Merge pull request #3544 from timg236/usb-msd-pwr-off-time

USB_MSD_PWR_OFF_TIME is Pi4 only

documentation/asciidoc/computers/raspberry-pi/eeprom-bootloader.adoc

@@ -499,9 +499,22 @@ Default: `2000` (2 seconds)
 [[USB_MSD_PWR_OFF_TIME]]
 ==== `USB_MSD_PWR_OFF_TIME`
 
-During USB mass storage boot, power to the USB ports is switched off for a short time to ensure the correct operation of USB mass storage devices. Most devices work correctly using the default setting: change this only if you have problems booting from a particular device. Setting `USB_MSD_PWR_OFF_TIME=0` will prevent power to the USB ports being switched off during USB mass storage boot.
+Raspberry Pi 4 only.
 
-Minimum: `250`
+When the Pi is rebooted power USB power is switched off by the hardware. A short power off time can
+cause problems with some USB devices so this parameter may be used to force a longer power off
+as though the cable was physically removed.
+
+On Pi4 v1.3 and older the configurable/long power off requires the XHCI controller to be enabled
+so there is actually a short power off followed by a longer configurable power off. The longer
+configurable power off may be skipped by setting this parameter to zero.
+
+On newer revisions the hardware ensures that USB power is off from reboot and the bootloader
+only enables power after this timeout has elapsed. This is happens after memory is initialised
+ensuring that USB power is off for at least two seconds. Therefore, this parameter generally
+has no effect on newer hardware revisions.
+
+Minimum: `0`
 
 Maximum: `5000`