rtc-dm355evm.c
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/*
* rtc-dm355evm.c - access battery-backed counter in MSP430 firmware
*
* Copyright (c) 2008 by David Brownell
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version
* 2 of the License, or (at your option) any later version.
*/
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/rtc.h>
#include <linux/platform_device.h>
#include <linux/i2c/dm355evm_msp.h>
#include <linux/module.h>
/*
* The MSP430 firmware on the DM355 EVM uses a watch crystal to feed
* a 1 Hz counter. When a backup battery is supplied, that makes a
* reasonable RTC for applications where alarms and non-NTP drift
* compensation aren't important.
*
* The only real glitch is the inability to read or write all four
* counter bytes atomically: the count may increment in the middle
* of an operation, causing trouble when the LSB rolls over.
*
* This driver was tested with firmware revision A4.
*/
union evm_time {
u8 bytes[4];
u32 value;
};
static int dm355evm_rtc_read_time(struct device *dev, struct rtc_time *tm)
{
union evm_time time;
int status;
int tries = 0;
do {
/*
* Read LSB(0) to MSB(3) bytes. Defend against the counter
* rolling over by re-reading until the value is stable,
* and assuming the four reads take at most a few seconds.
*/
status = dm355evm_msp_read(DM355EVM_MSP_RTC_0);
if (status < 0)
return status;
if (tries && time.bytes[0] == status)
break;
time.bytes[0] = status;
status = dm355evm_msp_read(DM355EVM_MSP_RTC_1);
if (status < 0)
return status;
if (tries && time.bytes[1] == status)
break;
time.bytes[1] = status;
status = dm355evm_msp_read(DM355EVM_MSP_RTC_2);
if (status < 0)
return status;
if (tries && time.bytes[2] == status)
break;
time.bytes[2] = status;
status = dm355evm_msp_read(DM355EVM_MSP_RTC_3);
if (status < 0)
return status;
if (tries && time.bytes[3] == status)
break;
time.bytes[3] = status;
} while (++tries < 5);
dev_dbg(dev, "read timestamp %08x\n", time.value);
rtc_time_to_tm(le32_to_cpu(time.value), tm);
return 0;
}
static int dm355evm_rtc_set_time(struct device *dev, struct rtc_time *tm)
{
union evm_time time;
unsigned long value;
int status;
rtc_tm_to_time(tm, &value);
time.value = cpu_to_le32(value);
dev_dbg(dev, "write timestamp %08x\n", time.value);
/*
* REVISIT handle non-atomic writes ... maybe just retry until
* byte[1] sticks (no rollover)?
*/
status = dm355evm_msp_write(time.bytes[0], DM355EVM_MSP_RTC_0);
if (status < 0)
return status;
status = dm355evm_msp_write(time.bytes[1], DM355EVM_MSP_RTC_1);
if (status < 0)
return status;
status = dm355evm_msp_write(time.bytes[2], DM355EVM_MSP_RTC_2);
if (status < 0)
return status;
status = dm355evm_msp_write(time.bytes[3], DM355EVM_MSP_RTC_3);
if (status < 0)
return status;
return 0;
}
static struct rtc_class_ops dm355evm_rtc_ops = {
.read_time = dm355evm_rtc_read_time,
.set_time = dm355evm_rtc_set_time,
};
/*----------------------------------------------------------------------*/
static int dm355evm_rtc_probe(struct platform_device *pdev)
{
struct rtc_device *rtc;
rtc = devm_rtc_device_register(&pdev->dev, pdev->name,
&dm355evm_rtc_ops, THIS_MODULE);
if (IS_ERR(rtc)) {
dev_err(&pdev->dev, "can't register RTC device, err %ld\n",
PTR_ERR(rtc));
return PTR_ERR(rtc);
}
platform_set_drvdata(pdev, rtc);
return 0;
}
/*
* I2C is used to talk to the MSP430, but this platform device is
* exposed by an MFD driver that manages I2C communications.
*/
static struct platform_driver rtc_dm355evm_driver = {
.probe = dm355evm_rtc_probe,
.driver = {
.owner = THIS_MODULE,
.name = "rtc-dm355evm",
},
};
module_platform_driver(rtc_dm355evm_driver);
MODULE_LICENSE("GPL");