rtc: ds1305/ds1306 driver

Support the Dallas/Maxim DS1305 and DS1306 RTC chips.  These use SPI, and
support alarms, NVRAM, and a trickle charger for use when their backup
power supply is a supercap or rechargeable cell.

This basic driver doesn't yet support suspend/resume or wakealarms.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 893 insertions, 0 deletions

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index beffb834c440..90ab73825401 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -282,6 +282,16 @@ config RTC_DRV_M41T94
           This driver can also be built as a module. If so, the module
           will be called rtc-m41t94.
 
+config RTC_DRV_DS1305
+        tristate "Dallas/Maxim DS1305/DS1306"
+        help
+          Select this driver to get support for the Dallas/Maxim DS1305
+          and DS1306 real time clock chips.  These support a trickle
+          charger, alarms, and NVRAM in addition to the clock.
+
+          This driver can also be built as a module. If so, the module
+          will be called rtc-ds1305.
+
 config RTC_DRV_MAX6902
         tristate "Maxim MAX6902"
         help
diff --git a/drivers/rtc/Makefile b/drivers/rtc/Makefile
index b0e1af54f800..18622ef84cab 100644
--- a/drivers/rtc/Makefile
+++ b/drivers/rtc/Makefile
@@ -24,6 +24,7 @@ obj-$(CONFIG_RTC_DRV_BFIN)	+= rtc-bfin.o
 obj-$(CONFIG_RTC_DRV_CMOS)      += rtc-cmos.o
 obj-$(CONFIG_RTC_DRV_DS1216)    += rtc-ds1216.o
 obj-$(CONFIG_RTC_DRV_DS1302)    += rtc-ds1302.o
+obj-$(CONFIG_RTC_DRV_DS1305)    += rtc-ds1305.o
 obj-$(CONFIG_RTC_DRV_DS1307)    += rtc-ds1307.o
 obj-$(CONFIG_RTC_DRV_DS1374)    += rtc-ds1374.o
 obj-$(CONFIG_RTC_DRV_DS1511)    += rtc-ds1511.o
diff --git a/drivers/rtc/rtc-ds1305.c b/drivers/rtc/rtc-ds1305.c
new file mode 100644
index 000000000000..b91d02a3ace9
--- /dev/null
+++ b/drivers/rtc/rtc-ds1305.c
@@ -0,0 +1,847 @@
+/*
+ * rtc-ds1305.c -- driver for DS1305 and DS1306 SPI RTC chips
+ *
+ * Copyright (C) 2008 David Brownell
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ */
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/bcd.h>
+#include <linux/rtc.h>
+#include <linux/workqueue.h>
+
+#include <linux/spi/spi.h>
+#include <linux/spi/ds1305.h>
+
+
+/*
+ * Registers ... mask DS1305_WRITE into register address to write,
+ * otherwise you're reading it.  All non-bitmask values are BCD.
+ */
+#define DS1305_WRITE            0x80
+
+
+/* RTC date/time ... the main special cases are that we:
+ *  - Need fancy "hours" encoding in 12hour mode
+ *  - Don't rely on the "day-of-week" field (or tm_wday)
+ *  - Are a 21st-century clock (2000 <= year < 2100)
+ */
+#define DS1305_RTC_LEN          7               /* bytes for RTC regs */
+
+#define DS1305_SEC              0x00            /* register addresses */
+#define DS1305_MIN              0x01
+#define DS1305_HOUR             0x02
+#       define DS1305_HR_12             0x40    /* set == 12 hr mode */
+#       define DS1305_HR_PM             0x20    /* set == PM (12hr mode) */
+#define DS1305_WDAY             0x03
+#define DS1305_MDAY             0x04
+#define DS1305_MON              0x05
+#define DS1305_YEAR             0x06
+
+
+/* The two alarms have only sec/min/hour/wday fields (ALM_LEN).
+ * DS1305_ALM_DISABLE disables a match field (some combos are bad).
+ *
+ * NOTE that since we don't use WDAY, we limit ourselves to alarms
+ * only one day into the future (vs potentially up to a week).
