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Message-ID: <20190426104458.gx4qqtdl7kumphzt@mobilestation>
Date: Fri, 26 Apr 2019 13:45:00 +0300
From: Serge Semin <fancer.lancer@...il.com>
To: Richard Leitner <richard.leitner@...data.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Serge Semin <Sergey.Semin@...latforms.ru>,
linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [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@...il.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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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