[PATCH v6 6/7] mfd: nct6694: Introduce regmap-based transport abstraction

[a0282524688@gmail.com]

From: Ming Yu <a0282524688@gmail.com>

Wrap the USB transport behind a regmap_bus so that sub-device drivers
talk to the firmware exclusively through nct6694_{read,write}_msg(),
without referencing the underlying transport. This unblocks adding the
upcoming HIF (eSPI) backend without touching any sub-device driver.

Encode the firmware addressing scheme (host control, module id and
16-bit offset) into a single 32-bit regmap register so that struct
nct6694_cmd_header maps cleanly onto regmap_bulk_{read,write}():

    bits [31:24]  host control
    bits [23:16]  module id
    bits [15:0]   offset (low byte = command, high byte = selector)

Add nct6694_write_read_msg() for the few commands (e.g. the I2C
"deliver" command) that send a request and read the reply back in a
single transaction, and convert the I2C transfer path to use it.

The USB backend gains a scratch xfer_buf because nct6694_usb_write_msg()
writes the firmware response back into its payload buffer, which is
incompatible with the const buffer handed to regmap_bus->write. The
per-transport access_lock is removed: regmap already serialises bus
accesses on its own.

Signed-off-by: Ming Yu <a0282524688@gmail.com>
---
Changes in v6:
- New patch. Replaces the v5 function-pointer abstraction with a
  regmap_bus based transport: the firmware command header is packed into
  a single 32-bit regmap register and sub-device drivers use the regmap
  bulk accessors. Adds nct6694_write_read_msg() for request/response
  commands and drops the per-transport access_lock (regmap already
  serialises bus accesses).

 drivers/i2c/busses/i2c-nct6694.c |   2 +-
 drivers/mfd/Kconfig              |   1 +
 drivers/mfd/nct6694-usb.c        | 118 +++++++++++++++++++++----------
 include/linux/mfd/nct6694.h      |  63 ++++++++++++++---
 4 files changed, 134 insertions(+), 50 deletions(-)

diff --git a/drivers/i2c/busses/i2c-nct6694.c b/drivers/i2c/busses/i2c-nct6694.c
index ef3329f34246..7e32dab6e759 100644
--- a/drivers/i2c/busses/i2c-nct6694.c
+++ b/drivers/i2c/busses/i2c-nct6694.c
@@ -77,7 +77,7 @@ static int nct6694_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int
 		deliver->addr = i2c_8bit_addr_from_msg(msg_temp);
 		if (msg_temp->flags & I2C_M_RD) {
 			deliver->r_cnt = msg_temp->len;
-			ret = nct6694_write_msg(data->nct6694, &cmd_hd, deliver);
+			ret = nct6694_write_read_msg(data->nct6694, &cmd_hd, deliver);
 			if (ret < 0)
 				return ret;
 
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index 5506a0adf3ec..742fc26e6ff7 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -1166,6 +1166,7 @@ config MFD_MENF21BMC
 config MFD_NCT6694
 	tristate
 	select MFD_CORE
+	select REGMAP
 	help
 	  Core MFD support for the Nuvoton NCT6694 peripheral expander.
 	  This provides the common APIs and shared structures used by all
diff --git a/drivers/mfd/nct6694-usb.c b/drivers/mfd/nct6694-usb.c
index 2289ebfde7fa..8e7f3b07de53 100644
--- a/drivers/mfd/nct6694-usb.c
+++ b/drivers/mfd/nct6694-usb.c
@@ -9,6 +9,7 @@
  * CAN, WDT, HWMON and RTC management.
  */
 
+#include <linux/bitfield.h>
 #include <linux/bits.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>
@@ -17,7 +18,9 @@
 #include <linux/mfd/core.h>
 #include <linux/mfd/nct6694.h>
 #include <linux/module.h>
+#include <linux/regmap.h>
 #include <linux/slab.h>
+#include <linux/unaligned.h>
 #include <linux/usb.h>
 