+ *
+ * NOTE ALSO that while we could generate once-a-second IRQs (UIE), we
+ * don't currently support them.  We'd either need to do it only when
+ * no alarm is pending (not the standard model), or to use the second
+ * alarm (implying that this is a DS1305 not DS1306, *and* that either
+ * it's wired up a second IRQ we know, or that INTCN is set)
+ */
+#define DS1305_ALM_LEN          4               /* bytes for ALM regs */
+#define DS1305_ALM_DISABLE      0x80
+
+#define DS1305_ALM0(r)          (0x07 + (r))    /* register addresses */
+#define DS1305_ALM1(r)          (0x0b + (r))
+
+
+/* three control registers */
+#define DS1305_CONTROL_LEN      3               /* bytes of control regs */
+
+#define DS1305_CONTROL          0x0f            /* register addresses */
+#       define DS1305_nEOSC             0x80    /* low enables oscillator */
+#       define DS1305_WP                0x40    /* write protect */
+#       define DS1305_INTCN             0x04    /* clear == only int0 used */
+#       define DS1306_1HZ               0x04    /* enable 1Hz output */
+#       define DS1305_AEI1              0x02    /* enable ALM1 IRQ */
+#       define DS1305_AEI0              0x01    /* enable ALM0 IRQ */
+#define DS1305_STATUS           0x10
+/* status has just AEIx bits, mirrored as IRQFx */
+#define DS1305_TRICKLE          0x11
+/* trickle bits are defined in <linux/spi/ds1305.h> */
+
+/* a bunch of NVRAM */
+#define DS1305_NVRAM_LEN        96              /* bytes of NVRAM */
+
+#define DS1305_NVRAM            0x20            /* register addresses */
+
+
+struct ds1305 {
+        struct spi_device       *spi;
+        struct rtc_device       *rtc;
+
+        struct work_struct      work;
+
+        unsigned long           flags;
+#define FLAG_EXITING    0
+
+        bool                    hr12;
+        u8                      ctrl[DS1305_CONTROL_LEN];
+};
+
+
+/*----------------------------------------------------------------------*/
+
+/*
+ * Utilities ...  tolerate 12-hour AM/PM notation in case of non-Linux
+ * software (like a bootloader) which may require it.
+ */
+
+static unsigned bcd2hour(u8 bcd)
+{
+        if (bcd & DS1305_HR_12) {
+                unsigned        hour = 0;
+
+                bcd &= ~DS1305_HR_12;
+                if (bcd & DS1305_HR_PM) {
+                        hour = 12;
+                        bcd &= ~DS1305_HR_PM;
+                }
+                hour += BCD2BIN(bcd);
+                return hour - 1;
+        }
+        return BCD2BIN(bcd);
+}
+
+static u8 hour2bcd(bool hr12, int hour)
+{
+        if (hr12) {
+                hour++;
+                if (hour <= 12)
+                        return DS1305_HR_12 | BIN2BCD(hour);
+                hour -= 12;
+                return DS1305_HR_12 | DS1305_HR_PM | BIN2BCD(hour);
+        }
+        return BIN2BCD(hour);
+}
+
+/*----------------------------------------------------------------------*/
+
+/*
+ * Interface to RTC framework
+ */
+
+#ifdef CONFIG_RTC_INTF_DEV
+
+/*
+ * Context: caller holds rtc->ops_lock (to protect ds1305->ctrl)
+ */
+static int ds1305_ioctl(struct device *dev, unsigned cmd, unsigned long arg)
+{
+        struct ds1305   *ds1305 = dev_get_drvdata(dev);
+        u8              buf[2];
+        int             status = -ENOIOCTLCMD;
+
+        buf[0] = DS1305_WRITE | DS1305_CONTROL;
+        buf[1] = ds1305->ctrl[0];
+
+        switch (cmd) {
+        case RTC_AIE_OFF:
+                status = 0;
+                if (!(buf[1] & DS1305_AEI0))
+                        goto done;
+                buf[1] &= ~DS1305_AEI0;
+                break;
+
+        case RTC_AIE_ON:
+                status = 0;
+                if (ds1305->ctrl[0] & DS1305_AEI0)
+                        goto done;
+                buf[1] |= DS1305_AEI0;
+                break;
+        }
+        if (status == 0) {
+                status = spi_write_then_read(ds1305->spi, buf, sizeof buf,
+                                NULL, 0);
+                if (status >= 0)
+                        ds1305->ctrl[0] = buf[1];
+        }
+
+done:
+        return status;
+}
+
+#else
+#define ds1305_ioctl    NULL
+#endif
+
+/*
+ * Get/set of date and time is pretty normal.
