[2/2] spi: Add generic SPI multiplexer

diff --git a/drivers/spi/Kconfig b/drivers/spi/Kconfig
index 870f7797b56b..90df945490d9 100644
--- a/drivers/spi/Kconfig
+++ b/drivers/spi/Kconfig
@@ -880,6 +880,18 @@  config SPI_ZYNQMP_GQSPI
 #
 # Add new SPI master controllers in alphabetical order above this line
 #
+#
+
+comment "SPI Multiplexer support"
+
+config SPI_MUX
+	tristate "SPI multiplexer support"
+	select MULTIPLEXER
+	help
+	  This adds support for SPI multiplexers. Each SPI mux will be
+	  accessible as a SPI controller, the devices behind the mux will appear
+	  to be chip selects on this controller. It is still necessary to
+	  select one or more specific mux-controller drivers.
 
 #
 # There are lots of SPI device types, with sensors and memory
diff --git a/drivers/spi/Makefile b/drivers/spi/Makefile
index bb49c9e6d0a0..5f7593c84210 100644
--- a/drivers/spi/Makefile
+++ b/drivers/spi/Makefile
@@ -11,6 +11,7 @@  obj-$(CONFIG_SPI_MASTER)		+= spi.o
 obj-$(CONFIG_SPI_MEM)			+= spi-mem.o
 obj-$(CONFIG_SPI_SPIDEV)		+= spidev.o
 obj-$(CONFIG_SPI_LOOPBACK_TEST)		+= spi-loopback-test.o
+obj-$(CONFIG_SPI_MUX)			+= spi-mux.o
 
