ARM: 5965/1: Fix soft lockup in at91 udc driver

Fix a potential soft lockup in the AT91 UDC driver by ensuring that
the UDC clock is enabled inside the interrupt handler. If the UDC clock is not enabled then the UDC registers cannot be written to
and the interrupt cannot be cleared or masked.

Note that this patch (and other parts of the existing AT91 UDC
driver) is potentially racy for preempt-rt kernels,
but is okay for mainline.

For more info see:

http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20100203/09cdb3b4/attachment.el

http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20100203/8443a1e4/attachment.el

Signed-off-by: Ryan Mallon <ryan@bluewatersys.com>
Acked-by: Harro Haan <hrhaan@gmail.com>
Tested-by: Remy Bohmer <linux@bohmer.net>
Acked-by: Andrew Victor <avictor.za@gmail.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

drivers/usb/gadget/at91_udc.c

@@ -1370,6 +1370,12 @@ static irqreturn_t at91_udc_irq (int irq, void *_udc)
 {
 	struct at91_udc		*udc = _udc;
 	u32			rescans = 5;
+	int			disable_clock = 0;
+
+	if (!udc->clocked) {
+		clk_on(udc);
+		disable_clock = 1;
+	}
 
 	while (rescans--) {
 		u32 status;
@@ -1458,6 +1464,9 @@ static irqreturn_t at91_udc_irq (int irq, void *_udc)
 		}
 	}
 
+	if (disable_clock)
+		clk_off(udc);
+
 	return IRQ_HANDLED;
 }