+ */
+
+static int ds1305_get_time(struct device *dev, struct rtc_time *time)
+{
+        struct ds1305   *ds1305 = dev_get_drvdata(dev);
+        u8              addr = DS1305_SEC;
+        u8              buf[DS1305_RTC_LEN];
+        int             status;
+
+        /* Use write-then-read to get all the date/time registers
+         * since dma from stack is nonportable
+         */
+        status = spi_write_then_read(ds1305->spi, &addr, sizeof addr,
+                        buf, sizeof buf);
+        if (status < 0)
+                return status;
+
+        dev_vdbg(dev, "%s: %02x %02x %02x, %02x %02x %02x %02x\n",
+                "read", buf[0], buf[1], buf[2], buf[3],
+                buf[4], buf[5], buf[6]);
+
+        /* Decode the registers */
+        time->tm_sec = BCD2BIN(buf[DS1305_SEC]);
+        time->tm_min = BCD2BIN(buf[DS1305_MIN]);
+        time->tm_hour = bcd2hour(buf[DS1305_HOUR]);
+        time->tm_wday = buf[DS1305_WDAY] - 1;
+        time->tm_mday = BCD2BIN(buf[DS1305_MDAY]);
+        time->tm_mon = BCD2BIN(buf[DS1305_MON]) - 1;
+        time->tm_year = BCD2BIN(buf[DS1305_YEAR]) + 100;
+
+        dev_vdbg(dev, "%s secs=%d, mins=%d, "
+                "hours=%d, mday=%d, mon=%d, year=%d, wday=%d\n",
+                "read", time->tm_sec, time->tm_min,
+                time->tm_hour, time->tm_mday,
+                time->tm_mon, time->tm_year, time->tm_wday);
+
+        /* Time may not be set */
+        return rtc_valid_tm(time);
+}
+
+static int ds1305_set_time(struct device *dev, struct rtc_time *time)
+{
+        struct ds1305   *ds1305 = dev_get_drvdata(dev);
+        u8              buf[1 + DS1305_RTC_LEN];
+        u8              *bp = buf;
+
+        dev_vdbg(dev, "%s secs=%d, mins=%d, "
+                "hours=%d, mday=%d, mon=%d, year=%d, wday=%d\n",
+                "write", time->tm_sec, time->tm_min,
+                time->tm_hour, time->tm_mday,
+                time->tm_mon, time->tm_year, time->tm_wday);
+
+        /* Write registers starting at the first time/date address. */
+        *bp++ = DS1305_WRITE | DS1305_SEC;
+
+        *bp++ = BIN2BCD(time->tm_sec);
+        *bp++ = BIN2BCD(time->tm_min);
+        *bp++ = hour2bcd(ds1305->hr12, time->tm_hour);
+        *bp++ = (time->tm_wday < 7) ? (time->tm_wday + 1) : 1;
+        *bp++ = BIN2BCD(time->tm_mday);
+        *bp++ = BIN2BCD(time->tm_mon + 1);
+        *bp++ = BIN2BCD(time->tm_year - 100);
+
+        dev_dbg(dev, "%s: %02x %02x %02x, %02x %02x %02x %02x\n",
+                "write", buf[1], buf[2], buf[3],
+                buf[4], buf[5], buf[6], buf[7]);
+
+        /* use write-then-read since dma from stack is nonportable */
+        return spi_write_then_read(ds1305->spi, buf, sizeof buf,
+                        NULL, 0);
+}
+
+/*
+ * Get/set of alarm is a bit funky:
+ *
+ * - First there's the inherent raciness of getting the (partitioned)
+ *   status of an alarm that could trigger while we're reading parts
+ *   of that status.
+ *
+ * - Second there's its limited range (we could increase it a bit by
+ *   relying on WDAY), which means it will easily roll over.
+ *
+ * - Third there's the choice of two alarms and alarm signals.
+ *   Here we use ALM0 and expect that nINT0 (open drain) is used;
+ *   that's the only real option for DS1306 runtime alarms, and is
+ *   natural on DS1305.
+ *
+ * - Fourth, there's also ALM1, and a second interrupt signal:
+ *     + On DS1305 ALM1 uses nINT1 (when INTCN=1) else nINT0;
+ *     + On DS1306 ALM1 only uses INT1 (an active high pulse)
+ *       and it won't work when VCC1 is active.
+ *
+ *   So to be most general, we should probably set both alarms to the
+ *   same value, letting ALM1 be the wakeup event source on DS1306
+ *   and handling several wiring options on DS1305.
+ *
+ * - Fifth, we support the polled mode (as well as possible; why not?)
+ *   even when no interrupt line is wired to an IRQ.
+ */
+
+/*
+ * Context: caller holds rtc->ops_lock (to protect ds1305->ctrl)
+ */
+static int ds1305_get_alarm(struct device *dev, struct rtc_wkalrm *alm)
+{
+        struct ds1305   *ds1305 = dev_get_drvdata(dev);
+        struct spi_device *spi = ds1305->spi;
+        u8              addr;
+        int             status;
+        u8              buf[DS1305_ALM_LEN];
+
+        /* Refresh control register cache BEFORE reading ALM0 registers,
+         * since reading alarm registers acks any pending IRQ.  That
+         * makes returning "pending" status a bit of a lie, but that bit
+         * of EFI status is at best fragile anyway (given IRQ handlers).
