6];
>  	int ret;
>  
> +	/* Set Alarm 0 Mask to match all */
> +	/* Read first, so we can leave other bits untouched */
> +	ret = mcp795_rtcc_read(dev, MCP795_REG_ALM0_DAY, tmp, 1);
> +	if (ret)
> +		return ret;
> +	/* Set Alarm 0 Mask to match all */
> +	tmp[0] = (tmp[0] & 0x8F) | (bin2bcd(MCP795_ALM_MASK_ALL) << 4);

If possible, I wouldn't match all and simply drop weekday, there is no
guarantee that userspace will provide a valid weekday and you could then
drop the previous workaround while setting weekday.

> +	ret = mcp795_rtcc_write(dev, MCP795_REG_ALM0_DAY, tmp, 1);
> +	if (ret)
> +		return ret;
> +
>  	/* Read current time from RTC hardware */
>  	ret = mcp795_read_time(dev, &now_tm);
>  	if (ret)
> @@ -291,12 +469,14 @@ static int mcp795_set_alarm(struct device *dev, struct rtc_wkalrm *alm)
>  	alm->time.tm_isdst	= -1;
>  	alm->time.tm_yday	= -1;
>  
> +	/* set alarm match: seconds, minutes, hour, day, date and month */
>  	tmp[0] = (tmp[0] & 0x80) | bin2bcd(alm->time.tm_sec);
>  	tmp[1] = (tmp[1] & 0x80) | bin2bcd(alm->time.tm_min);
>  	tmp[2] = (tmp[2] & 0xE0) | bin2bcd(alm->time.tm_hour);
>  	tmp[3] = (tmp[3] & 0x80) | bin2bcd(alm->time.tm_wday + 1);
> -	/* set alarm match: seconds, minutes, hour, day, date and month */
> -	tmp[3] |= (MCP795_ALM0C2_BIT | MCP795_ALM0C1_BIT | MCP795_ALM0C0_BIT);
> +	/* ALMxIF cleared in the update function */
> +	tmp[3] |= (MCP795_ALM_SEC_T_C2_BIT | MCP795_ALM_SEC_T_C1_BIT
> +				| MCP795_ALM_SEC_T_C0_BIT);
>  	tmp[4] = (tmp[4] & 0xC0) | bin2bcd(alm->time.tm_mday);
>  	tmp[5] = (tmp[5] & 0xE0) | bin2bcd(alm->time.tm_mon + 1);
>  
> @@ -309,10 +489,14 @@ static int mcp795_set_alarm(struct device *dev, struct rtc_wkalrm *alm)
>  		ret = mcp795_update_alarm(dev, true);
>  		if (ret)
>  			return ret;
> +		alm->pending = 1;

Why is that needed?

>  		dev_dbg(dev, "Alarm IRQ armed\n");
>  	}
> -	dev_dbg(dev, "Set alarm: %ptRdr(%d) %ptRt\n",
> -		&alm->time, alm->time.tm_wday, &alm->time);
> +
> +	/* do not print year, it's not implemented in the RTC hardware */
> +	dev_dbg(dev, "Set alarm: %02d-%02dT%02d:%02d:%02d\n", alm->time.tm_mon + 1,
> +		alm->time.tm_mday, alm->time.tm_hour, alm->time.tm_min, alm->time.tm_sec);
> +

This is a debug print, I don't think it is an issue to print year
anyway.

>  	return 0;
>  }
>  
> @@ -321,8 +505,7 @@ static int mcp795_read_alarm(struct device *dev, struct rtc_wkalrm *alm)
>  	u8 data[6];
>  	int ret;
>  
> -	ret = mcp795_rtcc_read(
> -			dev, MCP795_REG_ALM0_SECONDS, data, sizeof(data));
> +	ret = mcp795_rtcc_read(dev, MCP795_REG_ALM0_SECONDS, data, sizeof(data));

