spi: Add driver_override SPI device attribute

This attribute works the same was as the identically named attribute
for PCI, AMBA, and platform devices.  For reference, see:

commit 3cf38571 ("ARM: 8256/1: driver coamba: add device binding
path 'driver_override'")
commit 3d713e0e ("driver core: platform: add device binding path
'driver_override'")
commit 782a985d ("PCI: Introduce new device binding path using
pci_dev.driver_override")

If the name of a driver is written to this attribute, then the device
will bind to the named driver and only the named driver.

The device will bind to the driver even if the driver does not list the
device in its id table.  This behavior is different than the driver's
bind attribute, which only allows binding to devices that are listed as
supported by the driver.

It can be used to bind a generic driver, like spidev, to a device.
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -51,6 +51,7 @@ static void spidev_release(struct device *dev)
 		spi->controller->cleanup(spi);
 
 	spi_controller_put(spi->controller);
+	kfree(spi->driver_override);
 	kfree(spi);
 }
 
@@ -68,6 +69,51 @@ modalias_show(struct device *dev, struct device_attribute *a, char *buf)
 }
 static DEVICE_ATTR_RO(modalias);
 
+static ssize_t driver_override_store(struct device *dev,
+				     struct device_attribute *a,
+				     const char *buf, size_t count)
+{
+	struct spi_device *spi = to_spi_device(dev);
+	const char *end = memchr(buf, '\n', count);
+	const size_t len = end ? end - buf : count;
+	const char *driver_override, *old;
+
+	/* We need to keep extra room for a newline when displaying value */
+	if (len >= (PAGE_SIZE - 1))
+		return -EINVAL;
+
+	driver_override = kstrndup(buf, len, GFP_KERNEL);
+	if (!driver_override)
+		return -ENOMEM;
+
+	device_lock(dev);
+	old = spi->driver_override;
+	if (len) {
+		spi->driver_override = driver_override;
+	} else {
+		/* Emptry string, disable driver override */
+		spi->driver_override = NULL;
+		kfree(driver_override);
+	}
+	device_unlock(dev);
+	kfree(old);
+
+	return count;
+}
+
+static ssize_t driver_override_show(struct device *dev,
+				    struct device_attribute *a, char *buf)
+{
+	const struct spi_device *spi = to_spi_device(dev);
+	ssize_t len;
+
+	device_lock(dev);
+	len = snprintf(buf, PAGE_SIZE, "%s\n", spi->driver_override ? : "");
+	device_unlock(dev);
+	return len;
+}
+static DEVICE_ATTR_RW(driver_override);
+
 #define SPI_STATISTICS_ATTRS(field, file)				\
 static ssize_t spi_controller_##field##_show(struct device *dev,	\
 					     struct device_attribute *attr, \
@@ -149,6 +195,7 @@ SPI_STATISTICS_SHOW(transfers_split_maxsize, "%lu");
 
 static struct attribute *spi_dev_attrs[] = {
 	&dev_attr_modalias.attr,
+	&dev_attr_driver_override.attr,
 	NULL,
 };
 
@@ -296,6 +343,10 @@ static int spi_match_device(struct device *dev, struct device_driver *drv)
 	const struct spi_device	*spi = to_spi_device(dev);
 	const struct spi_driver	*sdrv = to_spi_driver(drv);
 
+	/* Check override first, and if set, only use the named driver */
+	if (spi->driver_override)
+		return strcmp(spi->driver_override, drv->name) == 0;
+
 	/* Attempt an OF style match */
 	if (of_driver_match_device(dev, drv))
 		return 1;

include/linux/spi/spi.h

@@ -159,6 +159,7 @@ struct spi_device {
 	void			*controller_state;
 	void			*controller_data;
 	char			modalias[SPI_NAME_SIZE];
+	const char		*driver_override;
 	int			cs_gpio;	/* chip select gpio */
 
 	/* the statistics */