+         */
+        addr = DS1305_CONTROL;
+        status = spi_write_then_read(spi, &addr, sizeof addr,
+                        ds1305->ctrl, sizeof ds1305->ctrl);
+        if (status < 0)
+                return status;
+
+        alm->enabled = !!(ds1305->ctrl[0] & DS1305_AEI0);
+        alm->pending = !!(ds1305->ctrl[1] & DS1305_AEI0);
+
+        /* get and check ALM0 registers */
+        addr = DS1305_ALM0(DS1305_SEC);
+        status = spi_write_then_read(spi, &addr, sizeof addr,
+                        buf, sizeof buf);
+        if (status < 0)
+                return status;
+
+        dev_vdbg(dev, "%s: %02x %02x %02x %02x\n",
+                "alm0 read", buf[DS1305_SEC], buf[DS1305_MIN],
+                buf[DS1305_HOUR], buf[DS1305_WDAY]);
+
+        if ((DS1305_ALM_DISABLE & buf[DS1305_SEC])
+                        || (DS1305_ALM_DISABLE & buf[DS1305_MIN])
+                        || (DS1305_ALM_DISABLE & buf[DS1305_HOUR]))
+                return -EIO;
+
+        /* Stuff these values into alm->time and let RTC framework code
+         * fill in the rest ... and also handle rollover to tomorrow when
+         * that's needed.
+         */
+        alm->time.tm_sec = BCD2BIN(buf[DS1305_SEC]);
+        alm->time.tm_min = BCD2BIN(buf[DS1305_MIN]);
+        alm->time.tm_hour = bcd2hour(buf[DS1305_HOUR]);
+        alm->time.tm_mday = -1;
+        alm->time.tm_mon = -1;
+        alm->time.tm_year = -1;
+        /* next three fields are unused by Linux */
+        alm->time.tm_wday = -1;
+        alm->time.tm_mday = -1;
+        alm->time.tm_isdst = -1;
+
+        return 0;
+}
+
+/*
+ * Context: caller holds rtc->ops_lock (to protect ds1305->ctrl)
+ */
+static int ds1305_set_alarm(struct device *dev, struct rtc_wkalrm *alm)
+{
+        struct ds1305   *ds1305 = dev_get_drvdata(dev);
+        struct spi_device *spi = ds1305->spi;
+        unsigned long   now, later;
+        struct rtc_time tm;
+        int             status;
+        u8              buf[1 + DS1305_ALM_LEN];
+
+        /* convert desired alarm to time_t */
+        status = rtc_tm_to_time(&alm->time, &later);
+        if (status < 0)
+                return status;
+
+        /* Read current time as time_t */
+        status = ds1305_get_time(dev, &tm);
+        if (status < 0)
+                return status;
+        status = rtc_tm_to_time(&tm, &now);
+        if (status < 0)
+                return status;
+
+        /* make sure alarm fires within the next 24 hours */
+        if (later <= now)
+                return -EINVAL;
+        if ((later - now) > 24 * 60 * 60)
+                return -EDOM;
+
+        /* disable alarm if needed */
+        if (ds1305->ctrl[0] & DS1305_AEI0) {
+                ds1305->ctrl[0] &= ~DS1305_AEI0;
+
+                buf[0] = DS1305_WRITE | DS1305_CONTROL;
+                buf[1] = ds1305->ctrl[0];
+                status = spi_write_then_read(ds1305->spi, buf, 2, NULL, 0);
+                if (status < 0)
+                        return status;
+        }
+
+        /* write alarm */
+        buf[0] = DS1305_WRITE | DS1305_ALM0(DS1305_SEC);
+        buf[1 + DS1305_SEC] = BIN2BCD(alm->time.tm_sec);
+        buf[1 + DS1305_MIN] = BIN2BCD(alm->time.tm_min);
+        buf[1 + DS1305_HOUR] = hour2bcd(ds1305->hr12, alm->time.tm_hour);
+        buf[1 + DS1305_WDAY] = DS1305_ALM_DISABLE;
+
+        dev_dbg(dev, "%s: %02x %02x %02x %02x\n",
+                "alm0 write", buf[1 + DS1305_SEC], buf[1 + DS1305_MIN],
+                buf[1 + DS1305_HOUR], buf[1 + DS1305_WDAY]);
+
+        status = spi_write_then_read(spi, buf, sizeof buf, NULL, 0);
+        if (status < 0)
