Re: [PATCH v6 18/22] usb: dwc2: host: Schedule periodic right away if it's time

[Doug Anderson]

Kever,

On Sun, Jan 31, 2016 at 8:36 PM, Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> Kever,
>
> On Sun, Jan 31, 2016 at 7:32 PM, Kever Yang <kever.yang@xxxxxxxxxxxxxx> wrote:
>> Doug,
>>
>>
>> On 02/01/2016 06:09 AM, Doug Anderson wrote:
>>>
>>> Kever,
>>>
>>> On Sun, Jan 31, 2016 at 1:36 AM, Kever Yang <kever.yang@xxxxxxxxxxxxxx>
>>> wrote:
>>>>
>>>> Doug,
>>>>
>>>>
>>>> On 01/29/2016 10:20 AM, Douglas Anderson wrote:
>>>>>
>>>>> In dwc2_hcd_qh_deactivate() we will put some things on the
>>>>> periodic_sched_ready list.  These things won't be taken off the ready
>>>>> list until the next SOF, which might be a little late.  Let's put them
>>>>> on right away.
>>>>>
>>>>> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
>>>>> Tested-by: Heiko Stuebner <heiko@xxxxxxxxx>
>>>>> Tested-by: Stefan Wahren <stefan.wahren@xxxxxxxx>
>>>>> ---
>>>>> Changes in v6:
>>>>> - Add Heiko's Tested-by.
>>>>> - Add Stefan's Tested-by.
>>>>>
>>>>> Changes in v5: None
>>>>> Changes in v4:
>>>>> - Schedule periodic right away if it's time new for v4.
>>>>>
>>>>> Changes in v3: None
>>>>> Changes in v2: None
>>>>>
>>>>>    drivers/usb/dwc2/hcd_queue.c | 18 ++++++++++++++++--
>>>>>    1 file changed, 16 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/drivers/usb/dwc2/hcd_queue.c b/drivers/usb/dwc2/hcd_queue.c
>>>>> index 9b3c435339ee..3abb34a5fc5b 100644
>>>>> --- a/drivers/usb/dwc2/hcd_queue.c
>>>>> +++ b/drivers/usb/dwc2/hcd_queue.c
>>>>> @@ -1080,12 +1080,26 @@ void dwc2_hcd_qh_deactivate(struct dwc2_hsotg
>>>>> *hsotg, struct dwc2_qh *qh,
>>>>>           * Note: we purposely use the frame_number from the "hsotg"
>>>>> structure
>>>>>           * since we know SOF interrupt will handle future frames.
>>>>>           */
>>>>> -       if (dwc2_frame_num_le(qh->next_active_frame,
>>>>> hsotg->frame_number))
>>>>> +       if (dwc2_frame_num_le(qh->next_active_frame,
>>>>> hsotg->frame_number))
>>>>> {
>>>>> +               enum dwc2_transaction_type tr_type;
>>>>> +
>>>>> +               /*
>>>>> +                * We're bypassing the SOF handler which is normally
>>>>> what
>>>>> puts
>>>>> +                * us on the ready list because we're in a hurry and
>>>>> need
>>>>> to
>>>>> +                * try to catch up.
>>>>> +                */
>>>>> +               dwc2_sch_vdbg(hsotg, "QH=%p IMM ready fn=%04x,
>>>>> nxt=%04x\n",
>>>>> +                             qh, frame_number, qh->next_active_frame);
>>>>>                  list_move_tail(&qh->qh_list_entry,
>>>>>                                 &hsotg->periodic_sched_ready);
>>>>> -       else
>>>>> +
>>>>> +               tr_type = dwc2_hcd_select_transactions(hsotg);
>>>>
>>>> Do we need to add select_transactions call here? If we get into this
>>>> function in interrupt
>>>> and once we put the qh in ready queue, the qh can be handled in this
>>>> frame
>>>> again by the
>>>> later function call of dwc_hcd_select_transactions, so what we need to to
>>>> here is put
>>>> it in ready list instead of inactive queue, and wait for the schedule.
>>>
>>> I'm not sure I understand.  Can you restate?
>>>
>>>
>>> I'll try to explain more in the meantime...
>>>
>>> Both before and after my change, this function would place something
>>> on the ready queue if the next_active_frame <= the frame number as of
>>> last SOF interrupt (aka hsotg->frame_number).  Otherwise it goes on
>>> the inactive queue.  Assuming that the previous change ("usb: dwc2:
>>> host: Manage frame nums better in scheduler") worked properly then
>>> next_active_frame shouldn't be less than (hsotg->frame_number - 1).
>>> Remember that next_active_frame is always 1 before the wire frame, so
>>> if "next_active_frame == hsotg->frame_number - 1" it means that we
>>> need to get the transfer on the wire _right away_.  If
>>> "next_active_frame == hsotg->frame_number" the transfer doesn't need
>>> to go on the wire right away, but since dwc2 can be prepped one frame
>>> in advance it doesn't hurt to give it to the hardware right away if
>>> there's space.
>>>
>>> As I understand it, if we stick something on the ready queue it won't
>>> generally get looked at until the next SOF interrupt.  That means
>>> we'll be too late if "next_active_frame == hsotg->frame_number - 1"
>>> and we'll possibly be too late (depending on interrupt latency) if
>>> "next_active_frame == hsotg->frame_number"
>>>
>> I understand this patch and agree with your point of schedule the
>> periodic right away instead of at least next frame.
>> My point is, there are only two call to dwc2_hcd_qh_deactivate(), from
>> dwc2_hcd_urb_dequeue() and dwc2_release_channel(), we don't need
>> to do the schedule for dequeue, and there is one
>> dwc2_hcd_select_transactions() call at the end of dwc2_release_channel(),
>> maybe we don't need another dwc2_hcd_select_transactions() here.
>>
>> I think the duration from this point to the function call of
>> dwc2_hcd_select_transactions()
>> in dwc2_release_channel() will be the main factor for us to decide if
>> we need to add a function call of  dwc2_hcd_select_transactions() here.
>
> Oh, now I get what you're saying!
>
> A) You've got dwc2_release_channel() -> dwc2_deactivate_qh() ->
> dwc2_hcd_qh_deactivate()
> ...and always in that case we'll do a select / queue, so we don't need it there.
>
> B) You've got dwc2_hcd_urb_dequeue() -> dwc2_hcd_qh_deactivate()
>
> ...but why don't we need it for dwc2_hcd_urb_dequeue()?  Yes, you're
> not continuing a split so timing isn't quite as urgent, but you still
> might have an INT or ISOC packet that's scheduled with an interval of
> 1.  We still might want to schedule right away if there are remaining
> QTDs, right?

I ran out of time to fully test today, but I couldn't actually get a
case where we needed to schedule right away for B).  ...so given your
point about the the select / queue already present in case A, we could
probably just drop this patch ("usb: dwc2: host: Schedule periodic
right away if it's time") and if we can find a case where it's needed
in case B we can add the select / queue there.

Sound OK?  I'll try to do more testing tomorrow...

-Doug