[PATCH v2 4/6] regmap: sdw-mbq: Add support for SDCA deferred controls

[Charles Keepax <ckeepax@opensource.cirrus.com>]

The SDCA specification allows for controls to be deferred. In the case
of a deferred control the device will return COMMAND_IGNORED to the
8-bit operation that would cause the value to commit. Which is the
final 8-bits on a write, or the first 8-bits on a read. In the case of
receiving a defer, the regmap will poll the SDCA function busy bit,
after which the transaction will be retried, returning an error if the
function busy does not clear within a chip specific timeout. Since
this is common SDCA functionality which is the 99% use-case for MBQs
it makes sense to incorporate this functionality into the register
map. If no MBQ configuration is specified, the behaviour will default
to the existing behaviour.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
---

Changes since v1:
 - Initialise ret in regmap_sdw_mbq_poll_busy() to avoid potentially
   returning an uninitialised value.

Thanks,
Charles

 drivers/base/regmap/regmap-sdw-mbq.c | 129 ++++++++++++++++++++++++---
 include/linux/regmap.h               |  15 ++++
 2 files changed, 130 insertions(+), 14 deletions(-)

diff --git a/drivers/base/regmap/regmap-sdw-mbq.c b/drivers/base/regmap/regmap-sdw-mbq.c
index 1bd2773b11a45..86644bbd07100 100644
--- a/drivers/base/regmap/regmap-sdw-mbq.c
+++ b/drivers/base/regmap/regmap-sdw-mbq.c
@@ -2,12 +2,15 @@
 // Copyright(c) 2020 Intel Corporation.
 
 #include <linux/bits.h>
+#include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/errno.h>
+#include <linux/iopoll.h>
 #include <linux/module.h>
 #include <linux/regmap.h>
 #include <linux/soundwire/sdw.h>
 #include <linux/soundwire/sdw_registers.h>
+#include <sound/sdca_function.h>
 #include "internal.h"
 
 struct regmap_mbq_context {
@@ -16,6 +19,7 @@ struct regmap_mbq_context {
 	struct regmap_sdw_mbq_cfg cfg;
 
 	int val_size;
+	bool (*readable_reg)(struct device *dev, unsigned int reg);
 };
 
 static int regmap_sdw_mbq_size(struct regmap_mbq_context *ctx, unsigned int reg)
@@ -31,18 +35,48 @@ static int regmap_sdw_mbq_size(struct regmap_mbq_context *ctx, unsigned int reg)
 	return size;
 }
 
-static int regmap_sdw_mbq_write(void *context, unsigned int reg, unsigned int val)
+static bool regmap_sdw_mbq_deferrable(struct regmap_mbq_context *ctx, unsigned int reg)
+{
+	if (ctx->cfg.deferrable)
+		return ctx->cfg.deferrable(ctx->dev, reg);
+
+	return false;
+}
+
+static int regmap_sdw_mbq_poll_busy(struct sdw_slave *slave, unsigned int reg,
+				    struct regmap_mbq_context *ctx)
+{
+	struct device *dev = &slave->dev;
+	int val, ret = 0;
+
+	dev_dbg(dev, "Deferring transaction for 0x%x\n", reg);
+
+	reg = SDW_SDCA_CTL(SDW_SDCA_CTL_FUNC(reg), 0,
+			   SDCA_CTL_ENTITY_0_FUNCTION_STATUS, 0);
+
+	if (ctx->readable_reg(dev, reg)) {
+		ret = read_poll_timeout(sdw_read_no_pm, val,
+					val < 0 || !(val & SDCA_CTL_ENTITY_0_FUNCTION_BUSY),
+					ctx->cfg.timeout_us, ctx->cfg.retry_us,
+					false, slave, reg);
+		if (val < 0)
+			return val;
+		if (ret)
+			dev_err(dev, "Function busy timed out 0x%x: %d\n", reg, val);
+	} else {
+		fsleep(ctx->cfg.timeout_us);
+	}
+
+	return ret;
+}
+
+static int regmap_sdw_mbq_write_impl(struct sdw_slave *slave,
+				     unsigned int reg, unsigned int val,
+				     int mbq_size, bool deferrable)
 {
-	struct regmap_mbq_context *ctx = context;
-	struct device *dev = ctx->dev;
-	struct sdw_slave *slave = dev_to_sdw_dev(dev);
-	int mbq_size = regmap_sdw_mbq_size(ctx, reg);
 	int shift = mbq_size * BITS_PER_BYTE;
 	int ret;
 
-	if (mbq_size < 0)
-		return mbq_size;
-
 	while (--mbq_size > 0) {
 		shift -= BITS_PER_BYTE;
 
@@ -52,24 +86,58 @@ static int regmap_sdw_mbq_write(void *context, unsigned int reg, unsigned int va
 			return ret;
 	}
 
-	return sdw_write_no_pm(slave, reg, val & 0xff);
+	ret = sdw_write_no_pm(slave, reg, val & 0xff);
+	if (deferrable && ret == -ENODATA)
+		return -EAGAIN;
+
+	return ret;
 }
 