+                return status;
+
+        /* enable alarm if requested */
+        if (alm->enabled) {
+                ds1305->ctrl[0] |= DS1305_AEI0;
+
+                buf[0] = DS1305_WRITE | DS1305_CONTROL;
+                buf[1] = ds1305->ctrl[0];
+                status = spi_write_then_read(ds1305->spi, buf, 2, NULL, 0);
+        }
+
+        return status;
+}
+
+#ifdef CONFIG_PROC_FS
+
+static int ds1305_proc(struct device *dev, struct seq_file *seq)
+{
+        struct ds1305   *ds1305 = dev_get_drvdata(dev);
+        char            *diodes = "no";
+        char            *resistors = "";
+
+        /* ctrl[2] is treated as read-only; no locking needed */
+        if ((ds1305->ctrl[2] & 0xf0) == DS1305_TRICKLE_MAGIC) {
+                switch (ds1305->ctrl[2] & 0x0c) {
+                case DS1305_TRICKLE_DS2:
+                        diodes = "2 diodes, ";
+                        break;
+                case DS1305_TRICKLE_DS1:
+                        diodes = "1 diode, ";
+                        break;
+                default:
+                        goto done;
+                }
+                switch (ds1305->ctrl[2] & 0x03) {
+                case DS1305_TRICKLE_2K:
+                        resistors = "2k Ohm";
+                        break;
+                case DS1305_TRICKLE_4K:
+                        resistors = "4k Ohm";
+                        break;
+                case DS1305_TRICKLE_8K:
+                        resistors = "8k Ohm";
+                        break;
+                default:
+                        diodes = "no";
+                        break;
+                }
+        }
+
+done:
+        return seq_printf(seq,
+                        "trickle_charge\t: %s%s\n",
+                        diodes, resistors);
+}
+
+#else
+#define ds1305_proc     NULL
+#endif
+
+static const struct rtc_class_ops ds1305_ops = {
+        .ioctl          = ds1305_ioctl,
+        .read_time      = ds1305_get_time,
+        .set_time       = ds1305_set_time,
+        .read_alarm     = ds1305_get_alarm,
+        .set_alarm      = ds1305_set_alarm,
+        .proc           = ds1305_proc,
+};
+
+static void ds1305_work(struct work_struct *work)
+{
+        struct ds1305   *ds1305 = container_of(work, struct ds1305, work);
+        struct mutex    *lock = &ds1305->rtc->ops_lock;
+        struct spi_device *spi = ds1305->spi;
+        u8              buf[3];
+        int             status;
+
+        /* lock to protect ds1305->ctrl */
+        mutex_lock(lock);
+
+        /* Disable the IRQ, and clear its status ... for now, we "know"
+         * that if more than one alarm is active, they're in sync.
+         * Note that reading ALM data registers also clears IRQ status.
+         */
+        ds1305->ctrl[0] &= ~(DS1305_AEI1 | DS1305_AEI0);
+        ds1305->ctrl[1] = 0;
+
+        buf[0] = DS1305_WRITE | DS1305_CONTROL;
+        buf[1] = ds1305->ctrl[0];
+        buf[2] = 0;
+
+        status = spi_write_then_read(spi, buf, sizeof buf,
+                        NULL, 0);
+        if (status < 0)
+                dev_dbg(&spi->dev, "clear irq --> %d\n", status);
+
+        mutex_unlock(lock);
+
+        if (!test_bit(FLAG_EXITING, &ds1305->flags))
+                enable_irq(spi->irq);
+
+        /* rtc_update_irq() requires an IRQ-disabled context */
+        local_irq_disable();
+        rtc_update_irq(ds1305->rtc, 1, RTC_AF | RTC_IRQF);
+        local_irq_enable();
+}
+
+/*
+ * This "real" IRQ handler hands off to a workqueue mostly to allow
+ * mutex locking for ds1305->ctrl ... unlike I2C, we could issue async
+ * I/O requests in IRQ context (to clear the IRQ status).
