linux-kernel - Re: [RFC] usb: host: xhci: Remove the watchdog timer and use command timer to watch stop endpoint command

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Message-ID: <583FD4C6.7020306@linux.intel.com>
Date:   Thu, 1 Dec 2016 15:44:06 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     mathias.nyman@...el.com, Greg KH <gregkh@...uxfoundation.org>,
        USB <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [RFC] usb: host: xhci: Remove the watchdog timer and use command
 timer to watch stop endpoint command

Hi,

On 12/01/2016 03:35 PM, Baolin Wang wrote:
> On 1 December 2016 at 14:35, Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>> Hi,
>>
>> On 12/01/2016 02:04 PM, Baolin Wang wrote:
>>> Hi Baolu,
>>>
>>> On 1 December 2016 at 13:45, Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>>>> Hi,
>>>>
>>>> On 11/30/2016 05:02 PM, Baolin Wang wrote:
>>>>> If the hardware never responds to the stop endpoint command, the
>>>>> URBs will never be completed, and we might hang the USB subsystem.
>>>>> The original watchdog timer is used to watch if one stop endpoint
>>>>> command is timeout, if timeout, then the watchdog timer will set
>>>>> XHCI_STATE_DYING, try to halt the xHCI host, and give back all
>>>>> pending URBs.
>>>>>
>>>>> But now we already have one command timer to control command timeout,
>>>>> thus we can also use the command timer to watch the stop endpoint
>>>>> command, instead of one duplicate watchdog timer which need to be
>>>>> removed.
>>>>>
>>>>> Meanwhile we don't need the 'stop_cmds_pending' flag to identy if
>>>>> this is the last stop endpoint command of one endpoint. Since we
>>>>> can make sure we only set one stop endpoint command for one endpoint
>>>>> by 'EP_HALT_PENDING' flag in xhci_urb_dequeue() function. Thus remove
>>>>> this flag.
>>>> I am afraid you can't do this. "stop_cmds_pending" was added
>>>> to fix the problem described in the comments that you want to
>>>> remove. But I didn't find any fix of this problem in your patch.
>>> Now we can not pending another stop endpoint command for the same one
>>> endpoint, since will check 'EP_HALT_PENDING' flag in
>>> xhci_urb_dequeue() function to avoid this. But after some
>>> investigation, I think I missed the stop endpoint command in
>>> xhci_stop_device() which did not check the 'EP_HALT_PENDING' flag,
>>> maybe need to add 'EP_HALT_PENDING' flag checking in
>>> xhci_stop_device() function. DId I miss something else? Thanks.
>> Consider below three threads running on different CPUs at the same time.
>>
>> Thread A: xhci_handle_cmd_stop_ep()  --- in interrupt handler
>> Thread B: xhci_stop_endpoint_command_timeout() --- timer expired
>> Thread C: xhci_urb_dequeue --- called by usb core
>>
>> They are serialized by xhci->lock. Let's consider below sequence:
>>
>> Thread A:
>>     - delete xhci->cmd_timer), but will fail due to Thread B.
>>     - clear EP_HALT_PENDING bit
>>
>> Thread B:
>>     - halt the host controller
>>
>> Thread C:
>>     - set EP_HALT_PENDING bit
>>     - enqueue another stop endpoint command
>>     - add the timer back
> Ah, thanks for you comments.
> If thread B halted the host, then the thread C xhci_urb_dequeue() will
> check host state failed, then just return and can not set another stop
> endpoint command.

Not so simple. Thread B will release xhci->lock before xhci_halt().

> But from your example, I think your meaning is we
> should not halt the host by thread B, since we have handled the stop
> endpoint command event.
>
> So I think we need to check the xhci command of this stop endpoint
> command in xhci_stop_endpoint_command_timeout(), if the xhci command
> of this stop endpoint command is not in the command list (which means
> the stop endpoint command event has been handled), then just return
> and do not halt the controller. Right?
>

I'd like suggest you to put this change into a separated patch.
It's actually a different topic from the main purpose of this patch.

Best regards,
Lu Baolu