text: // SPDX-License-Identifier: GPL-2.0+
  /*
   * rtc-dm355evm.c - access battery-backed counter in MSP430 firmware
   *
   * Copyright (c) 2008 by David Brownell
   */
  #include <linux/kernel.h>
  #include <linux/init.h>
  #include <linux/rtc.h>
  #include <linux/platform_device.h>
  
  #include <linux/mfd/dm355evm_msp.h>
  #include <linux/module.h>
  
  
  /*
   * The MSP430 firmware on the DM355 EVM uses a watch crystal to feed
   * a 1 Hz counter.  When a backup battery is supplied, that makes a
   * reasonable RTC for applications where alarms and non-NTP drift
   * compensation aren't important.
   *
   * The only real glitch is the inability to read or write all four
   * counter bytes atomically:  the count may increment in the middle
   * of an operation, causing trouble when the LSB rolls over.
   *
   * This driver was tested with firmware revision A4.
   */
  union evm_time {
  	u8	bytes[4];
  	u32	value;
  };
  
  static int dm355evm_rtc_read_time(struct device *dev, struct rtc_time *tm)
  {
  	union evm_time	time;
  	int		status;
  	int		tries = 0;
  
  	do {
  		/*
  		 * Read LSB(0) to MSB(3) bytes.  Defend against the counter
  		 * rolling over by re-reading until the value is stable,
  		 * and assuming the four reads take at most a few seconds.
  		 */
  		status = dm355evm_msp_read(DM355EVM_MSP_RTC_0);
  		if (status < 0)
  			return status;
  		if (tries && time.bytes[0] == status)
  			break;
  		time.bytes[0] = status;
  
  		status = dm355evm_msp_read(DM355EVM_MSP_RTC_1);
  		if (status < 0)
  			return status;
  		if (tries && time.bytes[1] == status)
  			break;
  		time.bytes[1] = status;
  
  		status = dm355evm_msp_read(DM355EVM_MSP_RTC_2);
  		if (status < 0)
  			return status;
  		if (tries && time.bytes[2] == status)
  			break;
  		time.bytes[2] = status;
  
  		status = dm355evm_msp_read(DM355EVM_MSP_RTC_3);
  		if (status < 0)
  			return status;
  		if (tries && time.bytes[3] == status)
  			break;
  		time.bytes[3] = status;
  
  	} while (++tries < 5);
  
  	dev_dbg(dev, "read timestamp %08x\n", time.value);
  
  	rtc_time64_to_tm(le32_to_cpu(time.value), tm);
  	return 0;
  }
  
  static int dm355evm_rtc_set_time(struct device *dev, struct rtc_time *tm)
  {
  	union evm_time	time;
  	unsigned long	value;
  	int		status;
  
  	value = rtc_tm_to_time64(tm);
  	time.value = cpu_to_le32(value);
  
  	dev_dbg(dev, "write timestamp %08x\n", time.value);
  
  	/*
  	 * REVISIT handle non-atomic writes ... maybe just retry until
  	 * byte[1] sticks (no rollover)?
  	 */
  	status = dm355evm_msp_write(time.bytes[0], DM355EVM_MSP_RTC_0);
  	if (status < 0)
  		return status;
  
  	status = dm355evm_msp_write(time.bytes[1], DM355EVM_MSP_RTC_1);
  	if (status < 0)
  		return status;
  
  	status = dm355evm_msp_write(time.bytes[2], DM355EVM_MSP_RTC_2);
  	if (status < 0)
  		return status;
  
  	status = dm355evm_msp_write(time.bytes[3], DM355EVM_MSP_RTC_3);
  	if (status < 0)
  		return status;
  
  	return 0;
  }
  
  static const struct rtc_class_ops dm355evm_rtc_ops = {
  	.read_time	= dm355evm_rtc_read_time,
  	.set_time	= dm355evm_rtc_set_time,
  };
  
  /*----------------------------------------------------------------------*/
  
  static int dm355evm_rtc_probe(struct platform_device *pdev)
  {
  	struct rtc_device *rtc;
  
  	rtc = devm_rtc_allocate_device(&pdev->dev);
  	if (IS_ERR(rtc))
  		return PTR_ERR(rtc);
  
  	platform_set_drvdata(pdev, rtc);
  
  	rtc->ops = &dm355evm_rtc_ops;
  	rtc->range_max = U32_MAX;
  
  	return rtc_register_device(rtc);
  }
  
  /*
   * I2C is used to talk to the MSP430, but this platform device is
   * exposed by an MFD driver that manages I2C communications.
   */
  static struct platform_driver rtc_dm355evm_driver = {
  	.probe		= dm355evm_rtc_probe,
  	.driver		= {
  		.name	= "rtc-dm355evm",
  	},
  };
  
  module_platform_driver(rtc_dm355evm_driver);
  
  MODULE_LICENSE("GPL");
  