tieqep: fix unhandled fault on eQEP register access

Call pm_runtime_get_sync() at the beginning of any functions that will
read or write to the memory mapped eQEP registers.  This is to ensure
that the eQEP peripheral is running and its clock is enabled.

Before this patch, an attempt to read the position file via sysfs would
results in a segmentation fault.  The kernel log would be contain this
error:

[ 2591.653471] Unhandled fault: external abort on non-linefetch (0x1028) at 0xfa304180
[ 2591.915165] [<bf005310>] (eqep_get_position [tieqep]) from [<c08930d0>] (dev_attr_show+0x2c/0x58)

More details:
https://gist.github.com/pdp7/fe07082d23f2bfbc362c733a7b0aea72

BeagleBoard mailing list thread:
https://groups.google.com/d/msg/beagleboard/_TdTH7oPEXE/MNvU-mY6DgAJ

Author: pdp7

drivers/misc/tieqep.c

@@ -241,9 +241,13 @@ static ssize_t eqep_get_enabled(struct device *dev, struct device_attribute *att
 {
 	/* Get the instance structure */
 	struct eqep_chip *eqep = dev_get_drvdata(dev);
+	u16 enabled = 0;
+
+	/* Increment the device usage count and run pm_runtime_resume() */
+	pm_runtime_get_sync(dev);
 
 	/* Read the qep control register and mask all but the enabled bit */
-	u16 enabled = readw(eqep->mmio_base + QEPCTL) & PHEN;
+	enabled = readw(eqep->mmio_base + QEPCTL) & PHEN;
 
 	/* Return the target in string format */
 	return sprintf(buf, "%u\n", (enabled) ? 1 : 0);
@@ -262,6 +266,8 @@ static ssize_t eqep_set_enabled(struct device *dev, struct device_attribute *att
 	if ((rc = kstrtou8(buf, 0, &enabled)))
 		return rc;
 
+	/* Increment the device usage count and run pm_runtime_resume() */
+	pm_runtime_get_sync(dev);
 	/* Get the existing state of QEPCTL */
 	val = readw(eqep->mmio_base + QEPCTL);
 
@@ -286,6 +292,8 @@ static ssize_t eqep_get_position(struct device *dev, struct device_attribute *at
 	struct eqep_chip *eqep = dev_get_drvdata(dev);
 
 	s32 position = 0;
+	/* Increment the device usage count and run pm_runtime_resume() */
+	pm_runtime_get_sync(dev);
 
 	if (eqep->op_mode == TIEQEP_MODE_ABSOLUTE) {
 		position = readl(eqep->mmio_base + QPOSCNT);
@@ -308,6 +316,8 @@ static ssize_t eqep_set_position(struct device *dev, struct device_attribute *at
 	if ((rc = kstrtos32(buf, 0, &position)))
 		return rc;
 
+	/* Increment the device usage count and run pm_runtime_resume() */
+	pm_runtime_get_sync(dev);
 	/*
 	 * If we are in absolute mode, set the position of the encoder,
 	 * discard relative mode because thats pointless
@@ -327,6 +337,8 @@ static ssize_t eqep_get_timer_period(struct device *dev, struct device_attribute
 	struct eqep_chip *eqep = dev_get_drvdata(dev);
 	u64 period;
 
+	/* Increment the device usage count and run pm_runtime_resume() */
+	pm_runtime_get_sync(dev);
 	/* Convert from counts per interrupt back into period_ns */
 	period = readl(eqep->mmio_base + QUPRD);
 	period = period * NSEC_PER_SEC;
@@ -348,6 +360,8 @@ static ssize_t eqep_set_timer_period(struct device *dev, struct device_attribute
 	if ((rc = kstrtou64(buf, 0, &period)))
 		return rc;
 
+	/* Increment the device usage count and run pm_runtime_resume() */
+	pm_runtime_get_sync(dev);
 	/* Disable the unit timer before modifying its period register */
 	tmp = readw(eqep->mmio_base + QEPCTL);
 	tmp &= ~(UTE | QCLM);
@@ -395,6 +409,8 @@ static ssize_t eqep_set_mode(struct device *dev, struct device_attribute *attr,
 
 	dev_dbg(dev, "eqep_set_mode:%d\n", tmp_mode);
 
+	/* Increment the device usage count and run pm_runtime_resume() */
+	pm_runtime_get_sync(dev);
 	val = readw(eqep->mmio_base + QEPCTL);
 
 	if (tmp_mode == TIEQEP_MODE_ABSOLUTE) {