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usbehci, usbxhci: add *noehcihandoff= and *noxhcihandoff= parameters

on Samsung ATIV Smart PC Pro XE00T1C-A01CL, the EHCI handoff
causes the system to freeze in UEFI mode as soon as we assert
the os semaphore bit.

until a general solution is found, provide these parameters to
disable the handoff for now as it seems to otherwise work fine.

--- a/sys/src/9/pc/usbehcipc.c

+++ b/sys/src/9/pc/usbehcipc.c

@@ -41,7 +41,7 @@

 	off = ehciecap(ctlr, Clegacy);

 	if(off == -1)

 		return;

-	if(pcicfgr8(ctlr->pcidev, off+CLbiossem) != 0){

+	if(getconf("*noehcihandoff") == nil && pcicfgr8(ctlr->pcidev, off+CLbiossem) != 0){

 		dprint("ehci %#p: bios active, taking over...\n", ctlr->capio);

 		pcicfgw8(ctlr->pcidev, off+CLossem, 1);

 		for(i = 0; i < 100; i++){

--- a/sys/src/9/pc/usbxhci.c

+++ b/sys/src/9/pc/usbxhci.c

@@ -393,11 +393,13 @@

 	if((r = xecp(ctlr, 1, nil)) == nil)

 		return;

-	r[0] |= 1<<24;		/* request ownership */

-	for(i = 0; (r[0] & (1<<16)) != 0 && i<100; i++)

-		tsleep(&up->sleep, return0, nil, 10);

+	if(getconf("*noxhcihandoff") == nil){

+		r[0] |= 1<<24;		/* request ownership */

+		for(i = 0; (r[0] & (1<<16)) != 0 && i<100; i++)

+			tsleep(&up->sleep, return0, nil, 10);

+		r[0] &= ~(1<<16);	/* in case of timeout */

+	}

 	r[1] = 0;		/* disable SMI interrupts */

-	r[0] &= ~(1<<16);	/* in case of timeout */

 }

 static void