linux-kernel - Re: [PATCH 3/3] USB: Disable LPM on WD19's Realtek Hub during setting its ports to U0

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Date:   Fri, 3 Jan 2020 10:21:49 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
cc:     mathias.nyman@...el.com, <gregkh@...uxfoundation.org>,
        <acelan.kao@...onical.com>, <linux-usb@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] USB: Disable LPM on WD19's Realtek Hub during setting
 its ports to U0

On Fri, 3 Jan 2020, Kai-Heng Feng wrote:

> Realtek Hub (0bda:0x0487) used in Dell Dock WD19 sometimes drops off the
> bus when bringing underlying ports from U3 to U0.
> 
> After some expirements and guessworks, the hub itself needs to be U0
> during setting its port's link state back to U0.
> 
> So add a new quirk to let the hub disables LPM on setting U0 for its
> downstream facing ports.
> 
> Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
> ---
>  drivers/usb/core/hub.c     | 12 ++++++++++--
>  drivers/usb/core/quirks.c  |  7 +++++++
>  include/linux/usb/quirks.h |  3 +++
>  3 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> index f229ad6952c0..35a035781c5a 100644
> --- a/drivers/usb/core/hub.c
> +++ b/drivers/usb/core/hub.c
> @@ -3533,9 +3533,17 @@ int usb_port_resume(struct usb_device *udev, pm_message_t msg)
>  	}
>  
>  	/* see 7.1.7.7; affects power usage, but not budgeting */
> -	if (hub_is_superspeed(hub->hdev))
> +	if (hub_is_superspeed(hub->hdev)) {
> +		if (hub->hdev->quirks & USB_QUIRK_DISABLE_LPM_ON_U0) {
> +			usb_lock_device(hub->hdev);
> +			usb_unlocked_disable_lpm(hub->hdev);
> +		}
>  		status = hub_set_port_link_state(hub, port1, USB_SS_PORT_LS_U0);
> -	else
> +		if (hub->hdev->quirks & USB_QUIRK_DISABLE_LPM_ON_U0) {
> +			usb_unlocked_enable_lpm(hub->hdev);
> +			usb_unlock_device(hub->hdev);

The locking here seems questionable.  Doesn't this code sometimes get
called with the hub already locked?  Or with the child device locked
(in which case locking the hub would violate the normal locking order:  
parent first, child second)?

> +		}
> +	} else
>  		status = usb_clear_port_feature(hub->hdev,
>  				port1, USB_PORT_FEAT_SUSPEND);
>  	if (status) {
> diff --git a/drivers/usb/core/quirks.c b/drivers/usb/core/quirks.c
> index 6b6413073584..69474d0d2b38 100644
> --- a/drivers/usb/core/quirks.c
> +++ b/drivers/usb/core/quirks.c
> @@ -131,6 +131,9 @@ static int quirks_param_set(const char *val, const struct kernel_param *kp)
>  			case 'o':
>  				flags |= USB_QUIRK_HUB_SLOW_RESET;
>  				break;
> +			case 'p':
> +				flags |= USB_QUIRK_DISABLE_LPM_ON_U0;
> +				break;

The new 'p' flag needs to be documented.

Alan Stern