usb: xhci: guarantee that IMAN register is flushed

Add read call to guarantee that the write to the IMAN register has
been flushed.

xHCI specification 1.2, section 5.5.2.1, Note:
"Most systems have write buffers that minimize overhead, but this may
 require a read operation to guarantee that the write has been flushed
 from the posted buffer."

Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20250515135621.335595-17-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-ring.c

@@ -3087,6 +3087,9 @@ static void xhci_clear_interrupt_pending(struct xhci_interrupter *ir)
 		irq_pending = readl(&ir->ir_set->irq_pending);
 		irq_pending |= IMAN_IP;
 		writel(irq_pending, &ir->ir_set->irq_pending);
+
+		/* Read operation to guarantee the write has been flushed from posted buffers */
+		readl(&ir->ir_set->irq_pending);
 	}
 }
 

drivers/usb/host/xhci.c

@@ -335,6 +335,8 @@ int xhci_enable_interrupter(struct xhci_interrupter *ir)
 	iman |= IMAN_IE;
 	writel(iman, &ir->ir_set->irq_pending);
 
+	/* Read operation to guarantee the write has been flushed from posted buffers */
+	readl(&ir->ir_set->irq_pending);
 	return 0;
 }
 
@@ -350,6 +352,7 @@ int xhci_disable_interrupter(struct xhci_interrupter *ir)
 	iman &= ~IMAN_IE;
 	writel(iman, &ir->ir_set->irq_pending);
 
+	readl(&ir->ir_set->irq_pending);
 	return 0;
 }