linux-imx

/*

 * 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/mfd/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);