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Subject: Re: [PATCH 3/3] USB: Disable LPM on WD19's Realtek Hub during setting
 its ports to U0
From:   Kai-Heng Feng <kai.heng.feng@canonical.com>
In-Reply-To: <Pine.LNX.4.44L0.2001031018290.1560-100000@iolanthe.rowland.org>
Date:   Sat, 4 Jan 2020 00:25:34 +0800
Cc:     Mathias Nyman <mathias.nyman@intel.com>,
        gregkh@linuxfoundation.org, acelan.kao@canonical.com,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
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Message-Id: <B3E0C8C0-3E45-42B3-86D9-E34D669E1F4A@canonical.com>
References: <Pine.LNX.4.44L0.2001031018290.1560-100000@iolanthe.rowland.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Sender: linux-kernel-owner@vger.kernel.org
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Hi Alan,

> On Jan 3, 2020, at 23:21, Alan Stern <stern@rowland.harvard.edu> wrote:
> 
> 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@canonical.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)?

Maybe introduce a new lock? The lock however will only be used by this specific hub.
But I still want the LPM can be enabled for this hub.

> 
>> +		}
>> +	} 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.

Yes I missed that, will add in v2.

Kai-Heng

> 
> Alan Stern