 #define NCT6694_VENDOR_ID	0x0416
@@ -34,10 +37,10 @@ union __packed nct6694_usb_msg {
 };
 
 struct nct6694_usb_data {
-	struct mutex access_lock;
 	struct urb *int_in_urb;
 	struct usb_device *udev;
 	union nct6694_usb_msg *usb_msg;
+	void *xfer_buf;
 	__le32 *int_buffer;
 };
 
@@ -101,29 +104,15 @@ static int nct6694_usb_err_handling(struct nct6694 *nct6694, unsigned char err_s
 	return -EIO;
 }
 
-/**
- * nct6694_usb_read_msg() - Read message from NCT6694 device
- * @nct6694: NCT6694 device pointer
- * @cmd_hd: command header structure
- * @buf: buffer to store the response data
- *
- * Sends a command to the NCT6694 device and reads the response.
- * The command header is specified in @cmd_hd, and the response
- * data is stored in @buf.
- *
- * Return: Negative value on error or 0 on success.
- */
-int nct6694_usb_read_msg(struct nct6694 *nct6694,
-			 const struct nct6694_cmd_header *cmd_hd,
-			 void *buf)
+static int nct6694_usb_read_msg(struct nct6694 *nct6694,
+				const struct nct6694_cmd_header *cmd_hd,
+				void *buf)
 {
 	struct nct6694_usb_data *udata = nct6694->priv;
 	union nct6694_usb_msg *msg = udata->usb_msg;
 	struct usb_device *udev = udata->udev;
 	int tx_len, rx_len, ret;
 
-	guard(mutex)(&udata->access_lock);
-
 	memcpy(&msg->cmd_header, cmd_hd, sizeof(*cmd_hd));
 	msg->cmd_header.hctrl = NCT6694_HCTRL_GET;
 
@@ -153,30 +142,16 @@ int nct6694_usb_read_msg(struct nct6694 *nct6694,
 
 	return nct6694_usb_err_handling(nct6694, msg->response_header.sts);
 }
-EXPORT_SYMBOL_GPL(nct6694_usb_read_msg);
 
-/**
- * nct6694_usb_write_msg() - Write message to NCT6694 device
- * @nct6694: NCT6694 device pointer
- * @cmd_hd: command header structure
- * @buf: buffer containing the data to be sent
- *
- * Sends a command to the NCT6694 device and writes the data
- * from @buf. The command header is specified in @cmd_hd.
- *
- * Return: Negative value on error or 0 on success.
- */
-int nct6694_usb_write_msg(struct nct6694 *nct6694,
-			  const struct nct6694_cmd_header *cmd_hd,
-			  void *buf)
+static int nct6694_usb_write_msg(struct nct6694 *nct6694,
+				 const struct nct6694_cmd_header *cmd_hd,
+				 void *buf)
 {
 	struct nct6694_usb_data *udata = nct6694->priv;
 	union nct6694_usb_msg *msg = udata->usb_msg;
 	struct usb_device *udev = udata->udev;
 	int tx_len, rx_len, ret;
 
-	guard(mutex)(&udata->access_lock);
-
 	memcpy(&msg->cmd_header, cmd_hd, sizeof(*cmd_hd));
 	msg->cmd_header.hctrl = NCT6694_HCTRL_SET;
 
