1// SPDX-License-Identifier: GPL-2.0+
  2/*
  3 * rtc-dm355evm.c - access battery-backed counter in MSP430 firmware
  4 *
  5 * Copyright (c) 2008 by David Brownell
  6 */
  7#include <linux/kernel.h>
  8#include <linux/init.h>
  9#include <linux/rtc.h>
 10#include <linux/platform_device.h>
 11
 12#include <linux/mfd/dm355evm_msp.h>
 13#include <linux/module.h>
 14
 15
 16/*
 17 * The MSP430 firmware on the DM355 EVM uses a watch crystal to feed
 18 * a 1 Hz counter.  When a backup battery is supplied, that makes a
 19 * reasonable RTC for applications where alarms and non-NTP drift
 20 * compensation aren't important.
 21 *
 22 * The only real glitch is the inability to read or write all four
 23 * counter bytes atomically:  the count may increment in the middle
 24 * of an operation, causing trouble when the LSB rolls over.
 25 *
 26 * This driver was tested with firmware revision A4.
 27 */
 28union evm_time {
 29	u8	bytes[4];
 30	u32	value;
 31};
 32
 33static int dm355evm_rtc_read_time(struct device *dev, struct rtc_time *tm)
 34{
 35	union evm_time	time;
 36	int		status;
 37	int		tries = 0;
 38
 39	do {
 40		/*
 41		 * Read LSB(0) to MSB(3) bytes.  Defend against the counter
 42		 * rolling over by re-reading until the value is stable,
 43		 * and assuming the four reads take at most a few seconds.
 44		 */
 45		status = dm355evm_msp_read(DM355EVM_MSP_RTC_0);
 46		if (status < 0)
 47			return status;
 48		if (tries && time.bytes[0] == status)
 49			break;
 50		time.bytes[0] = status;
 51
 52		status = dm355evm_msp_read(DM355EVM_MSP_RTC_1);
 53		if (status < 0)
 54			return status;
 55		if (tries && time.bytes[1] == status)
 56			break;
 57		time.bytes[1] = status;
 58
 59		status = dm355evm_msp_read(DM355EVM_MSP_RTC_2);
 60		if (status < 0)
 61			return status;
 62		if (tries && time.bytes[2] == status)
 63			break;
 64		time.bytes[2] = status;
 65
 66		status = dm355evm_msp_read(DM355EVM_MSP_RTC_3);
 67		if (status < 0)
 68			return status;
 69		if (tries && time.bytes[3] == status)
 70			break;
 71		time.bytes[3] = status;
 72
 73	} while (++tries < 5);
 74
 75	dev_dbg(dev, "read timestamp %08x\n", time.value);
 76
 77	rtc_time64_to_tm(le32_to_cpu(time.value), tm);
 78	return 0;
 79}
 80
 81static int dm355evm_rtc_set_time(struct device *dev, struct rtc_time *tm)
 82{
 83	union evm_time	time;
 84	unsigned long	value;
 85	int		status;
 86
 87	value = rtc_tm_to_time64(tm);
 88	time.value = cpu_to_le32(value);
 89
 90	dev_dbg(dev, "write timestamp %08x\n", time.value);
 91
 92	/*
 93	 * REVISIT handle non-atomic writes ... maybe just retry until
 94	 * byte[1] sticks (no rollover)?
 95	 */
 96	status = dm355evm_msp_write(time.bytes[0], DM355EVM_MSP_RTC_0);
 97	if (status < 0)
 98		return status;
 99
100	status = dm355evm_msp_write(time.bytes[1], DM355EVM_MSP_RTC_1);
101	if (status < 0)
102		return status;
103
104	status = dm355evm_msp_write(time.bytes[2], DM355EVM_MSP_RTC_2);
105	if (status < 0)
106		return status;
107
108	status = dm355evm_msp_write(time.bytes[3], DM355EVM_MSP_RTC_3);
109	if (status < 0)
110		return status;
111
112	return 0;
113}
114
115static const struct rtc_class_ops dm355evm_rtc_ops = {
116	.read_time	= dm355evm_rtc_read_time,
117	.set_time	= dm355evm_rtc_set_time,
118};
119
120/*----------------------------------------------------------------------*/
121
122static int dm355evm_rtc_probe(struct platform_device *pdev)
123{
124	struct rtc_device *rtc;
125
126	rtc = devm_rtc_allocate_device(&pdev->dev);
127	if (IS_ERR(rtc))
128		return PTR_ERR(rtc);
129
130	platform_set_drvdata(pdev, rtc);
131
132	rtc->ops = &dm355evm_rtc_ops;
133	rtc->range_max = U32_MAX;
134
135	return rtc_register_device(rtc);
136}
137
138/*
139 * I2C is used to talk to the MSP430, but this platform device is
140 * exposed by an MFD driver that manages I2C communications.
141 */
142static struct platform_driver rtc_dm355evm_driver = {
143	.probe		= dm355evm_rtc_probe,
144	.driver		= {
145		.name	= "rtc-dm355evm",
146	},
147};
148
149module_platform_driver(rtc_dm355evm_driver);
150
151MODULE_LICENSE("GPL");