+ */
+static irqreturn_t ds1305_irq(int irq, void *p)
+{
+        struct ds1305           *ds1305 = p;
+
+        disable_irq(irq);
+        schedule_work(&ds1305->work);
+        return IRQ_HANDLED;
+}
+
+/*----------------------------------------------------------------------*/
+
+/*
+ * Interface for NVRAM
+ */
+
+static void msg_init(struct spi_message *m, struct spi_transfer *x,
+                u8 *addr, size_t count, char *tx, char *rx)
+{
+        spi_message_init(m);
+        memset(x, 0, 2 * sizeof(*x));
+
+        x->tx_buf = addr;
+        x->len = 1;
+        spi_message_add_tail(x, m);
+
+        x++;
+
+        x->tx_buf = tx;
+        x->rx_buf = rx;
+        x->len = count;
+        spi_message_add_tail(x, m);
+}
+
+static ssize_t
+ds1305_nvram_read(struct kobject *kobj, struct bin_attribute *attr,
+                char *buf, loff_t off, size_t count)
+{
+        struct spi_device       *spi;
+        u8                      addr;
+        struct spi_message      m;
+        struct spi_transfer     x[2];
+        int                     status;
+
+        spi = container_of(kobj, struct spi_device, dev.kobj);
+
+        if (unlikely(off >= DS1305_NVRAM_LEN))
+                return 0;
+        if (count >= DS1305_NVRAM_LEN)
+                count = DS1305_NVRAM_LEN;
+        if ((off + count) > DS1305_NVRAM_LEN)
+                count = DS1305_NVRAM_LEN - off;
+        if (unlikely(!count))
+                return count;
+
+        addr = DS1305_NVRAM + off;
+        msg_init(&m, x, &addr, count, NULL, buf);
+
+        status = spi_sync(spi, &m);
+        if (status < 0)
+                dev_err(&spi->dev, "nvram %s error %d\n", "read", status);
+        return (status < 0) ? status : count;
+}
+
+static ssize_t
+ds1305_nvram_write(struct kobject *kobj, struct bin_attribute *attr,
+                char *buf, loff_t off, size_t count)
+{
+        struct spi_device       *spi;
+        u8                      addr;
+        struct spi_message      m;
+        struct spi_transfer     x[2];
+        int                     status;
+
+        spi = container_of(kobj, struct spi_device, dev.kobj);
+
+        if (unlikely(off >= DS1305_NVRAM_LEN))
+                return -EFBIG;
+        if (count >= DS1305_NVRAM_LEN)
+                count = DS1305_NVRAM_LEN;
+        if ((off + count) > DS1305_NVRAM_LEN)
+                count = DS1305_NVRAM_LEN - off;
+        if (unlikely(!count))
+                return count;
+
+        addr = (DS1305_WRITE | DS1305_NVRAM) + off;
+        msg_init(&m, x, &addr, count, buf, NULL);
+
+        status = spi_sync(spi, &m);
+        if (status < 0)
+                dev_err(&spi->dev, "nvram %s error %d\n", "write", status);
+        return (status < 0) ? status : count;
+}
+
+static struct bin_attribute nvram = {
+        .attr.name      = "nvram",
+        .attr.mode      = S_IRUGO | S_IWUSR,
+        .attr.owner     = THIS_MODULE,
+        .read           = ds1305_nvram_read,
+        .write          = ds1305_nvram_write,
+        .size           = DS1305_NVRAM_LEN,
+};
+
+/*----------------------------------------------------------------------*/
+
+/*
+ * Interface to SPI stack
+ */
+
+static int __devinit ds1305_probe(struct spi_device *spi)
+{
+        struct ds1305                   *ds1305;
+        struct rtc_device               *rtc;
+        int                             status;
+        u8                              addr, value;
+        struct ds1305_platform_data     *pdata = spi->dev.platform_data;
+        bool                            write_ctrl = false;
+
+        /* Sanity check board setup data.  This may be hooked up
+         * in 3wire mode, but we don't care.  Note that unless
+         * there's an inverter in place, this needs SPI_CS_HIGH!
+         */
+        if ((spi->bits_per_word && spi->bits_per_word != 8)
+                        || (spi->max_speed_hz > 2000000)
+                        || !(spi->mode & SPI_CPHA))
+                return -EINVAL;
+
+        /* set up driver data */
+        ds1305 = kzalloc(sizeof *ds1305, GFP_KERNEL);
+        if (!ds1305)
+                return -ENOMEM;
+        ds1305->spi = spi;
+        spi_set_drvdata(spi, ds1305);
+
+        /* read and cache control registers */
+        addr = DS1305_CONTROL;
+        status = spi_write_then_read(spi, &addr, sizeof addr,
+                        ds1305->ctrl, sizeof ds1305->ctrl);
+        if (status < 0) {
+                dev_dbg(&spi->dev, "can't %s, %d\n",
+                                "read", status);
+                goto fail0;
+        }
+
+        dev_dbg(&spi->dev, "ctrl %s: %02x %02x %02x\n",
+                        "read", ds1305->ctrl[0],
+                        ds1305->ctrl[1], ds1305->ctrl[2]);
+
+        /* Sanity check register values ... partially compensating for the
+         * fact that SPI has no device handshake.  A pullup on MISO would
+         * make these tests fail; but not all systems will have one.  If
+         * some register is neither 0x00 nor 0xff, a chip is likely there.
