Re: [PATCH 2/2] usb: host: xhci: Handle the right timeout command

[Baolin Wang]

Hi Mathias,

On 12 December 2016 at 23:52, Mathias Nyman
<mathias.nyman@xxxxxxxxxxxxxxx> wrote:
> On 05.12.2016 09:51, Baolin Wang wrote:
>>
>> If a command event is found on the event ring during an interrupt,
>> we need to stop the command timer with del_timer(). Since del_timer()
>> can fail if the timer is running and waiting on the xHCI lock, then
>> it maybe get the wrong timeout command in xhci_handle_command_timeout()
>> if host fetched a new command and updated the xhci->current_cmd in
>> handle_cmd_completion(). For this situation, we need a way to signal
>> to the command timer that everything is fine and it should exit.
>
>
> Ah, right, this could actually happen.
>
>>
>>
>> We should introduce a counter (xhci->current_cmd_pending) for the number
>> of pending commands. If we need to cancel the command timer and
>> del_timer()
>> succeeds, we decrement the number of pending commands. If del_timer()
>> fails,
>> we leave the number of pending commands alone.
>>
>> For handling timeout command, in xhci_handle_command_timeout() we will
>> check
>> the counter after decrementing it, if the counter
>> (xhci->current_cmd_pending)
>> is 0, which means xhci->current_cmd is the right timeout command. If the
>> counter (xhci->current_cmd_pending) is greater than 0, which means current
>> timeout command has been handled by host and host has fetched new command
>> as
>> xhci->current_cmd, then just return and wait for new current command.
>
>
> A counter like this could work.
>
> Writing the abort bit can generate either ABORT+STOP, or just STOP
> event, this seems to cover both.
>
> quick check, case 1: timeout and cmd completion at the same time.
>
> cpu1                                    cpu2
>
> queue_command(first), p++ (=1)
> queue_command(more),
> --completion irq fires--                -- timer times out at same time--
> handle_cmd_completion()                 handle_cmd_timeout(),)
> lock(xhci_lock  )                       spin_on(xhci_lock)
> del_timer() fail, p (=1, nochange)
> cur_cmd = list_next(), p++ (=2)
> unlock(xhci_lock)
>                                         lock(xhci_lock)
>                                         p-- (=1)
>                                         if (p > 0), exit
> OK works
>
> case 2: normal timeout case with ABORT+STOP, no race.
>
> cpu1                                    cpu2
>
> queue_command(first), p++ (=1)
> queue_command(more),
>                                         handle_cmd_timeout()
>                                         p-- (P=0), don't exit
>                                         mod_timer(), p++ (P=1)
>                                         write_abort_bit()
> handle_cmd_comletion(ABORT)
> del_timer(), ok, p-- (p = 0)
> handle_cmd_completion(STOP)
> del_timer(), fail, (P=0)
> handle_stopped_cmd_ring()
> cur_cmd = list_next(), p++ (=1)
> mod_timer()
>
> OK, works, and same for just STOP case, with the only difference that
> during handle_cmd_completion(STOP) p is decremented (p--)

Yes, that's the cases what I want to handle, thanks for your explicit
explanation.

>
> So unless there is a way to find out if cur_cmd is valid in command timeout
> in command timeout with the help of existing flags and lists this would be a
> working
> solution.
>
> -Mathias
>



-- 
Baolin.wang
Best Regards