[PATCH 1/2] usb: xhci: add relaxed timing quirk bit

[Adam Wallis]

On 11/23/2017 5:59 AM, Mathias Nyman wrote:
> On 23.11.2017 01:32, Adam Wallis wrote:
>> On 11/22/2017 10:24 AM, Mathias Nyman wrote:
>> [..]
>>>
>>> We know have at least two hosts/platforms that need custom interrupt moderation
>>> values
>>>
>>> How about adding a u32 device property for xhci with the interrupt moderation
>>> interval in
>>> nanoseconds?  And also add a u32 imod_interval variable to struct xhci_hcd?
>>>   imod_interval can be set to the current default 40000ns (160*250ns) and
>>> overwritten if
>>> device_property_read_u32() returns something else.
>>>
>>
>> Isn't the 160 value quite aggressive anyway? Section 5.5.2.2 of the xHCI spec
>> says that maximum observable interrupt rate should never exceed 8000
>> interrupts/second. I believe the IMOD value in the most aggressive case would
>> then be 500 by this statement [ 1 / (250e-9 * 500) = 8000 irqs/second ]
>>
>> Perhaps I am misreading the spec or just doing the math wrong? With the default
>> value of 160, we are interrupting 25,000 irq/second...which is over 3 times the
>> maximum stated value (again, assuming I did the math right)
>>
>> Anyway, my preference would be to set the IMOD default val to 4000 (~1ms) per
>> the recommended value in Table 49 of the spec and allow platforms to adjust as
>> necessary from that point.
>>
>> Thoughts on this?
>>
> 
> I think current 40us is still reasonable, it's about ~3 times per
> usb microframe (125us) .8000 interrupts per second just covers the microframe rate,
> which is the shortest interval a interrupt transfer can require service.
> 
> 1ms interrupt interval is too long. In the worst case ~8 microframes could pass
> before the driver is aware of a error it needs to take care of.
> USB 3.1 devices can transfer 6 burst of 16 max sized packets (6 x 16 x 2014) bytes
> per microframe.
> 
> closer to 125us could probably work as well, but unless we are fixing some issue or
> getting some other bigger benefit out of it I don't think it's worth changing it
> just to see if it breaks stuff.

I agree - my patch will just stick with the current default as the fallback
value if no device property is submitted.

> 
> Thanks
> -Mathias
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-- 
Adam Wallis
Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project.