+         */
+        if ((ds1305->ctrl[0] & 0x38) != 0 || (ds1305->ctrl[1] & 0xfc) != 0) {
+                dev_dbg(&spi->dev, "RTC chip is not present\n");
+                status = -ENODEV;
+                goto fail0;
+        }
+        if (ds1305->ctrl[2] == 0)
+                dev_dbg(&spi->dev, "chip may not be present\n");
+
+        /* enable writes if needed ... if we were paranoid it would
+         * make sense to enable them only when absolutely necessary.
+         */
+        if (ds1305->ctrl[0] & DS1305_WP) {
+                u8              buf[2];
+
+                ds1305->ctrl[0] &= ~DS1305_WP;
+
+                buf[0] = DS1305_WRITE | DS1305_CONTROL;
+                buf[1] = ds1305->ctrl[0];
+                status = spi_write_then_read(spi, buf, sizeof buf, NULL, 0);
+
+                dev_dbg(&spi->dev, "clear WP --> %d\n", status);
+                if (status < 0)
+                        goto fail0;
+        }
+
+        /* on DS1305, maybe start oscillator; like most low power
+         * oscillators, it may take a second to stabilize
+         */
+        if (ds1305->ctrl[0] & DS1305_nEOSC) {
+                ds1305->ctrl[0] &= ~DS1305_nEOSC;
+                write_ctrl = true;
+                dev_warn(&spi->dev, "SET TIME!\n");
+        }
+
+        /* ack any pending IRQs */
+        if (ds1305->ctrl[1]) {
+                ds1305->ctrl[1] = 0;
+                write_ctrl = true;
+        }
+
+        /* this may need one-time (re)init */
+        if (pdata) {
+                /* maybe enable trickle charge */
+                if (((ds1305->ctrl[2] & 0xf0) != DS1305_TRICKLE_MAGIC)) {
+                        ds1305->ctrl[2] = DS1305_TRICKLE_MAGIC
+                                                | pdata->trickle;
+                        write_ctrl = true;
+                }
+
+                /* on DS1306, configure 1 Hz signal */
+                if (pdata->is_ds1306) {
+                        if (pdata->en_1hz) {
+                                if (!(ds1305->ctrl[0] & DS1306_1HZ)) {
+                                        ds1305->ctrl[0] |= DS1306_1HZ;
+                                        write_ctrl = true;
+                                }
+                        } else {
+                                if (ds1305->ctrl[0] & DS1306_1HZ) {
+                                        ds1305->ctrl[0] &= ~DS1306_1HZ;
+                                        write_ctrl = true;
+                                }
+                        }
+                }
+        }
+
+        if (write_ctrl) {
+                u8              buf[4];
+
+                buf[0] = DS1305_WRITE | DS1305_CONTROL;
+                buf[1] = ds1305->ctrl[0];
+                buf[2] = ds1305->ctrl[1];
+                buf[3] = ds1305->ctrl[2];
+                status = spi_write_then_read(spi, buf, sizeof buf, NULL, 0);
+                if (status < 0) {
+                        dev_dbg(&spi->dev, "can't %s, %d\n",
+                                        "write", status);
+                        goto fail0;
+                }
+
+                dev_dbg(&spi->dev, "ctrl %s: %02x %02x %02x\n",
+                                "write", ds1305->ctrl[0],
+                                ds1305->ctrl[1], ds1305->ctrl[2]);
+        }
+
+        /* see if non-Linux software set up AM/PM mode */
+        addr = DS1305_HOUR;
+        status = spi_write_then_read(spi, &addr, sizeof addr,
+                                &value, sizeof value);
+        if (status < 0) {
+                dev_dbg(&spi->dev, "read HOUR --> %d\n", status);
+                goto fail0;
+        }
+
+        ds1305->hr12 = (DS1305_HR_12 & value) != 0;
+        if (ds1305->hr12)
+                dev_dbg(&spi->dev, "AM/PM\n");
+
+        /* register RTC ... from here on, ds1305->ctrl needs locking */
+        rtc = rtc_device_register("ds1305", &spi->dev,
+                        &ds1305_ops, THIS_MODULE);
+        if (IS_ERR(rtc)) {
+                status = PTR_ERR(rtc);
+                dev_dbg(&spi->dev, "register rtc --> %d\n", status);
+                goto fail0;
+        }
+        ds1305->rtc = rtc;
+
+        /* Maybe set up alarm IRQ; be ready to handle it triggering right
+         * away.  NOTE that we don't share this.  The signal is active low,
+         * and we can't ack it before a SPI message delay.  We temporarily
+         * disable the IRQ until it's acked, which lets us work with more
+         * IRQ trigger modes (not all IRQ controllers can do falling edge).
