linux/drivers/rtc/rtc-dm355evm.c
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   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 __devinit dm355evm_rtc_probe(struct platform_device *pdev)
 127{
 128        struct rtc_device *rtc;
 129
 130        rtc = rtc_device_register(pdev->name,
 131                                  &pdev->dev, &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
 142static int __devexit dm355evm_rtc_remove(struct platform_device *pdev)
 143{
 144        struct rtc_device *rtc = platform_get_drvdata(pdev);
 145
 146        rtc_device_unregister(rtc);
 147        platform_set_drvdata(pdev, NULL);
 148        return 0;
 149}
 150
 151/*
 152 * I2C is used to talk to the MSP430, but this platform device is
 153 * exposed by an MFD driver that manages I2C communications.
 154 */
 155static struct platform_driver rtc_dm355evm_driver = {
 156        .probe          = dm355evm_rtc_probe,
 157        .remove         = __devexit_p(dm355evm_rtc_remove),
 158        .driver         = {
 159                .owner  = THIS_MODULE,
 160                .name   = "rtc-dm355evm",
 161        },
 162};
 163
 164module_platform_driver(rtc_dm355evm_driver);
 165
 166MODULE_LICENSE("GPL");
 167
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