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>

3 files changed, 858 insertions, 0 deletions

diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index beffb834c44..90ab7382540 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 b0e1af54f80..18622ef84ca 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 00000000000..b91d02a3ace
--- /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");