net: ionic: add dma_wmb() before ringing TX doorbell

JIRA: https://issues.redhat.com/browse/RHEL-121486

commit d261f5b
Author: Mohammad Heib <mheib@redhat.com>
Date:   Fri Oct 31 17:52:02 2025 +0200

    net: ionic: add dma_wmb() before ringing TX doorbell

    The TX path currently writes descriptors and then immediately writes to
    the MMIO doorbell register to notify the NIC.  On weakly ordered
    architectures, descriptor writes may still be pending in CPU or DMA
    write buffers when the doorbell is issued, leading to the device
    fetching stale or incomplete descriptors.

    Add a dma_wmb() in ionic_txq_post() to ensure all descriptor writes are
    visible to the device before the doorbell MMIO write.

    Fixes: 0f3154e ("ionic: Add Tx and Rx handling")
    Signed-off-by: Mohammad Heib <mheib@redhat.com>
    Link: https://patch.msgid.link/20251031155203.203031-1-mheib@redhat.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Jakub Ramaseuski <jramaseu@redhat.com>

Author: Jakub Ramaseuski

drivers/net/ethernet/pensando/ionic/ionic_txrx.c

@@ -29,6 +29,10 @@ static void ionic_tx_clean(struct ionic_queue *q,
 
 static inline void ionic_txq_post(struct ionic_queue *q, bool ring_dbell)
 {
+	/* Ensure TX descriptor writes reach memory before NIC reads them.
+	 * Prevents device from fetching stale descriptors.
+	 */
+	dma_wmb();
 	ionic_q_post(q, ring_dbell);
 }