High CPU load produced by USB (DW2)

[Marek Vasut]

On 02/16/2018 06:59 AM, Minas Harutyunyan wrote:
> On 2/15/2018 5:20 PM, Mirza Krak wrote:
>> On 14 February 2018 at 13:07, Minas Harutyunyan
>> <Minas.Harutyunyan at synopsys.com> wrote:
>>> On 2/14/2018 12:57 PM, Mirza Krak wrote:
>>>> On 8 February 2018 at 14:53, Minas Harutyunyan
>>>> <Minas.Harutyunyan at synopsys.com> wrote:
>>
>> < snip >
>>
>>>
>>> I reviewed your interrupt count log again. About 140,000 interrupts in 2
>>> seconds, obviously it's not SOF only interrupts. More probably, its NAK
>>> respond interrupts to SSPLIT/CSPLIT transactions. For this case I can
>>> recommend you to apply patch from Douglas Anderson: "[PATCH v2] usb:
>>> dwc2: host: Don't retry NAKed transactions right away" which already
>>> merged to 4.16-rc1.
>>>
>>> In your setups you see different behavior on different HUBs. Your HUBs
>>> have different "TT think time": 8 and 32. In USB2.0 spec "TT think time"
>>> described as follow "TT requires at most 8/32 FS bit times of inter
>>> transaction gap on a full-/low-speed downstream bus". So, your "worst"
>>> HUB with "TT think time"=8 sending more frequently SSPLIT/CSPLIT
>>> transactions which replied by NAK. As result you see about 4 time more
>>> interrupts comparing to "good" HUB. Could you please check interrupts
>>> count for "good" HUB and check "4 time" hypothesis.
>>
>> Did some further testing. The "good" HUB is actually as bad as the
>> "bad" HUB, and it was my setup that caused the different behavior. Did
>> not use the same devices etc.
>>
>> Once I made sure that the configuration and setup was the same on both
>> board I could see that the behaved similarly. And that is the
>> following interrupt load:
>>
>> - BT USB (FS) = ~80k interrupts / second
>> - Keyboard (FS) = ~80k interrupts / second
>> - WiFI USB (HS) = ~8k interrupts / second
>>
>> After applying the suggested patch [1], it is steady around 8k
>> interrupts / second no matter what device I connect (HS/FS,
>> HUB/NO-HUB). Which is acceptable and usable.
> 
> Great!

So 8k IRQs per second is what I should expect from this HW with HS
device attached, that's normal and cannot be reduced ?

-- 
Best regards,
Marek Vasut