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 __devinit dm355evm_rtc_probe(struct platform_device *pdev)
{
    struct rtc_device *rtc;

    rtc = rtc_device_register(pdev->name,
                  &pdev->dev, &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;
}

static int __devexit dm355evm_rtc_remove(struct platform_device *pdev)
{
    struct rtc_device *rtc = platform_get_drvdata(pdev);

    rtc_device_unregister(rtc);
    platform_set_drvdata(pdev, NULL);
    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,
    .remove     = __devexit_p(dm355evm_rtc_remove),
    .driver     = {
        .owner  = THIS_MODULE,
        .name   = "rtc-dm355evm",
    },
};

module_platform_driver(rtc_dm355evm_driver);

MODULE_LICENSE("GPL");