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 <a.motakis@virtualopensystems.com>
Reviewed-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

3 files changed, 68 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-bus-amba b/Documentation/ABI/testing/sysfs-bus-amba
new file mode 100644
index 000000000000..e7b54677cfbe
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-amba
@@ -0,0 +1,20 @@
+What:           /sys/bus/amba/devices/.../driver_override
+Date:           September 2014
+Contact:        Antonios Motakis <a.motakis@virtualopensystems.com>
+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.
diff --git a/drivers/amba/bus.c b/drivers/amba/bus.c
index 52ddd9fbb55e..f0099360039e 100644
--- a/drivers/amba/bus.c
+++ b/drivers/amba/bus.c
@@ -18,6 +18,7 @@
 #include <linux/pm_domain.h>
 #include <linux/amba/bus.h>
 #include <linux/sizes.h>
+#include <linux/limits.h>
 
 #include <asm/irq.h>
 
@@ -43,6 +44,10 @@ static int amba_match(struct device *dev, struct device_driver *drv)
         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;
 }
 
@@ -59,6 +64,47 @@ static int amba_uevent(struct device *dev, struct kobj_uevent_env *env)
         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 ssize_t name##_show(struct device *_dev,                         \
                            struct device_attribute *attr, char *buf)    \
@@ -81,6 +127,7 @@ amba_attr_func(resource, "\t%016llx\t%016llx\t%016lx\n",
 static struct device_attribute amba_dev_attrs[] = {
         __ATTR_RO(id),
         __ATTR_RO(resource),
+        __ATTR_RW(driver_override),
         __ATTR_NULL,
 };
 
diff --git a/include/linux/amba/bus.h b/include/linux/amba/bus.h
index 0ab5f8e0dea2..50fc66868402 100644
--- a/include/linux/amba/bus.h
+++ b/include/linux/amba/bus.h
@@ -33,6 +33,7 @@ struct amba_device {
         struct clk              *pclk;
         unsigned int            periphid;
         unsigned int            irq[AMBA_NR_IRQS];
+        char                    *driver_override;
 };
 
 struct amba_driver {