@@ -212,7 +187,67 @@ int nct6694_usb_write_msg(struct nct6694 *nct6694,
 
 	return nct6694_usb_err_handling(nct6694, msg->response_header.sts);
 }
-EXPORT_SYMBOL_GPL(nct6694_usb_write_msg);
+
+static int nct6694_usb_regmap_read(void *context, const void *reg_buf,
+				   size_t reg_size, void *val_buf,
+				   size_t val_size)
+{
+	struct nct6694 *nct6694 = context;
+	u32 reg = get_unaligned_be32(reg_buf);
+	const struct nct6694_cmd_header cmd_hd = {
+		.mod = FIELD_GET(NCT6694_REG_MOD, reg),
+		.offset = cpu_to_le16(FIELD_GET(NCT6694_REG_OFFSET, reg)),
+		.len = cpu_to_le16(val_size),
+	};
+
+	/*
+	 * A SET transaction is a request/response exchange: send the payload
+	 * already present in @val_buf and read the response back into it.
+	 */
+	if (FIELD_GET(NCT6694_REG_HCTRL, reg) == NCT6694_HCTRL_SET)
+		return nct6694_usb_write_msg(nct6694, &cmd_hd, val_buf);
+
+	return nct6694_usb_read_msg(nct6694, &cmd_hd, val_buf);
+}
+
+static int nct6694_usb_regmap_write(void *context, const void *data,
+				    size_t count)
+{
+	struct nct6694 *nct6694 = context;
+	struct nct6694_usb_data *udata = nct6694->priv;
+	u32 reg = get_unaligned_be32(data);
+	size_t len = count - sizeof(reg);
+	const struct nct6694_cmd_header cmd_hd = {
+		.mod = FIELD_GET(NCT6694_REG_MOD, reg),
+		.offset = cpu_to_le16(FIELD_GET(NCT6694_REG_OFFSET, reg)),
+		.len = cpu_to_le16(len),
+	};
+
+	if (len > NCT6694_MAX_PACKET_SIZE)
+		return -EINVAL;
+
+	/*
+	 * nct6694_usb_write_msg() reads the firmware response back into the
+	 * payload buffer, so copy the const regmap data into a scratch buffer
+	 * it can write to.
+	 */
+	memcpy(udata->xfer_buf, data + sizeof(reg), len);
+
+	return nct6694_usb_write_msg(nct6694, &cmd_hd, udata->xfer_buf);
+}
+
+static const struct regmap_bus nct6694_usb_regmap_bus = {
+	.read = nct6694_usb_regmap_read,
+	.write = nct6694_usb_regmap_write,
+};
+
+static const struct regmap_config nct6694_usb_regmap_config = {
+	.reg_bits = 32,
+	.val_bits = 8,
+	.reg_stride = 1,
+	.max_raw_read = NCT6694_MAX_PACKET_SIZE,
+	.max_raw_write = NCT6694_MAX_PACKET_SIZE,
+};
 
 static void nct6694_usb_int_callback(struct urb *urb)
 {
@@ -270,6 +305,10 @@ static int nct6694_usb_probe(struct usb_interface *iface,
 	if (!udata->usb_msg)
 		return -ENOMEM;
 
+	udata->xfer_buf = devm_kzalloc(dev, NCT6694_MAX_PACKET_SIZE, GFP_KERNEL);
+	if (!udata->xfer_buf)
+		return -ENOMEM;
+
 	udata->int_buffer = devm_kzalloc(dev, sizeof(*udata->int_buffer), GFP_KERNEL);
 	if (!udata->int_buffer)
 		return -ENOMEM;
@@ -283,9 +322,12 @@ static int nct6694_usb_probe(struct usb_interface *iface,
 	nct6694->dev = dev;
 	nct6694->priv = udata;
 
-	ret = devm_mutex_init(dev, &udata->access_lock);
-	if (ret)
+	nct6694->regmap = devm_regmap_init(dev, &nct6694_usb_regmap_bus, nct6694,
+					   &nct6694_usb_regmap_config);
+	if (IS_ERR(nct6694->regmap)) {
+		ret = PTR_ERR(nct6694->regmap);
 		goto err_urb;
+	}
 
 	interface = iface->cur_altsetting;
 
diff --git a/include/linux/mfd/nct6694.h b/include/linux/mfd/nct6694.h
index 853b1530755d..d655d726d4de 100644
--- a/include/linux/mfd/nct6694.h
+++ b/include/linux/mfd/nct6694.h
@@ -9,7 +9,9 @@
 #ifndef __MFD_NCT6694_H
 #define __MFD_NCT6694_H
 
+#include <linux/bitfield.h>
 #include <linux/idr.h>
+#include <linux/regmap.h>
 #include <linux/spinlock.h>
 #include <linux/types.h>
 
@@ -29,6 +31,9 @@ struct mfd_cell;
 #define NCT6694_HCTRL_SET	0x40
 #define NCT6694_HCTRL_GET	0x80
 
