shithub: plan9front

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ref: d09e5a7af724cbb2df7a066a805cf6c8303a4b39
parent: 2380532ff58c2afd666bc9c40dbb7a65c77c204c
author: Michael Forney <>
date: Fri Nov 11 20:11:09 EST 2022

usbxhci: wait for reset to complete before continuing initialization

The xhci spec says that the HCRST bit is cleared once the reset is
complete, and that no operational or runtime registers shall be
written while the bit is set.

Waiting for the reset to complete fixes initialization of some host
controllers, such as the ASMedia ASM1142, which otherwise enter an
endless init-recover loop.

Also, add a comment about the reason for the 1ms delay. This appears
to work around a system hang bug in intel chipsets through the 400
series (see errata 15 in [0]).


--- a/sys/src/9/port/usbxhci.c
+++ b/sys/src/9/port/usbxhci.c
@@ -481,7 +481,10 @@
 		tsleep(&up->sleep, return0, nil, 10);
 	ctlr->opr[USBCMD] = HCRST;
+	/* some intel controllers require 1ms delay after reset */
+	for(i=0; (ctlr->opr[USBCMD] & HCRST) != 0 && i<100; i++)
+		tsleep(&up->sleep, return0, nil, 10);
 	for(i=0; (ctlr->opr[USBSTS] & (CNR|HCH)) != HCH && i<100; i++)
 		tsleep(&up->sleep, return0, nil, 10);