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");
152