linux-kernel - Infinite looping in omap2430.c USB driver

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Date:	Sat, 7 Jul 2012 08:39:49 +1000
From:	NeilBrown <neilb@...e.de>
To:	Felipe Balbi <balbi@...com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-omap@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Infinite looping in omap2430.c USB driver

Hello `./scripts/get_maintainer.pl -f drivers/usb/musb/omap2430.c`

omap2430_musb_set_vbus in omap2430.c contains:

			while (musb_readb(musb->mregs, MUSB_DEVCTL) & 0x80) {

				cpu_relax();

				if (time_after(jiffies, timeout)) {
					dev_err(musb->controller,
					"configured as A device timeout");
					ret = -EINVAL;
					break;
				}
			}

having set
	unsigned long timeout = jiffies + msecs_to_jiffies(1000);

so it can busy-loop for up to 1 second.  Probably not ideal, but if it works
I wouldn't complain.

The
	if (int_usb & MUSB_INTR_SESSREQ) {
branch of musb_stage0_irq() called from musb_interrupt (from
generic_interrupt) calls this:

	if (musb->int_usb)
		retval |= musb_stage0_irq(musb, musb->int_usb,
				devctl, power);

so the busy loop can happen in an interrupt handler (not a threaded interrupt
handler), which is probably less ideal.

However this can be called with interrupt disabled, as happens at least
during resume when resume_irqs() calls:

		raw_spin_lock_irqsave(&desc->lock, flags);
		__enable_irq(desc, irq, true);
		raw_spin_unlock_irqrestore(&desc->lock, flags);

and an interrupt is found to be IRQS_PENDING.

In this case interrupts are disabled so 'jiffies' never changes so this loop
can continue forever.

This happens on my (GTA04) phone fairly regularly - between 1 in 10 and 1 in
30 resumes. The musb-hdrc interrupt is pending and reports

[ 4957.624176] musb-hdrc musb-hdrc: ** IRQ peripheral usb0040 tx0000 rx0000

'usb0040' is MUSB_INTR_SESSREQ.  I think this is triggered by detecting a
voltage change on the USB ID pin - is that right?  A short-to-earth would be
a request to switch to host mode, which is why it tries to enable VBUS.
Maybe there is some electrical noise which is being picked up?

In any case I get the interrupt despite nothing being plugged in, and the 0x80
bit of MUSB_DEVCTL never gets cleared.

I've added a simple loop counter which aborts the loop after 1000 loops -
this takes about 5 seconds, but includes some printks which probably slow it
down.

In 2 out of 2 cases, subsequent messages show that the hsmmc driver for the
uSD card that holds my root filesystem is messed up.  It seems to be waiting
for a request that is never going to complete.
So maybe the hsmmc is causing the noise that triggers the musb issue.

I can send a patch which add a loop count if you like, but I suspect you can
come up with a much better approach.

Thanks,
NeilBrown

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