[PATCH v2 0/2] SPI bus multiplexing

[PATCH v2 0/2] SPI bus multiplexing

[Chris Packham]

This is a revisit of two earlier efforts to add more complex mutliplexing to
SPI busses.

https://patchwork.kernel.org/patch/2706151/
https://patchwork.kernel.org/patch/10897255/

This version makes use of the general purpose mux controller which simplifies
things greatly.

One outstanding problem is the need for the spi-max-frequency on the mux
device. This is not used for anything and is just needed to satisfy the spi
driver infrastructure which expects all spi devices to have this property.

Chris Packham (2):
  dt-bindings: spi: Document binding for generic SPI multiplexer
  spi: Add generic SPI multiplexer

 .../devicetree/bindings/spi/spi-mux.yaml      |  84 ++++++++
 drivers/spi/Kconfig                           |  12 ++
 drivers/spi/Makefile                          |   1 +
 drivers/spi/spi-mux.c                         | 189 ++++++++++++++++++
 4 files changed, 286 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/spi/spi-mux.yaml
 create mode 100644 drivers/spi/spi-mux.c

-- 
2.25.0

[PATCH v2 1/2] dt-bindings: spi: Document binding for generic SPI multiplexer

[Chris Packham]

Add binding documentation for the spi-mux driver. This allows a generic
multiplexer to be used to provide access to multiple SPI devices.

Signed-off-by: Chris Packham <chris.packham-6g8wRflRTwXFdCa3tKVlE6U/zSkkHjvu@public.gmane.org>
---

Notes:
    Changes in v2:
    - update license
    - make dt_binding_check clean

 .../devicetree/bindings/spi/spi-mux.yaml      | 84 +++++++++++++++++++
 1 file changed, 84 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/spi/spi-mux.yaml

diff --git a/Documentation/devicetree/bindings/spi/spi-mux.yaml b/Documentation/devicetree/bindings/spi/spi-mux.yaml
new file mode 100644
index 000000000000..4bdb31e2257e
--- /dev/null
+++ b/Documentation/devicetree/bindings/spi/spi-mux.yaml
@@ -0,0 +1,84 @@
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/spi-mux.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Generic SPI Multiplexer
+
+description: |
+  This binding describes a SPI bus multiplexer to route the SPI chip select
+  signals. This can be used when you need more devices than the SPI controller
+  has chip selects available. An example setup is shown in ASCII art; the actual
+  setting of the multiplexer to a channel needs to be done by a specific SPI mux
+  driver.
+
+        MOSI /--------------------------------+--------+--------+--------\
+        MISO |/------------------------------+|-------+|-------+|-------\|
+         SCL ||/----------------------------+||------+||------+||------\||
+             |||                            |||      |||      |||      |||
+      +------------+                        |||      |||      |||      |||
+      | SoC  |||   |                      +-+++-+  +-+++-+  +-+++-+  +-+++-+
+      |      |||   |                      | dev |  | dev |  | dev |  | dev |
+      |   +--+++-+ | CS-X  +------+\      +--+--+  +--+--+  +--+--+  +--+--+
+      |   | SPI  +-|-------+ Mux  |\\   CS-0 |        |        |        |
+      |   +------+ |       +--+---+\\\-------/   CS-1 |        |        |
+      |            |          |    \\\----------------/   CS-2 |        |
+      |   +------+ |          |     \\-------------------------/   CS-3 |
+      |   | ?    +-|----------/      \----------------------------------/
+      |   +------+ |
+      +------------+
+
+allOf:
+  - $ref: "/schemas/spi/spi-controller.yaml#"
+
+maintainers:
+  - Chris Packham <chris.packham-6g8wRflRTwXFdCa3tKVlE6U/zSkkHjvu@public.gmane.org>
+
+properties:
+  compatible:
+    const: spi-mux
+
+  mux-controls:
+    maxItems: 1
+
+required:
+   - compatible
+   - reg
+   - spi-max-frequency
+   - mux-controls
+
+examples:
+   - |
+     mux: mux-controller {
+       compatible = "gpio-mux";
+       #mux-control-cells = <0>;
+
+       mux-gpios = <&gpio0 3 GPIO_ACTIVE_HIGH>;
+     };
+
+     spi {
+       spi-mux@0 {
+         compatible = "spi-mux";
+         #address-cells = <1>;
+         #size-cells = <0>;
+         reg = <0>;
+         spi-max-frequency = <100000000>;
+
+         mux-controls = <&mux>
+
+         spi-flash@0 {
+           compatible = "jedec,spi-nor";
+           #address-cells = <1>;
+           #size-cells = <1>;
+           reg = <0>;
+           spi-max-frequency = <40000000>;
+         };
+
+         spi-device@1 {
+           compatible = "lineartechnology,ltc2488";
+           reg = <1>;
+           spi-max-frequency = <10000000>;
+         };
+       };
+     };
-- 
2.25.0

