High CPU load produced by USB (DW2)

Minas Harutyunyan Minas.Harutyunyan at synopsys.com
Mon Feb 19 00:19:40 PST 2018

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.


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