libpayload: usb: Add interval attribute to endpoints

Read bInterval from endpoint descriptors and store it in our endpoint_t
struct. The interval is encoded dependently on the device' speed and the
endpoint's type. Therefore, it will be normalized to the binary logarithm
of the number of microframes, i.e.
  t = 125us * 2^interval

The interval attribute will be used in the xHCI driver.

Change-Id: I65a8eda6145faf34666800789f0292e640a8141b
Signed-off-by: Nico Huber <>
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <>
2 files changed