[RFC PATCH 03/12] usb: xhci: simplify USBSTS register reset

[Niklas Neronin <niklas.neronin@linux.intel.com>]

The current code clears the USB Status Register (USBSTS) and then sets
the Enable Interrupt (EINT) bit to '1'. A mask (~0x1fff) is used to avoid
modifying reserved bits 31:13, but it also inadvertently excludes reserved
bits 7:5, 1. These fields are defined as RsvdZ, so writing '0' to them has
no effect.

According to the xHCI specification, section 5.1.1:
"System software shall mask all reserved fields (Rsvd, RsvdP or RsvdZ) to
 '0' before evaluating a register or data structure value. This will enable
 current system software to run with future xHCI implementations that
 define the reserved fields."

Given this, it is safe to write zero to all bits of USBSTS except EINT.
This allows the mask and the register read to be removed, simplifying the
code, avoiding potential future issues and removing unexplained hex.

USB Status Register; xHCI Specification, version 1.2 section 5.4.2

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
---
 drivers/usb/host/xhci.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
index a288f59c604c..ad095768ed48 100644
--- a/drivers/usb/host/xhci.c
+++ b/drivers/usb/host/xhci.c
@@ -705,7 +705,6 @@ EXPORT_SYMBOL_GPL(xhci_run);
  */
 void xhci_stop(struct usb_hcd *hcd)
 {
-	u32 temp;
 	struct xhci_hcd *xhci = hcd_to_xhci(hcd);
 	struct xhci_interrupter *ir = xhci->interrupters[0];
 
@@ -740,8 +739,7 @@ void xhci_stop(struct usb_hcd *hcd)
 
 	xhci_dbg_trace(xhci, trace_xhci_dbg_init,
 			"// Disabling event ring interrupts");
-	temp = readl(&xhci->op_regs->status);
-	writel((temp & ~0x1fff) | STS_EINT, &xhci->op_regs->status);
+	writel(STS_EINT, &xhci->op_regs->status);
 	xhci_disable_interrupter(xhci, ir);
 
 	xhci_dbg_trace(xhci, trace_xhci_dbg_init, "cleaning up memory");
@@ -1174,8 +1172,7 @@ int xhci_resume(struct xhci_hcd *xhci, bool power_lost, bool is_auto_resume)
 			return retval;
 
 		xhci_dbg(xhci, "// Disabling event ring interrupts\n");
-		temp = readl(&xhci->op_regs->status);
-		writel((temp & ~0x1fff) | STS_EINT, &xhci->op_regs->status);
+		writel(STS_EINT, &xhci->op_regs->status);
 		xhci_disable_interrupter(xhci, xhci->interrupters[0]);
 
 		xhci_dbg(xhci, "cleaning up memory\n");
-- 
2.50.1