On Fri, Jan 31, 2020 at 4:34 AM Chris Packham
[off-list ref] wrote:

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.

Thanks for an update, my comments below.

...

 #
 # Add new SPI master controllers in alphabetical order above this line
 #

+#

Redundant line.

...

+/*
+ * General Purpose SPI multiplexer
+ */

I think Mark wants to have this in one line with C++ style of comments.

...

+#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>

Perhaps sorted?

...

+       /* do the transfer */
+       ret = spi_async(priv->spi, m);
+       return ret;

return spi_async(...);

...

+       priv->mux = devm_mux_control_get(&spi->dev, NULL);

+       ret = PTR_ERR_OR_ZERO(priv->mux);

This is a bit complicated.

+       if (ret) {

Why not simple do

  if (IS_ERR(priv->mux)) {
    ret = PTR_ERR(...);
    ...
  }

?

+               if (ret != -EPROBE_DEFER)
+                       dev_err(&spi->dev, "failed to get control-mux\n");
+               goto err_put_ctlr;
+       }

+       ctlr->dev.of_node = spi->dev.of_node;

I'm wondering why SPI core can't handle this by default (like GPIO
subsystem does).

+       ret = devm_spi_register_controller(&spi->dev, ctlr);
+       if (ret)
+               goto err_put_ctlr;
+

+       return ret;

return 0;

...

+static const struct of_device_id spi_mux_of_match[] = {
+       { .compatible = "spi-mux" },
+       { },

Comma is not needed in terminator line.

+};

-- 
With Best Regards,
Andy Shevchenko