Hello,
Recently we came across an embedded platform which has a very complex
USB infrastructure. In total, 17 USB devices are connected over various
USB hubs. Initial USB scanning on this platform takes an incredible
amounts of time ~22 seconds. This huge delay is unacceptable if we are
required to boot from USB and need to perform scanning on each boot-up.
However this huge USB scanning delay is only apparent in Barebox as
subsequent Linux drivers don't take as much time to enumerate the bus.
Thus we decided to investigate and try to optimize USB scanning time as
much as possible without loosing any functionality & retain stability.
Code analysis of USB hub driver revealed many calls to mdelay(). Some of
them unnecessary and some even longer than 1 second per port, statically
placed without any checks if required at all. These delays significantly
degrade USB scanning performance and must be reworked.
Investigation lead us also to U-boot which was already tested by us on
the same i.MX6 platform and didn't show such poor USB performance. We
found out U-boot already implements quasi parallel USB scanning
strategy and thus we decided to port this functionality to Barebox.
We took great deal of ideas and code from U-boot patch series by Stefan
Roese: https://lists.denx.de/pipermail/u-boot/2016-March/248493.html
Although we didn't achieve the same level of delay reduction as reported
by Mr. Roese (28.4 to 1.8 seconds), USB scanning in our case now takes
around 4.5 seconds (21.7 to 4.5 seconds) which is a significant
improvement!
We could take this patch series a step further by also porting patch 3/4
"usb: Don't reset the USB hub a 2nd time" from U-boot. But according to
comment in Barebox code: "This is a Windows scheme of initialization
sequence, with double reset of the device (Linux uses the same
sequence)" we wanted to be safe and left the double device reset
functionality intact for the sake of retaining stability over speed
performance.
All further details on changes are described in each patch description.
Best regards,
Primoz Fiser
Primoz Fiser (3):
usb: hub_port_reset(): Speedup hub reset handling
usb: usb_hub_port_connect_change(): Remove unnecessary delays
usb: Change power-on / scanning timeout handling
drivers/usb/core/hub.c | 184 +++++++++++++++++++++++++++++++++++------
include/usb/usb.h | 3 +
2 files changed, 162 insertions(+), 25 deletions(-)
--
2.17.1
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