[PATCH v2 2/2] spi: Add generic SPI multiplexer

[Chris Packham]

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-6g8wRflRTwXFdCa3tKVlE6U/zSkkHjvu@public.gmane.org>
---
 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

Re: [PATCH v2 1/2] dt-bindings: spi: Document binding for generic SPI multiplexer

[Rob Herring]

On Wed, Jan 22, 2020 at 7:24 PM Chris Packham
<chris.packham@alliedtelesis.co.nz> wrote:
>
> Add binding documentation for the spi-mux driver. This allows a generic
> multiplexer to be used to provide access to multiple SPI devices.
>
> Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
> ---
>
> Notes:
>     Changes in v2:
>     - update license
>     - make dt_binding_check clean

Sure about that?

>
>  .../devicetree/bindings/spi/spi-mux.yaml      | 84 +++++++++++++++++++
>  1 file changed, 84 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/spi/spi-mux.yaml
>
> diff --git a/Documentation/devicetree/bindings/spi/spi-mux.yaml b/Documentation/devicetree/bindings/spi/spi-mux.yaml
> new file mode 100644
> index 000000000000..4bdb31e2257e
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/spi/spi-mux.yaml
> @@ -0,0 +1,84 @@
> +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/spi/spi-mux.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Generic SPI Multiplexer
> +
> +description: |
> +  This binding describes a SPI bus multiplexer to route the SPI chip select
> +  signals. This can be used when you need more devices than the SPI controller
> +  has chip selects available. An example setup is shown in ASCII art; the actual
> +  setting of the multiplexer to a channel needs to be done by a specific SPI mux
> +  driver.
> +
> +        MOSI /--------------------------------+--------+--------+--------\
> +        MISO |/------------------------------+|-------+|-------+|-------\|
> +         SCL ||/----------------------------+||------+||------+||------\||
> +             |||                            |||      |||      |||      |||
> +      +------------+                        |||      |||      |||      |||
> +      | SoC  |||   |                      +-+++-+  +-+++-+  +-+++-+  +-+++-+
> +      |      |||   |                      | dev |  | dev |  | dev |  | dev |
> +      |   +--+++-+ | CS-X  +------+\      +--+--+  +--+--+  +--+--+  +--+--+
> +      |   | SPI  +-|-------+ Mux  |\\   CS-0 |        |        |        |
> +      |   +------+ |       +--+---+\\\-------/   CS-1 |        |        |
> +      |            |          |    \\\----------------/   CS-2 |        |
> +      |   +------+ |          |     \\-------------------------/   CS-3 |
> +      |   | ?    +-|----------/      \----------------------------------/
> +      |   +------+ |
> +      +------------+
> +
> +allOf:
> +  - $ref: "/schemas/spi/spi-controller.yaml#"
> +
> +maintainers:
> +  - Chris Packham <chris.packham@alliedtelesis.co.nz>
> +
> +properties:
> +  compatible:
> +    const: spi-mux
> +
> +  mux-controls:
> +    maxItems: 1
> +
> +required:
> +   - compatible
> +   - reg
> +   - spi-max-frequency
> +   - mux-controls
> +
> +examples:
> +   - |
> +     mux: mux-controller {
> +       compatible = "gpio-mux";
> +       #mux-control-cells = <0>;
> +
> +       mux-gpios = <&gpio0 3 GPIO_ACTIVE_HIGH>;
> +     };
> +
> +     spi {
> +       spi-mux@0 {
> +         compatible = "spi-mux";
> +         #address-cells = <1>;
> +         #size-cells = <0>;
> +         reg = <0>;
> +         spi-max-frequency = <100000000>;
> +
> +         mux-controls = <&mux>

