High CPU load produced by USB (DW2)

[Minas Harutyunyan]

On 2/19/2018 12:51 PM, Marek Vasut wrote:
> On 02/19/2018 09:19 AM, Minas Harutyunyan wrote:
>> On 2/17/2018 12:07 AM, Marek Vasut wrote:
>>> 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 ?
>>>
>> If core acting as Host in Buffer DMA mode then 8k IRQs per second (SOF
>> interrupts) is expected if connected device(s) has periodic endpoint.
> 
> Is there a way to reduce that or is that the absolute minimum in HS mode?
> 
We already discussed, in this email thread earlier, why SOF interrupts 
required and unmasked.
Only in case when connected device with CTRL+BLK EP's only (like flash 
drive) and directly connected to cores root HUB, SOF's will be masked.
Thanks,
Minas