ARM: 8256/1: driver coamba: add device binding path 'driver_override'

As already demonstrated with PCI [1] and the platform bus [2], a
driver_override property in sysfs can be used to bypass the id
matching of a device to a AMBA driver. This can be used by VFIO to
bind to any AMBA device requested by the user.

[1] http://lists-archives.com/linux-kernel/28030441-pci-introduce-new-device-binding-path-using-pci_dev-driver_override.html
[2] https://www.redhat.com/archives/libvir-list/2014-April/msg00382.html

Signed-off-by: Antonios Motakis <[e-mail]>
Reviewed-by: Kim Phillips <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

Documentation/ABI/testing/sysfs-bus-amba

···
+What:		/sys/bus/amba/devices/.../driver_override
+Date:		September 2014
+Contact:	Antonios Motakis <[e-mail]>
+Description:
+		This file allows the driver for a device to be specified which
+		will override standard OF, ACPI, ID table, and name matching.
+		When specified, only a driver with a name matching the value
+		written to driver_override will have an opportunity to bind to
+		the device. The override is specified by writing a string to the
+		driver_override file (echo vfio-amba > driver_override)	and may
+		be cleared with an empty string (echo > driver_override).
+		This returns the device to standard matching rules binding.
+		Writing to driver_override does not automatically unbind the
+		device from its current driver or make any attempt to
+		automatically load the specified driver. If no driver with a
+		matching name is currently loaded in the kernel, the device will
+		not bind to any driver. This also allows devices to opt-out of
+		driver binding using a driver_override name such as "none".
+		Only a single driver may be specified in the override, there is
+		no support for parsing delimiters.

drivers/amba/bus.c

···
 #include <linux/pm_domain.h>
 #include <linux/amba/bus.h>
 #include <linux/sizes.h>
+#include <linux/limits.h>
 
 #include <asm/irq.h>
 
···
 	struct amba_device *pcdev = to_amba_device(dev);
 	struct amba_driver *pcdrv = to_amba_driver(drv);
 
+	/* When driver_override is set, only bind to the matching driver */
+	if (pcdev->driver_override)
+		return !strcmp(pcdev->driver_override, drv->name);
+
 	return amba_lookup(pcdrv->id_table, pcdev) != NULL;
 }
 
···
 
 	retval = add_uevent_var(env, "MODALIAS=amba:d%08X", pcdev->periphid);
 	return retval;
+}
+
+static ssize_t driver_override_show(struct device *_dev,
+				    struct device_attribute *attr, char *buf)
+{
+	struct amba_device *dev = to_amba_device(_dev);
+
+	if (!dev->driver_override)
+		return 0;
+
+	return sprintf(buf, "%s\n", dev->driver_override);
+}
+
+static ssize_t driver_override_store(struct device *_dev,
+				     struct device_attribute *attr,
+				     const char *buf, size_t count)
+{
+	struct amba_device *dev = to_amba_device(_dev);
+	char *driver_override, *old = dev->driver_override, *cp;
+
+	if (count > PATH_MAX)
+		return -EINVAL;
+
+	driver_override = kstrndup(buf, count, GFP_KERNEL);
+	if (!driver_override)
+		return -ENOMEM;
+
+	cp = strchr(driver_override, '\n');
+	if (cp)
+		*cp = '\0';
+
+	if (strlen(driver_override)) {
+		dev->driver_override = driver_override;
+	} else {
+	       kfree(driver_override);
+	       dev->driver_override = NULL;
+	}
+
+	kfree(old);
+
+	return count;
 }
 
 #define amba_attr_func(name,fmt,arg...)					\
···
 static struct device_attribute amba_dev_attrs[] = {
 	__ATTR_RO(id),
 	__ATTR_RO(resource),
+	__ATTR_RW(driver_override),
 	__ATTR_NULL,
 };
 

include/linux/amba/bus.h

···
 	struct clk		*pclk;
 	unsigned int		periphid;
 	unsigned int		irq[AMBA_NR_IRQS];
+	char			*driver_override;
 };
 
 struct amba_driver {