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Subject[RFC PATCH v3] drivercore: Add driver probe deferral mechanism
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Allow drivers to report at probe time that they cannot get all the resources
required by the device, and should be retried at a later time.

This should completely solve the problem of getting devices
initialized in the right order. Right now this is mostly handled by
mucking about with initcall ordering which is a complete hack, and
doesn't even remotely handle the case where device drivers are in
modules. This approach completely sidesteps the issues by allowing
driver registration to occur in any order, and any driver can request
to be retried after a few more other drivers get probed.

v3: - Hold off workqueue scheduling until late_initcall so that the bulk
of driver probes are complete before we start retrying deferred devices.
- Tested with simple use cases. Still needs more testing though.
Using it to get rid of the gpio early_initcall madness, or to replace
the ASoC internal probe deferral code would be ideal.
v2: - added locking so it should no longer be utterly broken in that regard
- remove device from deferred list at device_del time.
- Still completely untested with any real use case, but has been
boot tested.

TODO: - Create a separate singlethread_workqueue so that drivers can't
mess things up by calling flush_work().
- Maybe this should be wrapped with a kconfig symbol so it can
be compiled out on systems that don't care.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Greg Kroah-Hartman <greg@kroah.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dilan Lee <dilee@nvidia.com>
Cc: Manjunath GKondaiah <manjunath.gkondaiah@linaro.org>
---
Hi Manjunath,

Here's the current state of the patch. The major think that needs to
be done is to convert it to use a separate workqueue as described in
the TODO above. It also needs some users adapted to it. One of the
gpio drivers would work; preferably one of the newer drivers that
doesn't have a lot of drivers depending on the early_initcall()
behaviour yet.

Mark Brown may also be able to suggest specific examples.

For everyone else; this is the current state of the patch. I think it
is in pretty good shape other than the TODO item above. I'm turning
it over to Manjunath to polish up for merging. I would appreciate
any feedback.

g.


drivers/base/base.h | 1
drivers/base/core.c | 2 +
drivers/base/dd.c | 134 ++++++++++++++++++++++++++++++++++++++++++++++++
include/linux/device.h | 5 ++
4 files changed, 141 insertions(+), 1 deletions(-)
diff --git a/drivers/base/base.h b/drivers/base/base.h
index a34dca0..9641309 100644
--- a/drivers/base/base.h
+++ b/drivers/base/base.h
@@ -105,6 +105,7 @@ extern void bus_remove_driver(struct device_driver *drv);

extern void driver_detach(struct device_driver *drv);
extern int driver_probe_device(struct device_driver *drv, struct device *dev);
+extern void driver_deferred_probe_del(struct device *dev);
static inline int driver_match_device(struct device_driver *drv,
struct device *dev)
{
diff --git a/drivers/base/core.c b/drivers/base/core.c
index bc8729d..0d37e18 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -588,6 +588,7 @@ void device_initialize(struct device *dev)
{
dev->kobj.kset = devices_kset;
kobject_init(&dev->kobj, &device_ktype);
+ INIT_LIST_HEAD(&dev->deferred_probe);
INIT_LIST_HEAD(&dev->dma_pools);
mutex_init(&dev->mutex);
lockdep_set_novalidate_class(&dev->mutex);
@@ -1119,6 +1120,7 @@ void device_del(struct device *dev)
device_remove_file(dev, &uevent_attr);
device_remove_attrs(dev);
bus_remove_device(dev);
+ driver_deferred_probe_del(dev);

/*
* Some platform devices are driven without driver attached
diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 6658da7..8be5b33 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -28,6 +28,129 @@
#include "base.h"
#include "power/power.h"

