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/mfd/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 */
32 union evm_time {
33 u8 bytes[4];
34 u32 value;
35 };
36
dm355evm_rtc_read_time(struct device * dev,struct rtc_time * tm)37 static 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
dm355evm_rtc_set_time(struct device * dev,struct rtc_time * tm)85 static 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
119 static const 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
dm355evm_rtc_probe(struct platform_device * pdev)126 static 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 */
146 static struct platform_driver rtc_dm355evm_driver = {
147 .probe = dm355evm_rtc_probe,
148 .driver = {
149 .name = "rtc-dm355evm",
150 },
151 };
152
153 module_platform_driver(rtc_dm355evm_driver);
154
155 MODULE_LICENSE("GPL");
156