-static int regmap_sdw_mbq_read(void *context, unsigned int reg, unsigned int *val)
+static int regmap_sdw_mbq_write(void *context, unsigned int reg, unsigned int val)
 {
 	struct regmap_mbq_context *ctx = context;
 	struct device *dev = ctx->dev;
 	struct sdw_slave *slave = dev_to_sdw_dev(dev);
+	bool deferrable = regmap_sdw_mbq_deferrable(ctx, reg);
 	int mbq_size = regmap_sdw_mbq_size(ctx, reg);
-	int shift = BITS_PER_BYTE;
-	int read;
+	int ret;
 
 	if (mbq_size < 0)
 		return mbq_size;
 
+	/*
+	 * Technically the spec does allow a device to set itself to busy for
+	 * internal reasons, but since it doesn't provide any information on
+	 * how to handle timeouts in that case, for now the code will only
+	 * process a single wait/timeout on function busy and a single retry
+	 * of the transaction.
+	 */
+	ret = regmap_sdw_mbq_write_impl(slave, reg, val, mbq_size, deferrable);
+	if (ret == -EAGAIN) {
+		ret = regmap_sdw_mbq_poll_busy(slave, reg, ctx);
+		if (ret)
+			return ret;
+
+		ret = regmap_sdw_mbq_write_impl(slave, reg, val, mbq_size, false);
+	}
+
+	return ret;
+}
+
+static int regmap_sdw_mbq_read_impl(struct sdw_slave *slave,
+				    unsigned int reg, unsigned int *val,
+				    int mbq_size, bool deferrable)
+{
+	int shift = BITS_PER_BYTE;
+	int read;
+
 	read = sdw_read_no_pm(slave, reg);
-	if (read < 0)
+	if (read < 0) {
+		if (deferrable && read == -ENODATA)
+			return -EAGAIN;
+
 		return read;
+	}
 
 	*val = read;
 
@@ -85,6 +153,37 @@ static int regmap_sdw_mbq_read(void *context, unsigned int reg, unsigned int *va
 	return 0;
 }
 
+static int regmap_sdw_mbq_read(void *context, unsigned int reg, unsigned int *val)
+{
+	struct regmap_mbq_context *ctx = context;
+	struct device *dev = ctx->dev;
+	struct sdw_slave *slave = dev_to_sdw_dev(dev);
+	bool deferrable = regmap_sdw_mbq_deferrable(ctx, reg);
+	int mbq_size = regmap_sdw_mbq_size(ctx, reg);
+	int ret;
+
+	if (mbq_size < 0)
+		return mbq_size;
+
+	/*
+	 * Technically the spec does allow a device to set itself to busy for
+	 * internal reasons, but since it doesn't provide any information on
+	 * how to handle timeouts in that case, for now the code will only
+	 * process a single wait/timeout on function busy and a single retry
+	 * of the transaction.
+	 */
+	ret = regmap_sdw_mbq_read_impl(slave, reg, val, mbq_size, deferrable);
+	if (ret == -EAGAIN) {
+		ret = regmap_sdw_mbq_poll_busy(slave, reg, ctx);
+		if (ret)
+			return ret;
+
+		ret = regmap_sdw_mbq_read_impl(slave, reg, val, mbq_size, false);
+	}
+
+	return ret;
+}
+
 static const struct regmap_bus regmap_sdw_mbq = {
 	.reg_read = regmap_sdw_mbq_read,
 	.reg_write = regmap_sdw_mbq_write,
@@ -119,11 +218,13 @@ regmap_sdw_mbq_gen_context(struct device *dev,
 		return ERR_PTR(-ENOMEM);
 
 	ctx->dev = dev;
-	ctx->val_size = config->val_bits / BITS_PER_BYTE;
 
 	if (mbq_config)
 		ctx->cfg = *mbq_config;
 
+	ctx->val_size = config->val_bits / BITS_PER_BYTE;
+	ctx->readable_reg = config->readable_reg;
+
 	return ctx;
 }
 
diff --git a/include/linux/regmap.h b/include/linux/regmap.h
index dd96a22f56578..198067d3cf10f 100644
--- a/include/linux/regmap.h
+++ b/include/linux/regmap.h
@@ -510,11 +510,26 @@ struct regmap_range_cfg {
  * struct regmap_sdw_mbq_cfg - Configuration for Multi-Byte Quantities
  *
  * @mbq_size: Callback returning the actual size of the given register.
+ * @deferrable: Callback returning true if the hardware can defer
+ *              transactions to the given register. Deferral should
+ *              only be used by SDCA parts and typically which controls
+ *              are deferrable will be specified in either as a hard
+ *              coded list or from the DisCo tables in the platform
+ *              firmware.
+ *
+ * @timeout_us: The time in microseconds after which waiting for a deferred
+ *              transaction should time out.
+ * @retry_us: The time in microseconds between polls of the function busy
+ *            status whilst waiting for an opportunity to retry a deferred
+ *            transaction.
  *
  * Provides additional configuration required for SoundWire MBQ register maps.
  */
 struct regmap_sdw_mbq_cfg {
 	int (*mbq_size)(struct device *dev, unsigned int reg);
+	bool (*deferrable)(struct device *dev, unsigned int reg);
+	unsigned long timeout_us;
+	unsigned long retry_us;
 };
 
 struct regmap_async;
-- 
2.39.5