[PATCH 2/2] usb: xhci: use writeq() for CA write on 64-bit architectures

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

Setting the Command Abort (CA) bit in the Command Ring Control Register
(CRCR) stops command execution by clearing the Command Ring Running (CRR).
Some controllers latch CRCR only when the upper 32 bits are written,
requiring a retry sequence when the initial lower 32-bit write does not
update CRR. While CRR=1, the controller ignores all CRP updates, so no
field other than CA may be modified.

On 64-bit architectures (CONFIG_64BIT=y), writeq() performs a single,
atomic 64-bit MMIO write. Using writeq() for the CA write ensures that all
64 bits reach the controller in one bus transaction, without giving
controller the opportunity to process the abort between 32-bit writes.

All xHCI 64-bit registers are accessed via lo_hi_writeq().
Earlier attempts to replace these with writeq()/readq() caused regressions
and were reverted [1]. The underlying cause was never identified [2].
It may have been a quirk in writeq() implementation or controller-specific
hardware behavior, both of which are likely no longer relevant after more
than a decade of kernel and hardware evolution.

To reduce risk, this change introduces writeq() only for the CA write path
while retaining the fallback upper 32-bit write. This keeps the change
contained and allows gradual validation across different hardware.

Link: https://git.kernel.org/torvalds/c/477632dff5c7 [1]
Link: https://marc.info/?t=139093294600002&r=1&w=2 [2]
Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
---
 drivers/usb/host/xhci-ring.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 10160e76df68..e7910ae2e488 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -499,7 +499,12 @@ static int xhci_abort_cmd_ring(struct xhci_hcd *xhci, unsigned long flags)
 	/* Preserve RsvdP (5:4), other writable bits read 0. */
 	crcr_lo = readl(&xhci->op_regs->cmd_ring);
 	crcr_lo |= CMD_RING_ABORT;
+
+#ifdef CONFIG_64BIT
+	writeq(crcr_lo, &xhci->op_regs->cmd_ring);
+#else
 	writel(crcr_lo, &xhci->op_regs->cmd_ring);
+#endif
 
 	/* In the future we should try to recover a -ETIMEDOUT with a host controller reset */
 	ret = xhci_handshake(&xhci->op_regs->cmd_ring, CMD_RING_RUNNING, 0, 3 * USEC_PER_SEC);
-- 
2.50.1