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

[Adam Wallis <awallis-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

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?

> XHCI_MTK_HOST could then use whatever preferred device propery interval value,
> and we can get rid of using XHCI_MTK_HOST quirk flag when setting up the IMODI
>  
> -Mathias
> 
> 
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Adam

-- 
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.
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