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    SubjectRe: [PATCH v2] usb: usb251xb: Lock i2c-bus segment the hub resides
    On Fri, Apr 26, 2019 at 01:32:56AM +0300, Serge Semin wrote:

    Ahh, sorry. Forget about this email. I'll resend it with the valid In-Reply-To
    message-id.

    -Sergey

    > 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 <fancer.lancer@gmail.com>
    > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    >
    > ---
    > Changelog v2
    > - Execute the check code only if GPIOLIB is enabled (reported by
    > a build robot).
    >
    > ---
    > drivers/usb/misc/usb251xb.c | 62 +++++++++++++++++++++++++++++++++++++
    > 1 file changed, 62 insertions(+)
    >
    > diff --git a/drivers/usb/misc/usb251xb.c b/drivers/usb/misc/usb251xb.c
    > index 04684849d683..cdc80e8c2d8a 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 +223,51 @@ static const struct usb251xb_data usb2517i_data = {
    > .product_str = "USB2517i",
    > };
    >
    > +#ifdef CONFIG_GPIOLIB
    > +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;
    > +}
    > +#else
    > +static int usb251x_check_gpio_chip(struct usb251xb *hub)
    > +{
    > + return 0;
    > +}
    > +#endif
    > +
    > static void usb251xb_reset(struct usb251xb *hub, int state)
    > {
    > if (!hub->gpio_reset)
    > return;
    >
    > + i2c_lock_bus(hub->i2c->adapter, I2C_LOCK_SEGMENT);
    > +
    > gpiod_set_value_cansleep(hub->gpio_reset, state);
    >
    > /* wait for hub recovery/stabilization */
    > @@ -234,6 +275,8 @@ static void usb251xb_reset(struct usb251xb *hub, int state)
    > usleep_range(500, 750); /* >=500us at power on */
    > else
    > 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 +664,25 @@ 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);
    > --
    > 2.21.0
    >

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