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Subject[PATCH] driver core: platform: add device binding path 'driver_override'
Needed by platform device drivers, such as the vfio-platform driver [1],
in order to bypass the existing OF, ACPI, id_table and name string matches,
and successfully be able to be bound to any device, like so:

echo vfio-platform > /sys/bus/platform/devices/fff51000.ethernet/driver_override
echo fff51000.ethernet > /sys/bus/platform/devices/fff51000.ethernet/driver/unbind
echo fff51000.ethernet > /sys/bus/platform/drivers_probe

This mimics "PCI: Introduce new device binding path using
pci_dev.driver_override" [2], which is an interface enhancement
for more deterministic PCI device binding, e.g., when in the
presence of hotplug.

[1] http://lkml.iu.edu/hypermail/linux/kernel/1402.1/00177.html
[2] http://lists-archives.com/linux-kernel/28030441-pci-introduce-new-device-binding-path-using-pci_dev-driver_override.html

Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
---
changes since RFC:
- fixed message Subject to properly reflect a new platform device
patch (instead of leaving it as a reply to the PCI version)
- addressed Guenter Roeck's comment
- updated [2] with link to later (Apr.4th) revision of the PCI patch
- updated documentation to address Christoffer Dall's comments to
previous version of [2]
- added a Suggested-by, and re-posted as a reply to the PCI patch,
should they be applied together

Documentation/ABI/testing/sysfs-bus-platform | 20 ++++++++++++
drivers/base/platform.c | 46 ++++++++++++++++++++++++++++
include/linux/platform_device.h | 1 +
3 files changed, 67 insertions(+)
create mode 100644 Documentation/ABI/testing/sysfs-bus-platform

diff --git a/Documentation/ABI/testing/sysfs-bus-platform b/Documentation/ABI/testing/sysfs-bus-platform
new file mode 100644
index 0000000..5172a61
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-platform
@@ -0,0 +1,20 @@
+What: /sys/bus/platform/devices/.../driver_override
+Date: April 2014
+Contact: Kim Phillips <kim.phillips@freescale.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-platform > \
+ 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/base/platform.c b/drivers/base/platform.c
index e714709..a0909cb 100644
--- a/drivers/base/platform.c
+++ b/drivers/base/platform.c
@@ -22,6 +22,7 @@
#include <linux/pm_runtime.h>
#include <linux/idr.h>
#include <linux/acpi.h>
+#include <linux/limits.h>

#include "base.h"
#include "power/power.h"
@@ -690,8 +691,49 @@ static ssize_t modalias_show(struct device *dev, struct device_attribute *a,
}
static DEVICE_ATTR_RO(modalias);

+static ssize_t driver_override_store(struct device *dev,
+ struct device_attribute *attr,
+ const char *buf, size_t count)
+{
+ struct platform_device *pdev = to_platform_device(dev);
+ char *driver_override, *old = pdev->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)) {
+ pdev->driver_override = driver_override;
+ } else {
+ kfree(driver_override);
+ pdev->driver_override = NULL;
+ }
+
+ kfree(old);
+
+ return count;
+}
+
+static ssize_t driver_override_show(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ struct platform_device *pdev = to_platform_device(dev);
+
+ return sprintf(buf, "%s\n", pdev->driver_override);
+}
+static DEVICE_ATTR_RW(driver_override);
+
+
static struct attribute *platform_dev_attrs[] = {
&dev_attr_modalias.attr,
+ &dev_attr_driver_override.attr,
NULL,
};
ATTRIBUTE_GROUPS(platform_dev);
@@ -747,6 +789,10 @@ static int platform_match(struct device *dev, struct device_driver *drv)
struct platform_device *pdev = to_platform_device(dev);
struct platform_driver *pdrv = to_platform_driver(drv);

+ /* When driver_override is set, only bind to the matching driver */
+ if (pdev->driver_override)
+ return !strcmp(pdev->driver_override, drv->name);
+
/* Attempt an OF style match first */
if (of_driver_match_device(dev, drv))
return 1;
diff --git a/include/linux/platform_device.h b/include/linux/platform_device.h
index 16f6654..153d303 100644
--- a/include/linux/platform_device.h
+++ b/include/linux/platform_device.h
@@ -28,6 +28,7 @@ struct platform_device {
struct resource *resource;

const struct platform_device_id *id_entry;
+ char *driver_override; /* Driver name to force a match */

/* MFD cell pointer */
struct mfd_cell *mfd_cell;
--
1.9.1


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