 # SPI master controller drivers (bus)
 obj-$(CONFIG_SPI_ALTERA)		+= spi-altera.o
diff --git a/drivers/spi/spi-mux.c b/drivers/spi/spi-mux.c
new file mode 100644
index 000000000000..8481067be5ae
--- /dev/null
+++ b/drivers/spi/spi-mux.c
@@ -0,0 +1,189 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * General Purpose SPI multiplexer
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/err.h>
+#include <linux/slab.h>
+#include <linux/spi/spi.h>
+#include <linux/mux/consumer.h>
+
+#define SPI_MUX_NO_CS	((unsigned int)-1)
+
+/**
+ * DOC: Driver description
+ *
+ * This driver supports a MUX on an SPI bus. This can be useful when you need
+ * more chip selects than the hardware peripherals support, or than are
+ * available in a particular board setup.
+ *
+ * The driver will create an additional SPI controller. Devices added under the
+ * mux will be handled as 'chip selects' on this controller.
+ */
+
+/**
+ * struct spi_mux_priv - the basic spi_mux structure
+ * @spi:		pointer to the device struct attached to the parent
+ *			spi controller
+ * @current_cs:		The current chip select set in the mux
+ * @child_mesg_complete: The mux replaces the complete callback in the child's
+ *			message to its own callback; this field is used by the
+ *			driver to store the child's callback during a transfer
+ * @child_mesg_context: Used to store the child's context to the callback
+ * @child_mesg_dev:	Used to store the spi_device pointer to the child
+ * @mux:		mux_control structure used to provide chip selects for
+ *			downstream spi devices
+ */
+struct spi_mux_priv {
+	struct spi_device	*spi;
+	unsigned int		current_cs;
+
+	void			(*child_mesg_complete)(void *context);
+	void			*child_mesg_context;
+	struct spi_device	*child_mesg_dev;
+	struct mux_control	*mux;
+};
+
+/* should not get called when the parent controller is doing a transfer */
+static int spi_mux_select(struct spi_device *spi)
+{
+	struct spi_mux_priv *priv = spi_controller_get_devdata(spi->controller);
+	int ret = 0;
+
+	if (priv->current_cs != spi->chip_select) {
+		dev_dbg(&priv->spi->dev,
+			"setting up the mux for cs %d\n",
+			spi->chip_select);
+
+		/* copy the child device's settings except for the cs */
+		priv->spi->max_speed_hz = spi->max_speed_hz;
+		priv->spi->mode = spi->mode;
+		priv->spi->bits_per_word = spi->bits_per_word;
+
+		ret = mux_control_select(priv->mux, spi->chip_select);
+		if (ret)
+			return ret;
+
+		priv->current_cs = spi->chip_select;
+	}
+
+	return ret;
+}
+
+static int spi_mux_setup(struct spi_device *spi)
+{
+	struct spi_mux_priv *priv = spi_controller_get_devdata(spi->controller);
+
+	/*
+	 * can be called multiple times, won't do a valid setup now but we will
+	 * change the settings when we do a transfer (necessary because we
+	 * can't predict from which device it will be anyway)
+	 */
+	return spi_setup(priv->spi);
+}
+
+static void spi_mux_complete_cb(void *context)
+{
+	struct spi_mux_priv *priv = (struct spi_mux_priv *)context;
+	struct spi_controller *ctlr = spi_get_drvdata(priv->spi);
+	struct spi_message *m = ctlr->cur_msg;
+
+	m->complete = priv->child_mesg_complete;
+	m->context = priv->child_mesg_context;
+	m->spi = priv->child_mesg_dev;
+	spi_finalize_current_message(ctlr);
+	mux_control_deselect(priv->mux);
+}
+
+static int spi_mux_transfer_one_message(struct spi_controller *ctlr,
+						struct spi_message *m)
+{
+	struct spi_mux_priv *priv = spi_controller_get_devdata(ctlr);
+	struct spi_device *spi = m->spi;
+	int ret;
+
+	ret = spi_mux_select(spi);
+	if (ret)
+		return ret;
+
+	/*
+	 * Replace the complete callback, context and spi_device with our own
+	 * pointers. Save originals
+	 */
+	priv->child_mesg_complete = m->complete;
+	priv->child_mesg_context = m->context;
+	priv->child_mesg_dev = m->spi;
+
+	m->complete = spi_mux_complete_cb;
+	m->context = priv;
+	m->spi = priv->spi;
+
+	/* do the transfer */
+	ret = spi_async(priv->spi, m);
+	return ret;
+}
+
+static int spi_mux_probe(struct spi_device *spi)
+{
+	struct spi_controller *ctlr;
+	struct spi_mux_priv *priv;
+	int ret;
+
+	ctlr = spi_alloc_master(&spi->dev, sizeof(*priv));
+	if (!ctlr)
+		return -ENOMEM;
+
+	spi_set_drvdata(spi, ctlr);
+	priv = spi_controller_get_devdata(ctlr);
+	priv->spi = spi;
+
+	priv->mux = devm_mux_control_get(&spi->dev, NULL);
+	ret = PTR_ERR_OR_ZERO(priv->mux);
+	if (ret) {
+		if (ret != -EPROBE_DEFER)
+			dev_err(&spi->dev, "failed to get control-mux\n");
+		goto err_put_ctlr;
+	}
+
+	priv->current_cs = SPI_MUX_NO_CS;
+
+	/* supported modes are the same as our parent's */
+	ctlr->mode_bits = spi->controller->mode_bits;
+	ctlr->flags = spi->controller->flags;
+	ctlr->transfer_one_message = spi_mux_transfer_one_message;
+	ctlr->setup = spi_mux_setup;
+	ctlr->num_chipselect = mux_control_states(priv->mux);
+	ctlr->bus_num = -1;
+	ctlr->dev.of_node = spi->dev.of_node;
+
+	ret = devm_spi_register_controller(&spi->dev, ctlr);
+	if (ret)
+		goto err_put_ctlr;
+
+	return ret;
+
+err_put_ctlr:
+	spi_controller_put(ctlr);
+
+	return ret;
+}
+
+static const struct of_device_id spi_mux_of_match[] = {
+	{ .compatible = "spi-mux" },
+	{ },
+};
+
+static struct spi_driver spi_mux_driver = {
+	.probe  = spi_mux_probe,
+	.driver = {
+		.name   = "spi-mux",
+		.of_match_table = spi_mux_of_match,
+	},
+};
+
+module_spi_driver(spi_mux_driver);
+
+MODULE_DESCRIPTION("SPI multiplexer");
+MODULE_LICENSE("GPL");