usb: Reduce USB scanning time

My current x86 platform (Bay Trail, not in mainline yet) has a quite complex USB infrastructure with many USB hubs. Here the USB scan takes an incredible huge amount of time:

starting USB...
USB0: USB EHCI 1.00
scanning bus 0 for devices... 9 USB Device(s) found

time: 28.415 seconds

This is of course not acceptable on platforms, where USB needs to get scanned at every bootup. As this increases the bootup time of this device by nearly 30 seconds!

This patch series greatly reduces the USB scanning time. This is done by multiple means:

- Remove or reduce delays and timeouts
- Remove a 2nd reset of the USB hubs
- Change USB port timeout handling and introduce quasi parallel USB port scanning

As a result, the USB scanning time is greatly reduced:

starting USB...
USB0: USB EHCI 1.00
scanning bus 0 for devices... 9 USB Device(s) found

time: 1.822 seconds

As you can see, the time is reduced from 28.4 to 1.8 seconds!

Please find more details to the changes in the patch description.

Testing and comments welcome!

Thanks,
Stefan

Changes in v5:
- Removed superfluous debug output from usb_scan_port()
- Replaced usb_hub_power_on() with usb_set_port_feature() in case of overcurrent detection. This is whats really needed here.
- Added a per-port overcurrent counter and stop re-scanning of a device that exceeds the PORT_OVERCURRENT_MAX_SCAN_COUNT
- Moved list_del() to end of loop in usb_scan_port()
- Moved timeout / delay variables from USB dev to hub struct as they are hub specific
- Moved state_get_skip_delays() to usb_hub_power_on() so that the timeouts are not set there

Changes in v4:
- Minor rewording / fixes of the commit text
- Add Acked-by / Tested-by from Hans and Stephen
- Minor rewording / fixes of the commit text
- Add Acked-by from Hans
- Moved check for query_delay into usb_scan_port() as suggested by Hans
- Correct list handling (drop INIT_LIST_HEAD)
- Added some missing free() calls
- Changed connect_timeout calculation as suggested by Stephen
- Moved usb_scan_list to other globals to be cleaned up in a later patch
- Added timeout check for non-functional ports (usb_get_port_status return error
- Reverted if logic in loop to remove an indentation level
- Moved debug() output
- Removed unnecessary if when already connected
- Added Hans's Acked-by
- Added Stephen's Tested-by
- Minor rewording / fixes of the commit text

Changes in v3:
- Changed small timeout from 10ms to 20ms as this results in a much faster USB scanning time (10ms too small and 20ms enough in many cases)
- Introduced scanning list containing all USB devices of one USB controller that need to get scanned
- Don't delay in usb_hub_power_on(). Instead skip querying these devices until the delay time is reached.

Changes in v2:
- Add Acked-by / Tested-by from Hans and Stephen
- Make this change unconditional
- Add Acked-by / Tested-by from Hans and Stephen
- Make this change unconditional
- Add Tested-by from Stephen
- Remove static USB port configuration patch (for now)

Stefan Roese (4):
usb: legacy_hub_port_reset(): Speedup hub reset handling
usb: Remove 200 ms delay in usb_hub_port_connect_change()
usb: Don't reset the USB hub a 2nd time
usb: Change power-on / scanning timeout handling

 common/usb.c     |  13 +--
 common/usb_hub.c | 329 ++++++++++++++++++++++++++++++++++++++-----------------
 include/usb.h    |   4 +
 3 files changed, 235 insertions(+), 111 deletions(-)