1/*
2 * rtc-dm355evm.c - access battery-backed counter in MSP430 firmware
3 *
4 * Copyright (c) 2008 by David Brownell
5 *
6 * This program is free software; you can redistribute it and/or
7 * modify it under the terms of the GNU General Public License
8 * as published by the Free Software Foundation; either version
9 * 2 of the License, or (at your option) any later version.
10 */
11#include <linux/kernel.h>
12#include <linux/init.h>
13#include <linux/rtc.h>
14#include <linux/platform_device.h>
15
16#include <linux/i2c/dm355evm_msp.h>
17#include <linux/module.h>
18
19
20/*
21 * The MSP430 firmware on the DM355 EVM uses a watch crystal to feed
22 * a 1 Hz counter.  When a backup battery is supplied, that makes a
23 * reasonable RTC for applications where alarms and non-NTP drift
24 * compensation aren't important.
25 *
26 * The only real glitch is the inability to read or write all four
27 * counter bytes atomically:  the count may increment in the middle
28 * of an operation, causing trouble when the LSB rolls over.
29 *
30 * This driver was tested with firmware revision A4.
31 */
32union evm_time {
33	u8	bytes[4];
34	u32	value;
35};
36
37static int dm355evm_rtc_read_time(struct device *dev, struct rtc_time *tm)
38{
39	union evm_time	time;
40	int		status;
41	int		tries = 0;
42
43	do {
44		/*
45		 * Read LSB(0) to MSB(3) bytes.  Defend against the counter
46		 * rolling over by re-reading until the value is stable,
47		 * and assuming the four reads take at most a few seconds.
48		 */
49		status = dm355evm_msp_read(DM355EVM_MSP_RTC_0);
50		if (status < 0)
51			return status;
52		if (tries && time.bytes[0] == status)
53			break;
54		time.bytes[0] = status;
55
56		status = dm355evm_msp_read(DM355EVM_MSP_RTC_1);
57		if (status < 0)
58			return status;
59		if (tries && time.bytes[1] == status)
60			break;
61		time.bytes[1] = status;
62
63		status = dm355evm_msp_read(DM355EVM_MSP_RTC_2);
64		if (status < 0)
65			return status;
66		if (tries && time.bytes[2] == status)
67			break;
68		time.bytes[2] = status;
69
70		status = dm355evm_msp_read(DM355EVM_MSP_RTC_3);
71		if (status < 0)
72			return status;
73		if (tries && time.bytes[3] == status)
74			break;
75		time.bytes[3] = status;
76
77	} while (++tries < 5);
78
79	dev_dbg(dev, "read timestamp %08x\n", time.value);
80
81	rtc_time_to_tm(le32_to_cpu(time.value), tm);
82	return 0;
83}
84
85static int dm355evm_rtc_set_time(struct device *dev, struct rtc_time *tm)
86{
87	union evm_time	time;
88	unsigned long	value;
89	int		status;
90
91	rtc_tm_to_time(tm, &value);
92	time.value = cpu_to_le32(value);
93
94	dev_dbg(dev, "write timestamp %08x\n", time.value);
95
96	/*
97	 * REVISIT handle non-atomic writes ... maybe just retry until
98	 * byte[1] sticks (no rollover)?
99	 */
100	status = dm355evm_msp_write(time.bytes[0], DM355EVM_MSP_RTC_0);
101	if (status < 0)
102		return status;
103
104	status = dm355evm_msp_write(time.bytes[1], DM355EVM_MSP_RTC_1);
105	if (status < 0)
106		return status;
107
108	status = dm355evm_msp_write(time.bytes[2], DM355EVM_MSP_RTC_2);
109	if (status < 0)
110		return status;
111
112	status = dm355evm_msp_write(time.bytes[3], DM355EVM_MSP_RTC_3);
113	if (status < 0)
114		return status;
115
116	return 0;
117}
118
119static struct rtc_class_ops dm355evm_rtc_ops = {
120	.read_time	= dm355evm_rtc_read_time,
121	.set_time	= dm355evm_rtc_set_time,
122};
123
124/*----------------------------------------------------------------------*/
125
126static int dm355evm_rtc_probe(struct platform_device *pdev)
127{
128	struct rtc_device *rtc;
129
130	rtc = devm_rtc_device_register(&pdev->dev, pdev->name,
131					&dm355evm_rtc_ops, THIS_MODULE);
132	if (IS_ERR(rtc)) {
133		dev_err(&pdev->dev, "can't register RTC device, err %ld\n",
134			PTR_ERR(rtc));
135		return PTR_ERR(rtc);
136	}
137	platform_set_drvdata(pdev, rtc);
138
139	return 0;
140}
141
142/*
143 * I2C is used to talk to the MSP430, but this platform device is
144 * exposed by an MFD driver that manages I2C communications.
145 */
146static struct platform_driver rtc_dm355evm_driver = {
147	.probe		= dm355evm_rtc_probe,
148	.driver		= {
149		.name	= "rtc-dm355evm",
150	},
151};
152
153module_platform_driver(rtc_dm355evm_driver);
154
155MODULE_LICENSE("GPL");
156