Missing semicolon.

> +
> +         spi-flash@0 {
> +           compatible = "jedec,spi-nor";
> +           #address-cells = <1>;
> +           #size-cells = <1>;
> +           reg = <0>;
> +           spi-max-frequency = <40000000>;
> +         };
> +
> +         spi-device@1 {
> +           compatible = "lineartechnology,ltc2488";
> +           reg = <1>;
> +           spi-max-frequency = <10000000>;
> +         };
> +       };
> +     };
> --
> 2.25.0
>

Re: [PATCH v2 1/2] dt-bindings: spi: Document binding for generic SPI multiplexer

[Chris Packham]

On Thu, 2020-01-23 at 07:51 -0600, Rob Herring wrote:
> On Wed, Jan 22, 2020 at 7:24 PM Chris Packham
> <chris.packham@alliedtelesis.co.nz> wrote:
> > 
> > Add binding documentation for the spi-mux driver. This allows a generic
> > multiplexer to be used to provide access to multiple SPI devices.
> > 
> > Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
> > ---
> > 
> > Notes:
> >     Changes in v2:
> >     - update license
> >     - make dt_binding_check clean
> 
> Sure about that?
> 

I was when I wrote that, but now I think I need to consult my spell
book.

It appears the incantation I should be using is

  make ARCH=arm defconfig
  make ARCH=arm dt_binding_check \
     DT_SCHEMA_FILES=Documentation/devicetree/bindings/spi/spi-mux.yaml

I can see the simple failures (not sure how I missed them). But one
that remains is:

Warning (spi_bus_bridge): /example-0/spi/spi-mux@0: node name for SPI
buses should be 'spi'

I could fix that by having

 spi {
    spi@0 {
       compatible = "spi-mux";
    };
 };

Is that what we want? Or should I be adding awareness of spi-muxes to
dtc?

