[PATCH 17/22] usb: xhci: preserve RW bits in xhci_port_state_to_neutral()

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

xhci_port_state_to_neutral() prepares a PORTSC value that can be
written back without changing port state by dropping bits with
side effects on write (e.g. Read-Write-1-to-Clear).

Currently the helper preserves Read-Only (RO) and Read/Write Sticky
(RWS) bits, but omits standard Read/Write (RW) bits. This does not
currently cause issues, since the only RW bit, Port Link State Write
Strobe (LWS), always reads as 0 and ignores 0 writes.

Preserve RW bits as well to make the helper consistent with its
intent and robust against future changes to PORTSC definitions.

Update the function comment accordingly.

Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
---
 drivers/usb/host/xhci-hub.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/drivers/usb/host/xhci-hub.c b/drivers/usb/host/xhci-hub.c
index 111db70642b1..0375929aac70 100644
--- a/drivers/usb/host/xhci-hub.c
+++ b/drivers/usb/host/xhci-hub.c
@@ -386,25 +386,22 @@ static unsigned int xhci_port_speed(int portsc)
 	return 0;
 }
 
-/**
- * xhci_port_state_to_neutral() - Clean up read portsc value back into writeable
- * @portsc: u32 port value read from portsc register to be cleanup up
+/*
+ * Given a PORTSC register value, return a "neutral" value that can be written
+ * back to the register without changing the current port state.
  *
- * Given a portsc, this function returns a value that would result in the
- * port being in the same state, if the value was written to the port status
- * control register.
- * Save Read Only (RO) bits and save read/write bits where
- * writing a 0 clears the bit and writing a 1 sets the bit (RWS).
- * For all other types (RW1S, RW1CS, RW, and RZ), writing a '0' has no effect.
+ * The function preserves:
+ *  - Read-Only (RO) bits
+ *  - Read/Write (RW) bits
+ *  - Read/Write Sticky (RWS) bits
  *
- * Return: u32 value that can be written back to portsc register without
- * changing port state.
+ * For other bit types (e.g. RW1S, RW1CS, and RsvdZ), writing 0 has no effect,
+ * so they are intentionally cleared in the returned value to avoid unintended
+ * side effects.
  */
-
 u32 xhci_port_state_to_neutral(u32 portsc)
 {
-	/* Save read-only status and port state */
-	return (portsc & PORTSC_RO_BITS) | (portsc & PORTSC_RWS_BITS);
+	return (portsc & PORTSC_RO_BITS) | (portsc & PORTSC_RWS_BITS) | (portsc & PORTSC_RW_BITS);
 }
 EXPORT_SYMBOL_GPL(xhci_port_state_to_neutral);
 
-- 
2.50.1