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Subject[PATCH] usb: usb251xb: Lock i2c-bus segment the hub resides
SMBus slave configuration is activated by CFG_SEL[1:0]=0x1 pins
state. This is the mode the hub is supposed to be to let this driver
work correctly. But a race condition might happen right after reset
is cleared due to CFG_SEL[0] pin being multiplexed with SMBus SCL
function. In case if the reset pin is handled by a i2c GPIO expander,
which is also placed at the same i2c-bus segment as the usb251x
SMB-interface connected to, then the hub reset clearance might
cause the CFG_SEL[0] being latched in unpredictable state. So
sometimes the hub configuration mode might be 0x1 (as expected),
but sometimes being 0x0, which doesn't imply to have the hub SMBus-slave
interface activated and consequently causes this driver failure.

In order to fix the problem we must make sure the GPIO-reset chip doesn't
reside the same i2c-bus segment as the SMBus-interface of the hub. If
it doesn't, we can safely block the segment for the time the reset is
cleared to prevent anyone generating a traffic at the i2c-bus SCL lane
connected to the CFG_SEL[0] pin. But if it does, nothing we can do, so
just return an error. If we locked the i2c-bus segment and tried to
communicate with the GPIO-expander, it would cause a deadlock. If we didn't
lock the i2c-bus segment, it would randomly cause the CFG_SEL[0] bit flip.

Signed-off-by: Serge Semin <>
drivers/usb/misc/usb251xb.c | 55 +++++++++++++++++++++++++++++++++++++
1 file changed, 55 insertions(+)

diff --git a/drivers/usb/misc/usb251xb.c b/drivers/usb/misc/usb251xb.c
index 04684849d683..939b3bedd4c8 100644
--- a/drivers/usb/misc/usb251xb.c
+++ b/drivers/usb/misc/usb251xb.c
@@ -12,6 +12,7 @@

#include <linux/delay.h>
#include <linux/gpio/consumer.h>
+#include <linux/gpio/driver.h>
#include <linux/i2c.h>
#include <linux/module.h>
#include <linux/nls.h>
@@ -222,11 +223,44 @@ static const struct usb251xb_data usb2517i_data = {
.product_str = "USB2517i",

+static int usb251xb_check_dev_children(struct device *dev, void *child)
+ if (dev->type == &i2c_adapter_type) {
+ return device_for_each_child(dev, child,
+ usb251xb_check_dev_children);
+ }
+ return (dev == child);
+static int usb251x_check_gpio_chip(struct usb251xb *hub)
+ struct gpio_chip *gc = gpiod_to_chip(hub->gpio_reset);
+ struct i2c_adapter *adap = hub->i2c->adapter;
+ int ret;
+ if (!hub->gpio_reset)
+ return 0;
+ if (!gc)
+ return -EINVAL;
+ ret = usb251xb_check_dev_children(&adap->dev, gc->parent);
+ if (ret) {
+ dev_err(hub->dev, "Reset GPIO chip is at the same i2c-bus\n");
+ return -EINVAL;
+ }
+ return 0;
static void usb251xb_reset(struct usb251xb *hub, int state)
if (!hub->gpio_reset)

+ i2c_lock_bus(hub->i2c->adapter, I2C_LOCK_SEGMENT);
gpiod_set_value_cansleep(hub->gpio_reset, state);

/* wait for hub recovery/stabilization */
@@ -234,6 +268,8 @@ static void usb251xb_reset(struct usb251xb *hub, int state)
usleep_range(500, 750); /* >=500us at power on */
usleep_range(1, 10); /* >=1us at power down */
+ i2c_unlock_bus(hub->i2c->adapter, I2C_LOCK_SEGMENT);

static int usb251xb_connect(struct usb251xb *hub)
@@ -621,6 +657,25 @@ static int usb251xb_probe(struct usb251xb *hub)

+ /*
+ * usb251x SMBus-slave SCL lane is muxed with CFG_SEL0 pin. So if anyone
+ * tries to work with the bus at the moment the hub reset is released,
+ * it may cause an invalid config being latched by usb251x. Particularly
+ * one of the config modes makes the hub loading a default registers
+ * value without SMBus-slave interface activation. If the hub
+ * accidentally gets this mode, this will cause the driver SMBus-
+ * functions failure. Normally we could just lock the SMBus-segment the
+ * hub i2c-interface resides for the device-specific reset timing. But
+ * the GPIO controller, which is used to handle the hub reset, might be
+ * placed at the same i2c-bus segment. In this case an error should be
+ * returned since we can't safely use the GPIO controller to clear the
+ * reset state (it may affect the hub configuration) and we can't lock
+ * the i2c-bus segment (it will cause a deadlock).
+ */
+ err = usb251x_check_gpio_chip(hub);
+ if (err)
+ return err;
err = usb251xb_connect(hub);
if (err) {
dev_err(dev, "Failed to connect hub (%d)\n", err);
 \ /
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