> > 
> >  .../devicetree/bindings/spi/spi-mux.yaml      | 84 +++++++++++++++++++
> >  1 file changed, 84 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/spi/spi-mux.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/spi/spi-mux.yaml b/Documentation/devicetree/bindings/spi/spi-mux.yaml
> > new file mode 100644
> > index 000000000000..4bdb31e2257e
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/spi/spi-mux.yaml
> > @@ -0,0 +1,84 @@
> > +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/spi/spi-mux.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: Generic SPI Multiplexer
> > +
> > +description: |
> > +  This binding describes a SPI bus multiplexer to route the SPI chip select
> > +  signals. This can be used when you need more devices than the SPI controller
> > +  has chip selects available. An example setup is shown in ASCII art; the actual
> > +  setting of the multiplexer to a channel needs to be done by a specific SPI mux
> > +  driver.
> > +
> > +        MOSI /--------------------------------+--------+--------+--------\
> > +        MISO |/------------------------------+|-------+|-------+|-------\|
> > +         SCL ||/----------------------------+||------+||------+||------\||
> > +             |||                            |||      |||      |||      |||
> > +      +------------+                        |||      |||      |||      |||
> > +      | SoC  |||   |                      +-+++-+  +-+++-+  +-+++-+  +-+++-+
> > +      |      |||   |                      | dev |  | dev |  | dev |  | dev |
> > +      |   +--+++-+ | CS-X  +------+\      +--+--+  +--+--+  +--+--+  +--+--+
> > +      |   | SPI  +-|-------+ Mux  |\\   CS-0 |        |        |        |
> > +      |   +------+ |       +--+---+\\\-------/   CS-1 |        |        |
> > +      |            |          |    \\\----------------/   CS-2 |        |
> > +      |   +------+ |          |     \\-------------------------/   CS-3 |
> > +      |   | ?    +-|----------/      \----------------------------------/
> > +      |   +------+ |
> > +      +------------+
> > +
> > +allOf:
> > +  - $ref: "/schemas/spi/spi-controller.yaml#"
> > +
> > +maintainers:
> > +  - Chris Packham <chris.packham@alliedtelesis.co.nz>
> > +
> > +properties:
> > +  compatible:
> > +    const: spi-mux
> > +
> > +  mux-controls:
> > +    maxItems: 1
> > +
> > +required:
> > +   - compatible
> > +   - reg
> > +   - spi-max-frequency
> > +   - mux-controls
> > +
> > +examples:
> > +   - |
> > +     mux: mux-controller {
> > +       compatible = "gpio-mux";
> > +       #mux-control-cells = <0>;
> > +
> > +       mux-gpios = <&gpio0 3 GPIO_ACTIVE_HIGH>;
> > +     };
> > +
> > +     spi {
> > +       spi-mux@0 {
> > +         compatible = "spi-mux";
> > +         #address-cells = <1>;
> > +         #size-cells = <0>;
> > +         reg = <0>;
> > +         spi-max-frequency = <100000000>;
> > +
> > +         mux-controls = <&mux>
> 
> Missing semicolon.
> 
> > +
> > +         spi-flash@0 {
> > +           compatible = "jedec,spi-nor";
> > +           #address-cells = <1>;
> > +           #size-cells = <1>;
> > +           reg = <0>;
> > +           spi-max-frequency = <40000000>;
> > +         };
> > +
> > +         spi-device@1 {
> > +           compatible = "lineartechnology,ltc2488";
> > +           reg = <1>;
> > +           spi-max-frequency = <10000000>;
> > +         };
> > +       };
> > +     };
> > --
> > 2.25.0
> > 

Re: [PATCH v2 1/2] dt-bindings: spi: Document binding for generic SPI multiplexer

[Rob Herring]

On Sun, Jan 26, 2020 at 2:24 PM Chris Packham
<Chris.Packham@alliedtelesis.co.nz> wrote:
>
> On Thu, 2020-01-23 at 07:51 -0600, Rob Herring wrote:
> > On Wed, Jan 22, 2020 at 7:24 PM Chris Packham
> > <chris.packham@alliedtelesis.co.nz> wrote:
> > >
> > > Add binding documentation for the spi-mux driver. This allows a generic
> > > multiplexer to be used to provide access to multiple SPI devices.
> > >
> > > Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
> > > ---
> > >
> > > Notes:
> > >     Changes in v2:
> > >     - update license
> > >     - make dt_binding_check clean
> >
> > Sure about that?
> >
>
> I was when I wrote that, but now I think I need to consult my spell
> book.
>
> It appears the incantation I should be using is
>
>   make ARCH=arm defconfig
>   make ARCH=arm dt_binding_check \
>      DT_SCHEMA_FILES=Documentation/devicetree/bindings/spi/spi-mux.yaml

Note that you need to run this also without DT_SCHEMA_FILES set so the
example is checked against all schema. With that, the 'spi' node is
going to need #address-cells and #size-cells.

>
> I can see the simple failures (not sure how I missed them). But one
> that remains is:
>
> Warning (spi_bus_bridge): /example-0/spi/spi-mux@0: node name for SPI
> buses should be 'spi'
>
> I could fix that by having
>
>  spi {
>     spi@0 {
>        compatible = "spi-mux";
>     };
>  };
>
> Is that what we want? Or should I be adding awareness of spi-muxes to
> dtc?

We should probably relax dtc, but for now I'd just use 'spi'.

Rob