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Subject[PATCH 2/2] spi: Add generic SPI multiplexer
Date
Add a SPI device driver that sits in-band and provides a SPI controller
which supports chip selects via a mux-control. This enables extra SPI
devices to be connected with limited native chip selects.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
---
drivers/spi/Kconfig | 12 +++
drivers/spi/Makefile | 1 +
drivers/spi/spi-mux.c | 189 ++++++++++++++++++++++++++++++++++++++++++
3 files changed, 202 insertions(+)
create mode 100644 drivers/spi/spi-mux.c

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");
--
2.25.0
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