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  */
28 union evm_time {
29 	u8	bytes[4];
30 	u32	value;
31 };
32 
dm355evm_rtc_read_time(struct device * dev,struct rtc_time * tm)33 static 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 
dm355evm_rtc_set_time(struct device * dev,struct rtc_time * tm)81 static 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 
115 static 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 
dm355evm_rtc_probe(struct platform_device * pdev)122 static 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 devm_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  */
142 static struct platform_driver rtc_dm355evm_driver = {
143 	.probe		= dm355evm_rtc_probe,
144 	.driver		= {
145 		.name	= "rtc-dm355evm",
146 	},
147 };
148 
149 module_platform_driver(rtc_dm355evm_driver);
150 
151 MODULE_LICENSE("GPL");
152