drivers/rtc/rtc-dm355evm.c

   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
  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
  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
 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");