[PATCH 1/3] fpga: dfl: add driver_override support

[Xu Yilun]

Add support for overriding the default matching of a dfl device to a dfl
driver. It follows the same way that can be used for PCI and platform
devices. This patch adds the 'driver_override' sysfs file. It can be
used by VFIO to bind dfl devices for user accessing.

Signed-off-by: Xu Yilun <yilun.xu@xxxxxxxxx>
---
 Documentation/ABI/testing/sysfs-bus-dfl | 20 ++++++++++++
 drivers/fpga/dfl.c                      | 54 ++++++++++++++++++++++++++++++++-
 include/linux/fpga/dfl-bus.h            |  2 ++
 3 files changed, 75 insertions(+), 1 deletion(-)

diff --git a/Documentation/ABI/testing/sysfs-bus-dfl b/Documentation/ABI/testing/sysfs-bus-dfl
index 23543be..cf1357a 100644
--- a/Documentation/ABI/testing/sysfs-bus-dfl
+++ b/Documentation/ABI/testing/sysfs-bus-dfl
@@ -13,3 +13,23 @@ Contact:	Xu Yilun <yilun.xu@xxxxxxxxx>
 Description:	Read-only. It returns feature identifier local to its DFL FIU
 		type.
 		Format: 0x%x
+
+What:		/sys/bus/dfl/devices/.../driver_override
+Date:		Sep 2020
+KernelVersion:	5.10
+Contact:	Xu Yilun <yilun.xu@xxxxxxxxx>
+Description:	This file allows the driver for a device to be specified. 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-mdev-dfl > 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/fpga/dfl.c b/drivers/fpga/dfl.c
index 02a6780..98f88ee 100644
--- a/drivers/fpga/dfl.c
+++ b/drivers/fpga/dfl.c
@@ -262,6 +262,10 @@ static int dfl_bus_match(struct device *dev, struct device_driver *drv)
 	struct dfl_driver *ddrv = to_dfl_drv(drv);
 	const struct dfl_device_id *id_entry;
 
+	/* When driver_override is set, only bind to the matching driver */
+	if (ddev->driver_override)
+		return !strcmp(ddev->driver_override, drv->name);
+
 	id_entry = ddrv->id_table;
 	if (id_entry) {
 		while (id_entry->feature_id) {
@@ -304,6 +308,53 @@ static int dfl_bus_uevent(struct device *dev, struct kobj_uevent_env *env)
 			      ddev->type, ddev->feature_id);
 }
 
+static ssize_t driver_override_show(struct device *dev,
+				    struct device_attribute *attr, char *buf)
+{
+	struct dfl_device *ddev = to_dfl_dev(dev);
+	ssize_t len;
+
+	device_lock(dev);
+	len = sprintf(buf, "%s\n", ddev->driver_override);
+	device_unlock(dev);
+	return len;
+}
+
+static ssize_t driver_override_store(struct device *dev,
+				     struct device_attribute *attr,
+				     const char *buf, size_t count)
+{
+	struct dfl_device *ddev = to_dfl_dev(dev);
+	char *driver_override, *old, *cp;
+
+	/* We need to keep extra room for a newline */
+	if (count >= (PAGE_SIZE - 1))
+		return -EINVAL;
+
+	driver_override = kstrndup(buf, count, GFP_KERNEL);
+	if (!driver_override)
+		return -ENOMEM;
+
+	cp = strchr(driver_override, '\n');
+	if (cp)
+		*cp = '\0';
+
+	device_lock(dev);
+	old = ddev->driver_override;
+	if (strlen(driver_override)) {
+		ddev->driver_override = driver_override;
+	} else {
+		kfree(driver_override);
+		ddev->driver_override = NULL;
+	}
+	device_unlock(dev);
+
+	kfree(old);
+
+	return count;
+}
+static DEVICE_ATTR_RW(driver_override);
+
 static ssize_t
 type_show(struct device *dev, struct device_attribute *attr, char *buf)
 {
@@ -325,6 +376,7 @@ static DEVICE_ATTR_RO(feature_id);
 static struct attribute *dfl_dev_attrs[] = {
 	&dev_attr_type.attr,
 	&dev_attr_feature_id.attr,
+	&dev_attr_driver_override.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(dfl_dev);
@@ -470,7 +522,7 @@ static int dfl_devs_add(struct dfl_feature_platform_data *pdata)
 
 int __dfl_driver_register(struct dfl_driver *dfl_drv, struct module *owner)
 {
-	if (!dfl_drv || !dfl_drv->probe || !dfl_drv->id_table)
+	if (!dfl_drv || !dfl_drv->probe)
 		return -EINVAL;
 
 	dfl_drv->drv.owner = owner;
diff --git a/include/linux/fpga/dfl-bus.h b/include/linux/fpga/dfl-bus.h
index 2a2b283..09a9ee1 100644
--- a/include/linux/fpga/dfl-bus.h
+++ b/include/linux/fpga/dfl-bus.h
@@ -43,6 +43,8 @@ struct dfl_device {
 	unsigned int num_irqs;
 	struct dfl_fpga_cdev *cdev;
 	const struct dfl_device_id *id_entry;
+
+	char *driver_override;
 };
 
 /**
-- 
2.7.4