+         */
+        if (spi->irq) {
+                INIT_WORK(&ds1305->work, ds1305_work);
+                status = request_irq(spi->irq, ds1305_irq,
+                                0, dev_name(&rtc->dev), ds1305);
+                if (status < 0) {
+                        dev_dbg(&spi->dev, "request_irq %d --> %d\n",
+                                        spi->irq, status);
+                        goto fail1;
+                }
+        }
+
+        /* export NVRAM */
+        status = sysfs_create_bin_file(&spi->dev.kobj, &nvram);
+        if (status < 0) {
+                dev_dbg(&spi->dev, "register nvram --> %d\n", status);
+                goto fail2;
+        }
+
+        return 0;
+
+fail2:
+        free_irq(spi->irq, ds1305);
+fail1:
+        rtc_device_unregister(rtc);
+fail0:
+        kfree(ds1305);
+        return status;
+}
+
+static int __devexit ds1305_remove(struct spi_device *spi)
+{
+        struct ds1305   *ds1305 = spi_get_drvdata(spi);
+
+        sysfs_remove_bin_file(&spi->dev.kobj, &nvram);
+
+        /* carefully shut down irq and workqueue, if present */
+        if (spi->irq) {
+                set_bit(FLAG_EXITING, &ds1305->flags);
+                free_irq(spi->irq, ds1305);
+                flush_scheduled_work();
+        }
+
+        rtc_device_unregister(ds1305->rtc);
+        spi_set_drvdata(spi, NULL);
+        kfree(ds1305);
+        return 0;
+}
+
+static struct spi_driver ds1305_driver = {
+        .driver.name    = "rtc-ds1305",
+        .driver.owner   = THIS_MODULE,
+        .probe          = ds1305_probe,
+        .remove         = __devexit_p(ds1305_remove),
+        /* REVISIT add suspend/resume */
+};
+
+static int __init ds1305_init(void)
+{
+        return spi_register_driver(&ds1305_driver);
+}
+module_init(ds1305_init);
+
+static void __exit ds1305_exit(void)
+{
+        spi_unregister_driver(&ds1305_driver);
+}
+module_exit(ds1305_exit);
+
+MODULE_DESCRIPTION("RTC driver for DS1305 and DS1306 chips");
+MODULE_LICENSE("GPL");
diff --git a/include/linux/spi/ds1305.h b/include/linux/spi/ds1305.h
new file mode 100644
index 000000000000..287ec830eab7
--- /dev/null
+++ b/include/linux/spi/ds1305.h
@@ -0,0 +1,35 @@
+#ifndef __LINUX_SPI_DS1305_H
+#define __LINUX_SPI_DS1305_H
+
+/*
+ * One-time configuration for ds1305 and ds1306 RTC chips.
+ *
+ * Put a pointer to this in spi_board_info.platform_data if you want to
+ * be sure that Linux (re)initializes this as needed ... after losing
+ * backup power, and potentially on the first boot.
+ */
+struct ds1305_platform_data {
+
+        /* Trickle charge configuration:  it's OK to leave out the MAGIC
+         * bitmask; mask in either DS1 or DS2, and then one of 2K/4k/8K.
+         */
+#define DS1305_TRICKLE_MAGIC    0xa0
+#define DS1305_TRICKLE_DS2      0x08    /* two diodes */
+#define DS1305_TRICKLE_DS1      0x04    /* one diode */
+#define DS1305_TRICKLE_2K       0x01    /* 2 KOhm resistance */
+#define DS1305_TRICKLE_4K       0x02    /* 4 KOhm resistance */
+#define DS1305_TRICKLE_8K       0x03    /* 8 KOhm resistance */
+        u8      trickle;
+
+        /* set only on ds1306 parts */
+        bool    is_ds1306;
+
+        /* ds1306 only:  enable 1 Hz output */
+        bool    en_1hz;
+
+        /* REVISIT:  the driver currently expects nINT0 to be wired
+         * as the alarm IRQ.  ALM1 may also need to be set up ...
+         */
+};
+
+#endif /* __LINUX_SPI_DS1305_H */