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.
Thanks
-Mathias
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