regmap: add iopoll-like polling macro

This patch adds a macro regmap_read_poll_timeout that works similar
to the readx_poll_timeout defined in linux/iopoll.h, except that this
can also return the error value returned by a failed regmap_read.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

include/linux/regmap.h

@@ -95,6 +95,45 @@ struct reg_sequence {
 #define	regmap_fields_force_update_bits(field, id, mask, val) \
 	regmap_fields_update_bits_base(field, id, mask, val, NULL, false, true)
 
+/**
+ * regmap_read_poll_timeout - Poll until a condition is met or a timeout occurs
+ * @map: Regmap to read from
+ * @addr: Address to poll
+ * @val: Unsigned integer variable to read the value into
+ * @cond: Break condition (usually involving @val)
+ * @sleep_us: Maximum time to sleep between reads in us (0
+ *            tight-loops).  Should be less than ~20ms since usleep_range
+ *            is used (see Documentation/timers/timers-howto.txt).
+ * @timeout_us: Timeout in us, 0 means never timeout
+ *
+ * Returns 0 on success and -ETIMEDOUT upon a timeout or the regmap_read
+ * error return value in case of a error read. In the two former cases,
+ * the last read value at @addr is stored in @val. Must not be called
+ * from atomic context if sleep_us or timeout_us are used.
+ *
+ * This is modelled after the readx_poll_timeout macros in linux/iopoll.h.
+ */
+#define regmap_read_poll_timeout(map, addr, val, cond, sleep_us, timeout_us) \
+({ \
+	ktime_t timeout = ktime_add_us(ktime_get(), timeout_us); \
+	int ret; \
+	might_sleep_if(sleep_us); \
+	for (;;) { \
+		ret = regmap_read((map), (addr), &(val)); \
+		if (ret) \
+			break; \
+		if (cond) \
+			break; \
+		if (timeout_us && ktime_compare(ktime_get(), timeout) > 0) { \
+			ret = regmap_read((map), (addr), &(val)); \
+			break; \
+		} \
+		if (sleep_us) \
+			usleep_range((sleep_us >> 2) + 1, sleep_us); \
+	} \
+	ret ?: ((cond) ? 0 : -ETIMEDOUT); \
+})
+
 #ifdef CONFIG_REGMAP
 
 enum regmap_endian {