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Subject[PATCH 01/10] soundwire: bus: fix race condition with probe_complete signaling
Date
The driver probe takes care of basic initialization and is invoked
when a Slave becomes attached, after a match between the Slave DevID
registers and ACPI/DT entries.

The update_status callback is invoked when a Slave state changes,
e.g. when it is assigned a non-zero Device Number and it reports with
an ATTACHED/ALERT state.

The state change detection is usually hardware-based and based on the
SoundWire frame rate (e.g. double-digit microseconds) while the probe
is a pure software operation, which may involve a kernel module
load. In corner cases, it's possible that the state changes before the
probe completes.

This patch suggests the use of wait_for_completion to avoid races on
startup, so that the update_status callback does not rely on invalid
pointers/data structures.

Signed-off-by: Rander Wang <rander.wang@intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
---
drivers/soundwire/bus.c | 25 ++++++++++++++++++++++---
drivers/soundwire/bus.h | 1 +
drivers/soundwire/bus_type.c | 5 +++++
drivers/soundwire/slave.c | 2 ++
4 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/drivers/soundwire/bus.c b/drivers/soundwire/bus.c
index 6106577fb3ed..4980dfd6f3a3 100644
--- a/drivers/soundwire/bus.c
+++ b/drivers/soundwire/bus.c
@@ -970,10 +970,29 @@ static int sdw_handle_slave_alerts(struct sdw_slave *slave)
static int sdw_update_slave_status(struct sdw_slave *slave,
enum sdw_slave_status status)
{
- if (slave->ops && slave->ops->update_status)
- return slave->ops->update_status(slave, status);
+ unsigned long time;

- return 0;
+ if (!slave->probed) {
+ /*
+ * the slave status update is typically handled in an
+ * interrupt thread, which can race with the driver
+ * probe, e.g. when a module needs to be loaded.
+ *
+ * make sure the probe is complete before updating
+ * status.
+ */
+ time = wait_for_completion_timeout(&slave->probe_complete,
+ msecs_to_jiffies(DEFAULT_PROBE_TIMEOUT));
+ if (!time) {
+ dev_err(&slave->dev, "Probe not complete, timed out\n");
+ return -ETIMEDOUT;
+ }
+ }
+
+ if (!slave->ops || !slave->ops->update_status)
+ return 0;
+
+ return slave->ops->update_status(slave, status);
}

/**
diff --git a/drivers/soundwire/bus.h b/drivers/soundwire/bus.h
index cb482da914da..acb8d11a4c84 100644
--- a/drivers/soundwire/bus.h
+++ b/drivers/soundwire/bus.h
@@ -5,6 +5,7 @@
#define __SDW_BUS_H

#define DEFAULT_BANK_SWITCH_TIMEOUT 3000
+#define DEFAULT_PROBE_TIMEOUT 2000

#if IS_ENABLED(CONFIG_ACPI)
int sdw_acpi_find_slaves(struct sdw_bus *bus);
diff --git a/drivers/soundwire/bus_type.c b/drivers/soundwire/bus_type.c
index 4a465f55039f..17f096dd6806 100644
--- a/drivers/soundwire/bus_type.c
+++ b/drivers/soundwire/bus_type.c
@@ -110,6 +110,11 @@ static int sdw_drv_probe(struct device *dev)
slave->bus->clk_stop_timeout = max_t(u32, slave->bus->clk_stop_timeout,
slave->prop.clk_stop_timeout);

+ slave->probed = true;
+ complete(&slave->probe_complete);
+
+ dev_dbg(dev, "probe complete\n");
+
return 0;
}

diff --git a/drivers/soundwire/slave.c b/drivers/soundwire/slave.c
index 19919975bb6d..08db0488e02d 100644
--- a/drivers/soundwire/slave.c
+++ b/drivers/soundwire/slave.c
@@ -47,6 +47,8 @@ static int sdw_slave_add(struct sdw_bus *bus,
slave->bus = bus;
slave->status = SDW_SLAVE_UNATTACHED;
slave->dev_num = 0;
+ init_completion(&slave->probe_complete);
+ slave->probed = false;

mutex_lock(&bus->bus_lock);
list_add_tail(&slave->node, &bus->slaves);
--
2.20.1
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