Unnecessary change

>  	if (ret)
>  		return ret;
>  
> @@ -336,8 +519,17 @@ static int mcp795_read_alarm(struct device *dev, struct rtc_wkalrm *alm)
>  	alm->time.tm_isdst	= -1;
>  	alm->time.tm_yday	= -1;
>  
> -	dev_dbg(dev, "Read alarm: %ptRdr(%d) %ptRt\n",
> -		&alm->time, alm->time.tm_wday, &alm->time);
> +	alm->pending = !!(data[3] & MCP795_ALM_IF_BIT);
> +
> +	ret = mcp795_rtcc_read(dev, MCP795_REG_CONTROL, data, 1);
> +	if (ret)
> +		return ret;
> +	alm->enabled = !!(data[0] & MCP795_ALM0_EN_BIT);
> +
> +	/* Do not print year, it's not implemented in the RTC hardware */
> +	dev_dbg(dev, "Read alarm: %02d-%02dT%02d:%02d:%02d\n", alm->time.tm_mon + 1,
> +		alm->time.tm_mday, alm->time.tm_hour, alm->time.tm_min, alm->time.tm_sec);
> +
>  	return 0;
>  }
>  
> @@ -381,6 +573,7 @@ static int mcp795_probe(struct spi_device *spi)
>  {
>  	struct rtc_device *rtc;
>  	int ret;
> +	u8 data;
>  
>  	spi->mode = SPI_MODE_0;
>  	spi->bits_per_word = 8;
> @@ -393,10 +586,10 @@ static int mcp795_probe(struct spi_device *spi)
>  	/* Start the oscillator but don't set the value of EXTOSC bit */
>  	mcp795_start_oscillator(&spi->dev, NULL);
>  	/* Clear the 12 hour mode flag*/
> -	mcp795_rtcc_set_bits(&spi->dev, 0x03, MCP795_24_BIT, 0);
> +	mcp795_rtcc_set_bits(&spi->dev, MCP795_REG_HOURS, MCP795_24_BIT, 0);
>  
>  	rtc = devm_rtc_device_register(&spi->dev, "rtc-mcp795",
> -					&mcp795_rtc_ops, THIS_MODULE);
> +				       &mcp795_rtc_ops, THIS_MODULE);

ditto

>  	if (IS_ERR(rtc))
>  		return PTR_ERR(rtc);
>  
> @@ -409,22 +602,40 @@ static int mcp795_probe(struct spi_device *spi)
>  		 * the interrupt.
>  		 */
>  		mcp795_rtcc_set_bits(&spi->dev, MCP795_REG_ALM0_DAY,
> -					MCP795_ALM0IF_BIT, 0);
> -		ret = devm_request_threaded_irq(&spi->dev, spi->irq, NULL,
> -				mcp795_irq, IRQF_TRIGGER_FALLING | IRQF_ONESHOT,
> -				dev_name(&rtc->dev), spi);
> +				     MCP795_ALM_IF_BIT, 0);
> +		ret = devm_request_threaded_irq(&spi->dev, spi->irq, NULL, mcp795_irq,
> +						IRQF_TRIGGER_FALLING | IRQF_ONESHOT,
> +						dev_name(&rtc->dev), spi);

ditto

>  		if (ret)
>  			dev_err(&spi->dev, "Failed to request IRQ: %d: %d\n",
> -						spi->irq, ret);
> +				spi->irq, ret);

ditto

>  		else
>  			device_init_wakeup(&spi->dev, true);
>  	}
> +
> +	/* If PWRFAIL BIT is set, it must be cleared */
> +	ret = mcp795_rtcc_read(&spi->dev, MCP795_REG_DAY, &data, 1);
> +	if (ret)
> +		return ret;
> +
> +	if (!!(data & MCP795_PWRFAIL_BIT)) {
> +		ret = mcp795_rtcc_set_bits(&spi->dev, MCP795_REG_DAY,
> +					   MCP795_PWRFAIL_BIT, 0);
> +		if (ret)
> +			return ret;

If this indicates that the time/date is invalid, then it must not be
cleared in the probe and instead in set_time also this information has
to be used in read_time

> +	}
> +	/* Set VBAT flag to 1 */
> +	ret = mcp795_rtcc_set_bits(&spi->dev, MCP795_REG_DAY, MCP795_VBATEN_BIT,
> +				   MCP795_VBATEN_BIT);

There is a proper interface to set the backup mode, please use it

> +	if (ret)
> +		return ret;
> +
>  	return 0;
>  }
>  
>  #ifdef CONFIG_OF
>  static const struct of_device_id mcp795_of_match[] = {
> -	{ .compatible = "maxim,mcp795" },
> +	{ .compatible = "microchip,mcp795" },
>  	{ }


You should really slice this up in separate changes to make reviewing
easier.

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

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