[PATCH v9 1/7] driver core: Add support for linking devices during device addition

Saravana Kannan
2019-07-31T22:18:40Z

When devices are added, the bus might want to create device links to track
functional dependencies between supplier and consumer devices. This
tracking of supplier-consumer relationship allows optimizing device probe
order and tracking whether all consumers of a supplier are active. The
add_links bus callback is added to support this.

However, when consumer devices are added, they might not have a supplier
device to link to despite needing mandatory resources/functionality from
one or more suppliers. A waiting_for_suppliers list is created to track
such consumers and retry linking them when new devices get added.

Signed-off-by: Saravana Kannan <saravanak@google.com>
---
drivers/base/core.c | 83 ++++++++++++++++++++++++++++++++++++++++++
include/linux/device.h | 14 +++++++
2 files changed, 97 insertions(+)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 950e3bd0f45c..62d416e667bd 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -44,6 +44,8 @@ early_param("sysfs.deprecated", sysfs_deprecated_setup);
#endif

/* Device links support. */
+static LIST_HEAD(wait_for_suppliers);
+static DEFINE_MUTEX(wfs_lock);

#ifdef CONFIG_SRCU
static DEFINE_MUTEX(device_links_lock);
@@ -416,6 +418,51 @@ struct device_link *device_link_add(struct device *consumer,
}
EXPORT_SYMBOL_GPL(device_link_add);

+/**
+ * device_link_wait_for_supplier - Mark device as waiting for supplier
+ * @consumer: Consumer device
+ *
+ * Marks the consumer device as waiting for suppliers to become available. The
+ * consumer device will never be probed until it's unmarked as waiting for
+ * suppliers. The caller is responsible for adding the link to the supplier
+ * once the supplier device is present.
+ *
+ * This function is NOT meant to be called from the probe function of the
+ * consumer but rather from code that creates/adds the consumer device.
+ */
+static void device_link_wait_for_supplier(struct device *consumer)
+{
+ mutex_lock(&wfs_lock);
+ list_add_tail(&consumer->links.needs_suppliers, &wait_for_suppliers);
+ mutex_unlock(&wfs_lock);
+}
+
+/**
+ * device_link_check_waiting_consumers - Try to unmark waiting consumers
+ *
+ * Loops through all consumers waiting on suppliers and tries to add all their
+ * supplier links. If that succeeds, the consumer device is unmarked as waiting
+ * for suppliers. Otherwise, they are left marked as waiting on suppliers,
+ *
+ * The add_links bus callback is expected to return 0 if it has found and added
+ * all the supplier links for the consumer device. It should return an error if
+ * it isn't able to do so.
+ *
+ * The caller of device_link_wait_for_supplier() is expected to call this once
+ * it's aware of potential suppliers becoming available.
+ */
+static void device_link_check_waiting_consumers(void)
+{
+ struct device *dev, *tmp;
+
+ mutex_lock(&wfs_lock);
+ list_for_each_entry_safe(dev, tmp, &wait_for_suppliers,
+ links.needs_suppliers)
+ if (!dev->bus->add_links(dev))
+ list_del_init(&dev->links.needs_suppliers);
+ mutex_unlock(&wfs_lock);
+}
+
static void device_link_free(struct device_link *link)
{
while (refcount_dec_not_one(&link->rpm_active))
@@ -550,6 +597,19 @@ int device_links_check_suppliers(struct device *dev)
struct device_link *link;
int ret = 0;

+ /*
+ * If a device is waiting for one or more suppliers (in
+ * wait_for_suppliers list), it is not ready to probe yet. So just
+ * return -EPROBE_DEFER without having to check the links with existing
+ * suppliers.
+ */
+ mutex_lock(&wfs_lock);
+ if (!list_empty(&dev->links.needs_suppliers)) {
+ mutex_unlock(&wfs_lock);
+ return -EPROBE_DEFER;
+ }
+ mutex_unlock(&wfs_lock);
+
device_links_write_lock();

list_for_each_entry(link, &dev->links.suppliers, c_node) {
@@ -834,6 +894,10 @@ static void device_links_purge(struct device *dev)
{
struct device_link *link, *ln;

+ mutex_lock(&wfs_lock);
+ list_del(&dev->links.needs_suppliers);
+ mutex_unlock(&wfs_lock);
+
/*
* Delete all of the remaining links from this device to any other
* devices (either consumers or suppliers).
@@ -1698,6 +1762,7 @@ void device_initialize(struct device *dev)
#endif
INIT_LIST_HEAD(&dev->links.consumers);
INIT_LIST_HEAD(&dev->links.suppliers);
+ INIT_LIST_HEAD(&dev->links.needs_suppliers);
dev->links.status = DL_DEV_NO_DRIVER;
}
EXPORT_SYMBOL_GPL(device_initialize);
@@ -2133,6 +2198,24 @@ int device_add(struct device *dev)
BUS_NOTIFY_ADD_DEVICE, dev);

kobject_uevent(&dev->kobj, KOBJ_ADD);
+
+ /*
+ * Check if any of the other devices (consumers) have been waiting for
+ * this device (supplier) to be added so that they can create a device
+ * link to it.
+ *
+ * This needs to happen after device_pm_add() because device_link_add()
+ * requires the supplier be registered before it's called.
+ *
+ * But this also needs to happe before bus_probe_device() to make sure
+ * waiting consumers can link to it before the driver is bound to the
+ * device and the driver sync_state callback is called for this device.
+ */
+ device_link_check_waiting_consumers();
+
+ if (dev->bus && dev->bus->add_links && dev->bus->add_links(dev))
+ device_link_wait_for_supplier(dev);
+
bus_probe_device(dev);
if (parent)
klist_add_tail(&dev->p->knode_parent,
diff --git a/include/linux/device.h b/include/linux/device.h
index 5093691f56e7..dbcf1de5e9fa 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -78,6 +78,17 @@ extern void bus_remove_file(struct bus_type *, struct bus_attribute *);
* -EPROBE_DEFER it will queue the device for deferred probing.
* @uevent: Called when a device is added, removed, or a few other things
* that generate uevents to add the environment variables.
+ * @add_links: Called, perhaps multiple times per device, after a device is
+ * added to this bus. The function is expected to create device
+ * links to all the suppliers of the input device that are
+ * available at the time this function is called. As in, the
+ * function should NOT stop at the first failed device link if
+ * other unlinked supplier devices are present in the system.
+ *
+ * Return 0 if device links have been successfully created to all
+ * the suppliers of this device. Return an error if some of the
+ * suppliers are not yet available and this function needs to be
+ * reattempted in the future.
* @probe: Called when a new device or driver add to this bus, and callback
* the specific driver's probe to initial the matched device.
* @remove: Called when a device removed from this bus.
@@ -122,6 +133,7 @@ struct bus_type {

int (*match)(struct device *dev, struct device_driver *drv);
int (*uevent)(struct device *dev, struct kobj_uevent_env *env);
+ int (*add_links)(struct device *dev);
int (*probe)(struct device *dev);
int (*remove)(struct device *dev);
void (*shutdown)(struct device *dev);
@@ -895,11 +907,13 @@ enum dl_dev_state {
* struct dev_links_info - Device data related to device links.
* @suppliers: List of links to supplier devices.
* @consumers: List of links to consumer devices.
+ * @needs_suppliers: Hook to global list of devices waiting for suppliers.
* @status: Driver status information.
*/
struct dev_links_info {
struct list_head suppliers;
struct list_head consumers;
+ struct list_head needs_suppliers;
enum dl_dev_state status;
};

--
2.22.0.709.g102302147b-goog