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ARM: 8256/1: driver coamba: add device binding path 'driver_override'
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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>
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Antonios Motakis authored and Russell King committed Feb 10, 2015
1 parent 8684014 commit 3cf3857
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20 changes: 20 additions & 0 deletions Documentation/ABI/testing/sysfs-bus-amba
Original file line number Diff line number Diff line change
@@ -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.
47 changes: 47 additions & 0 deletions drivers/amba/bus.c
Original file line number Diff line number Diff line change
Expand Up @@ -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>

Expand All @@ -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;
}

Expand All @@ -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) \
Expand All @@ -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,
};

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1 change: 1 addition & 0 deletions include/linux/amba/bus.h
Original file line number Diff line number Diff line change
Expand Up @@ -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 {
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