High CPU load produced by USB (DW2)

Marek Vasut marek.vasut at gmail.com
Mon Feb 19 00:51:44 PST 2018

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?

Best regards,
Marek Vasut

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