+/* Maximum message payload length exchanged with the firmware in one command */
+#define NCT6694_MAX_PACKET_SIZE	0x3F0
+
 enum nct6694_irq_id {
 	NCT6694_IRQ_GPIO0 = 0,
 	NCT6694_IRQ_GPIO1,
@@ -87,6 +92,7 @@ struct __packed nct6694_response_header {
 
 struct nct6694 {
 	struct device *dev;
+	struct regmap *regmap;
 	struct ida gpio_ida;
 	struct ida i2c_ida;
 	struct ida canfd_ida;
@@ -97,29 +103,64 @@ struct nct6694 {
 	void *priv;
 };
 
-int nct6694_core_probe(struct device *dev, struct nct6694 *nct6694,
-		       const struct mfd_cell *cells, int n_cells);
-void nct6694_core_remove(struct nct6694 *nct6694);
+/*
+ * The NCT6694 firmware is reached through messages that carry a module id
+ * together with an offset (or command/selector pair). These fields are packed
+ * into a single 32-bit regmap register so that the sub-device drivers can use
+ * the standard regmap bulk accessors, while each transport driver implements
+ * the actual I/O through its own regmap bus.
+ *
+ *   bits [31:24]  host control (NCT6694_HCTRL_GET / NCT6694_HCTRL_SET)
+ *   bits [23:16]  module id
+ *   bits [15:0]   offset (low byte = command, high byte = selector)
+ */
+#define NCT6694_REG_HCTRL	GENMASK(31, 24)
+#define NCT6694_REG_MOD		GENMASK(23, 16)
+#define NCT6694_REG_OFFSET	GENMASK(15, 0)
 
-int nct6694_usb_read_msg(struct nct6694 *nct6694,
-			 const struct nct6694_cmd_header *cmd_hd,
-			 void *buf);
-int nct6694_usb_write_msg(struct nct6694 *nct6694,
-			  const struct nct6694_cmd_header *cmd_hd,
-			  void *buf);
+static inline u32 nct6694_cmd_to_reg(const struct nct6694_cmd_header *cmd_hd,
+				     u8 hctrl)
+{
+	return FIELD_PREP(NCT6694_REG_HCTRL, hctrl) |
+	       FIELD_PREP(NCT6694_REG_MOD, cmd_hd->mod) |
+	       FIELD_PREP(NCT6694_REG_OFFSET, le16_to_cpu(cmd_hd->offset));
+}
 
 static inline int nct6694_read_msg(struct nct6694 *nct6694,
 				   const struct nct6694_cmd_header *cmd_hd,
 				   void *buf)
 {
-	return nct6694_usb_read_msg(nct6694, cmd_hd, buf);
+	return regmap_bulk_read(nct6694->regmap,
+				nct6694_cmd_to_reg(cmd_hd, NCT6694_HCTRL_GET),
+				buf, le16_to_cpu(cmd_hd->len));
 }
 
 static inline int nct6694_write_msg(struct nct6694 *nct6694,
 				    const struct nct6694_cmd_header *cmd_hd,
 				    void *buf)
 {
-	return nct6694_usb_write_msg(nct6694, cmd_hd, buf);
+	return regmap_bulk_write(nct6694->regmap,
+				 nct6694_cmd_to_reg(cmd_hd, NCT6694_HCTRL_SET),
+				 buf, le16_to_cpu(cmd_hd->len));
 }
 
+/*
+ * A few firmware commands (e.g. the I2C "deliver") transmit a request and
+ * read the reply back in a single transaction. Model this as a regmap read of
+ * a SET register: @buf carries the request on entry and holds the reply on
+ * return, so the firmware exchange stays a single message as before.
+ */
+static inline int nct6694_write_read_msg(struct nct6694 *nct6694,
+					 const struct nct6694_cmd_header *cmd_hd,
+					 void *buf)
+{
+	return regmap_bulk_read(nct6694->regmap,
+				nct6694_cmd_to_reg(cmd_hd, NCT6694_HCTRL_SET),
+				buf, le16_to_cpu(cmd_hd->len));
+}
+
+int nct6694_core_probe(struct device *dev, struct nct6694 *nct6694,
+		       const struct mfd_cell *cells, int n_cells);
+void nct6694_core_remove(struct nct6694 *nct6694);
+
 #endif
-- 
2.34.1