usb: xhci-mtk: relax TT periodic bandwidth allocation

usb: xhci-mtk: relax TT periodic bandwidth allocation

Currently xhci-mtk needs software-managed bandwidth allocation for
periodic endpoints, it allocates the microframe index for the first
start-split packet for each endpoint. As this index allocation logic
should avoid the conflicts with other full/low-speed periodic endpoints,
it uses the worst case byte budgets on high-speed bus bandwidth
For example, for an isochronos IN endpoint with 192 bytes budget,
it will consume the whole 4 u-frames(188 * 4) while the actual
full-speed bus budget should be just 192bytes.

This patch changes the low/full-speed bandwidth allocation logic
to use "approximate" best case budget for lower speed bandwidth
management. For the same endpoint from the above example, the
approximate best case budget is now reduced to (188 * 2) bytes.

Without this patch, many usb audio headsets with 3 interfaces
(audio input, audio output, and HID) cannot be configured
on xhci-mtk.

Signed-off-by: Ikjoon Jang <ikjn@chromium.org>
Link: https://lore.kernel.org/r/20210805133937.1.Ia8174b875bc926c12ce427a5a1415dea31cc35ae@changeid
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>