+/*
+ * Deferred Probe infrastructure.
+ *
+ * Sometimes driver probe order matters, but the kernel doesn't always have
+ * dependency information which means some drivers will get probed before a
+ * resource it depends on is available. For example, an SDHCI driver may
+ * first need a GPIO line from an i2c GPIO controller before it can be
+ * initialized. If a required resource is not available yet, a driver can
+ * request probing to be deferred by returning -EAGAIN from its probe hook
+ *
+ * Deferred probe maintains two lists of devices, a pending list and an active
+ * list. A driver returning -EAGAIN causes the device to be added to the
+ * pending list.
+ *
+ * The deferred_probe_mutex *must* be held any time the deferred_probe_*_list
+ * of the (struct device*)->deferred_probe pointers are manipulated
+ */
+static DEFINE_MUTEX(deferred_probe_mutex);
+static LIST_HEAD(deferred_probe_pending_list);
+static LIST_HEAD(deferred_probe_active_list);
+
+/**
+ * deferred_probe_work_func() - Retry probing devices in the active list.
+ */
+static void deferred_probe_work_func(struct work_struct *work)
+{
+ struct device *dev;
+ /*
+ * This bit is tricky. We want to process every device in the
+ * deferred list, but devices can be removed from the list at any
+ * time while inside this for-each loop. There are two things that
+ * need to be protected against:
+ * - if the device is removed from the deferred_probe_list, then we
+ * loose our place in the loop. Since any device can be removed
+ * asynchronously, list_for_each_entry_safe() wouldn't make things
+ * much better. Simplest solution is to restart walking the list
+ * whenever the current device gets removed. Not the most efficient,
+ * but is simple to implement and easy to audit for correctness.
+ * - if the device is unregistered, and freed, then there is a risk
+ * of a null pointer dereference. This code uses get/put_device()
+ * to ensure the device cannot disappear from under our feet.
+ */
+ mutex_lock(&deferred_probe_mutex);
+ while (!list_empty(&deferred_probe_active_list)) {
+ dev = list_first_entry(&deferred_probe_active_list,
+ typeof(*dev), deferred_probe);
+ list_del_init(&dev->deferred_probe);
+
+ get_device(dev);
+
+ /* Drop the mutex while probing each device; the probe path
+ * may manipulate the deferred list */
+ mutex_unlock(&deferred_probe_mutex);
+ dev_dbg(dev, "Retrying from deferred list\n");
+ bus_probe_device(dev);
+ mutex_lock(&deferred_probe_mutex);
+
+ put_device(dev);
+ }
+ mutex_unlock(&deferred_probe_mutex);
+}
+static DECLARE_WORK(deferred_probe_work, deferred_probe_work_func);
+
+static void driver_deferred_probe_add(struct device *dev)
+{
+ mutex_lock(&deferred_probe_mutex);
+ if (list_empty(&dev->deferred_probe)) {
+ dev_dbg(dev, "Added to deferred list\n");
+ list_add(&dev->deferred_probe, &deferred_probe_pending_list);
+ }
+ mutex_unlock(&deferred_probe_mutex);
+}
+
+void driver_deferred_probe_del(struct device *dev)
+{
+ mutex_lock(&deferred_probe_mutex);
+ if (!list_empty(&dev->deferred_probe)) {
+ dev_dbg(dev, "Removed from deferred list\n");
+ list_del_init(&dev->deferred_probe);
+ }
+ mutex_unlock(&deferred_probe_mutex);
+}
+
+static bool driver_deferred_probe_enable = false;
+/**
+ * driver_deferred_probe_trigger() - Kick off re-probing deferred devices
+ *
+ * This functions moves all devices from the pending list to the active
+ * list and schedules the deferred probe workqueue to process them. It
+ * should be called anytime a driver is successfully bound to a device.
+ */
+static void driver_deferred_probe_trigger(void)
+{
+ if (!driver_deferred_probe_enable)
+ return;
+
+ /* A successful probe means that all the devices in the pending list
+ * should be triggered to be reprobed. Move all the deferred devices
+ * into the active list so they can be retried by the workqueue */
+ mutex_lock(&deferred_probe_mutex);
+ list_splice_tail_init(&deferred_probe_pending_list,
+ &deferred_probe_active_list);
+ mutex_unlock(&deferred_probe_mutex);
+
+ /* Kick the re-probe thread. It may already be scheduled, but
+ * it is safe to kick it again. */
+ schedule_work(&deferred_probe_work);
+}
+
+/**
+ * deferred_probe_initcall() - Enable probing of deferred devices
+ *
+ * We don't want to get in the way when the bulk of drivers are getting probed.
+ * Instead, this initcall makes sure that deferred probing is delayed until
+ * late_initcall time.
+ */
+static int deferred_probe_initcall(void)
+{
+ driver_deferred_probe_enable = true;
+ driver_deferred_probe_trigger();
+ return 0;
+}
+late_initcall(deferred_probe_initcall);

static void driver_bound(struct device *dev)
{
@@ -42,6 +165,11 @@ static void driver_bound(struct device *dev)

klist_add_tail(&dev->p->knode_driver, &dev->driver->p->klist_devices);

+ /* Make sure the device is no longer in one of the deferred lists
+ * and kick off retrying all pending devices */
+ driver_deferred_probe_del(dev);
+ driver_deferred_probe_trigger();
+
if (dev->bus)
blocking_notifier_call_chain(&dev->bus->p->bus_notifier,
BUS_NOTIFY_BOUND_DRIVER, dev);
@@ -142,7 +270,11 @@ probe_failed:
driver_sysfs_remove(dev);
dev->driver = NULL;

- if (ret != -ENODEV && ret != -ENXIO) {
+ if (ret == -EAGAIN) {
+ /* Driver requested deferred probing */
+ dev_info(dev, "Driver %s requests probe deferral\n", drv->name);
+ driver_deferred_probe_add(dev);
+ } else if (ret != -ENODEV && ret != -ENXIO) {
/* driver matched but the probe failed */
printk(KERN_WARNING
"%s: probe of %s failed with error %d\n",
diff --git a/include/linux/device.h b/include/linux/device.h
index c20dfbf..dab93a4 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -506,6 +506,10 @@ struct device_dma_parameters {
* @mutex: Mutex to synchronize calls to its driver.
* @bus: Type of bus device is on.
* @driver: Which driver has allocated this
+ * @deferred_probe: entry in deferred_probe_list which is used to retry the
+ * binding of drivers which were unable to get all the resources
+ * needed by the device; typically because it depends on another
+ * driver getting probed first.
* @platform_data: Platform data specific to the device.
* Example: For devices on custom boards, as typical of embedded
* and SOC based hardware, Linux often uses platform_data to point
@@ -564,6 +568,7 @@ struct device {
struct bus_type *bus; /* type of bus device is on */
struct device_driver *driver; /* which driver has allocated this
device */
+ struct list_head deferred_probe;
void *platform_data; /* Platform specific data, device
core doesn't touch it */
struct dev_pm_info power;


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