RE: [PATCH v2 1/8] usb: dwc3: Correct the logic for checking TRB full in __dwc3_prepare_one_trb()

[Anurag Kumar Vulisha]

Hi Thinh,

>-----Original Message-----
>From: Thinh Nguyen [mailto:Thinh.Nguyen@xxxxxxxxxxxx]
>Sent: Thursday, September 06, 2018 7:28 AM
>To: Anurag Kumar Vulisha <anuragku@xxxxxxxxxx>; Thinh Nguyen
><Thinh.Nguyen@xxxxxxxxxxxx>; balbi@xxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx
>Cc: v.anuragkumar@xxxxxxxxx; linux-usb@xxxxxxxxxxxxxxx; linux-
>kernel@xxxxxxxxxxxxxxx
>Subject: Re: [PATCH v2 1/8] usb: dwc3: Correct the logic for checking TRB full in
>__dwc3_prepare_one_trb()
>
>Hi,
>
>On 9/5/2018 2:19 AM, Anurag Kumar Vulisha wrote:
>> Hi Thinh,
>>
>> Thanks for spending your time in reviewing this code, please find my
>> comments inline
>>
>>> -----Original Message-----
>>> From: Thinh Nguyen [mailto:Thinh.Nguyen@xxxxxxxxxxxx]
>>> Sent: Wednesday, September 05, 2018 11:04 AM
>>> To: Anurag Kumar Vulisha <anuragku@xxxxxxxxxx>; balbi@xxxxxxxxxx;
>>> gregkh@xxxxxxxxxxxxxxxxxxx
>>> Cc: v.anuragkumar@xxxxxxxxx; linux-usb@xxxxxxxxxxxxxxx; linux-
>>> kernel@xxxxxxxxxxxxxxx
>>> Subject: Re: [PATCH v2 1/8] usb: dwc3: Correct the logic for checking
>>> TRB full in
>>> __dwc3_prepare_one_trb()
>>>
>>> Hi Anurag,
>>>
>>>> +	trb->ctrl |= DWC3_TRB_CTRL_HWO;
>>>> +
>>>>  	if ((!no_interrupt && !chain) ||
>>>>  			(dwc3_calc_trbs_left(dep) == 0))
>>>>  		trb->ctrl |= DWC3_TRB_CTRL_IOC;
>>>> @@ -1000,8 +1002,6 @@ static void __dwc3_prepare_one_trb(struct
>>>> dwc3_ep
>>> *dep, struct dwc3_trb *trb,
>>>>  	if (usb_endpoint_xfer_bulk(dep->endpoint.desc) && dep->stream_capable)
>>>>  		trb->ctrl |= DWC3_TRB_CTRL_SID_SOFN(stream_id);
>>>>
>>>> -	trb->ctrl |= DWC3_TRB_CTRL_HWO;
>>>> -
>>>>  	trace_dwc3_prepare_trb(dep, trb);
>>>>  }
>>>>
>>> How do you reproduce this issue? We should not set HWO before setting
>>> other trb->ctrl bits. Can you provide a driver tracepoint? If there's
>>> an issue with the check if ((!no_interrupt && !chain) ||
>>> dwc3_calc_trbs_left == 0), then we may need to fix the check there.
>>>
>> This issue gets triggered very rarely on long runs when dep->trb_enqueue == dep-
>>trb_dequeue.
>> In __dwc3_prepare_one_trb() , IOC bit is set when no TRBs are
>> available, so that a complete event can be generated and TRBs can be
>> cleaned  after complete . Dwc3_calc_trbs_left() is called to determine
>> the available TRBs, which depends on the previous TRB's HWO bit set
>> when
>> dep->trb_enqueue == dep->trb_dequeue. There are chances where the
>> dep->dwc3_calc_trbs_left() wrongly
>> returns DWC3_TRB_NUM -1 instead of 0 , even though there are no
>> available TRBs. Please consider the below example
>>
>> 1.  Consider a TRB passed to __dwc3_prepare_one_trb() is the last available TRB in
>the pool.
>> 2.  __dwc3_prepare_one_trb() calls dwc3_ep_inc_enq() which increments the dep-
>>trb_enqueue
>>    before preparing the TRB and since the current TRB is the last available,
>incrementing
>>   dep->enqueue will make dep->enqueue == dep->dequeue 3. IOC bit is
>> set by __dwc3_prepare_one_trb()  only when dwc3_calc_trbs_left()
>> returns 0 (empty TRBs) 4. Since dep->enqueue == dep->dequeue and the previous
>TRB(the one which we are working)
>>     doesn't yet has the HWO bit set, dwc3_calc_trbs_left() returns DWC3_TRB_NUM
>-1 instead of
>>     zero (Though there are no available TRBs) 5. Since
>> Dwc3_calc_trbs_left() returns non-zero value, IOC bit is not set in
>__dwc3_prepare_one_trb()
>>    for the last TRB and no complete event is generated. Because of this no further
>TRBs are queued.
>>
>> To avoid the above mentioned issue, I moved the code logic for setting HWO bit
>before setting IOC bit.
>> I agree that HWO bit should always be set at the last, but I didn't find any better
>logic for fixing this.
>> Please suggest if any better way to handle this situation.
>>
>
>I haven't tested it, but you can try this:
>
>diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c index
>d7a327eaee12..37171d46390b 100644
>--- a/drivers/usb/dwc3/gadget.c
>+++ b/drivers/usb/dwc3/gadget.c
>@@ -924,8 +924,6 @@ static void __dwc3_prepare_one_trb(struct dwc3_ep *dep,
>struct dwc3_trb *trb,
>        struct usb_gadget       *gadget = &dwc->gadget;
>        enum usb_device_speed   speed = gadget->speed;
>
>-       dwc3_ep_inc_enq(dep);
>-
>        trb->size = DWC3_TRB_SIZE_LENGTH(length);
>        trb->bpl = lower_32_bits(dma);
>        trb->bph = upper_32_bits(dma);
>@@ -1004,7 +1002,7 @@ static void __dwc3_prepare_one_trb(struct dwc3_ep *dep,
>struct dwc3_trb *trb,
>        }
>
>        if ((!no_interrupt && !chain) ||
>-                       (dwc3_calc_trbs_left(dep) == 0))
>+                       (dwc3_calc_trbs_left(dep) == 1))
>                trb->ctrl |= DWC3_TRB_CTRL_IOC;
>
>        if (chain)
>@@ -1013,6 +1011,8 @@ static void __dwc3_prepare_one_trb(struct dwc3_ep *dep,
>struct dwc3_trb *trb,
>        if (usb_endpoint_xfer_bulk(dep->endpoint.desc) &&
>dep->stream_capable)
>                trb->ctrl |= DWC3_TRB_CTRL_SID_SOFN(stream_id);
>
>+       dwc3_ep_inc_enq(dep);
>+
>        trb->ctrl |= DWC3_TRB_CTRL_HWO;
>
>        trace_dwc3_prepare_trb(dep, trb);
>

Thanks for pointing out a solution , the fix looks good. I will test with this fix and resend the patches

Best Regards,
Anurag Kumar Vulisha