15.0 on RPi4, USB broken: uhub_reattach_port: port 2 reset failed, error=USB_ERR_TIMEOUT

hello,

i'm using 15.0 (f66a994d59) on an 4GB RPi4 with a USB<>SATA adapter for
the root disk:

usb_msc_auto_quirk: UQ_MSC_NO_GETMAXLUN set for USB mass storage device ASM1153USB3.0TOSATA ASM1153USB3.0TOSATA (0x174c:0x55aa)
ugen0.3: <ASM1153USB3.0TOSATA ASM1153USB3.0TOSATA> at usbus0
umass0 on uhub1
umass0: <ASM1153USB3.0TOSATA ASM1153USB3.0TOSATA, class 0/0, rev 2.10/1.00, addr 2> on usbus0
umass0:  SCSI over Bulk-Only; quirks = 0x0100
umass0:1:0: Attached to scbus1
da0 at umass-sim0 bus 0 scbus1 target 0 lun 0
da0: <ASM1153U ASM1153USB3.0TOS 0> Fixed Direct Access SPC-4 SCSI device
da0: Serial Number 123456789019
da0: 40.000MB/s transfers
da0: 228936MB (468862128 512 byte sectors)
da0: quirks=0x2<NO_6_BYTE>

when connected via USB 2, this works fine.  when connected via USB 3.0,
the device sometimes fails to attach on boot, causing mountroot to fail.
i can reproduce this reliably with both GENERIC-NODEBUG and a custom
modular kernel, and sometimes (but not every boot) with GENERIC.

when the problem happens, with USB_DEBUG enabled, the kernel logs:

uhub_reattach_port: port 2 reset failed, error=USB_ERR_TIMEOUT
uhub_reattach_port: device problem (USB_ERR_TIMEOUT), disabling port 2

however, if i boot with "boot -v", the device is reliably detected
correctly.  since -v shouldn't cause any functional changes, i suspect
this may be some kind of timing issue.

i've tried increasing some of the USB timings (hw.usb.timings.*) but
this didn't seem to have any effect.  is there anything else i could try
that might affect this, or is this perhaps a known issue?

	thanks, lexi.