[PATCH] usb: xhci: Simplify clearing the Event Interrupt bit

[Michal Pecio <michal.pecio@gmail.com>]

USBSTS is mostly RW1C, so to clear EINT we should write just this
one bit. Remove pointless code which ORs the bit with current value
of the register, even though the bit is already known to be set,
and writes the result back, which clears all active RW1C flags.

We used to inadvertently clear PCD and SRE in this way. PCD isn't
used by the driver and SRE is only used at resume, so clearing them
should make no difference. Don't clear them anymore.

Tested by connecting and mounting a storage device on a few HCs.

Before: xhci_irq USBSTS 0x00000018 EINT PCD -> 0x00000000
        xhci_irq USBSTS 0x00000008 EINT -> 0x00000000
After:  xhci_irq USBSTS 0x00000018 EINT PCD -> 0x00000010 PCD
        xhci_irq USBSTS 0x00000018 EINT PCD -> 0x00000010 PCD

Some flags are RsvdZ - should be written as zero regardless of the
value read, so technically it was a bug. But no problems are known.

Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
---
 drivers/usb/host/xhci-ring.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 246a2b1573ff..0338daba5f9a 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -3209,10 +3209,9 @@ irqreturn_t xhci_irq(struct usb_hcd *hcd)
 	/*
 	 * Clear the op reg interrupt status first,
 	 * so we can receive interrupts from other MSI-X interrupters.
-	 * Write 1 to clear the interrupt status.
+	 * USBSTS bits are write 1 to clear.
 	 */
-	status |= STS_EINT;
-	writel(status, &xhci->op_regs->status);
+	writel(STS_EINT, &xhci->op_regs->status);
 
 	/* This is the handler of the primary interrupter */
 	xhci_handle_events(xhci, xhci->interrupters[0], false);
-- 
2.48.1