mwifiex: Read a PCI register after writing the TX ring write pointer

On the 88W8897 PCIe+USB card the firmware randomly crashes after setting
the TX ring write pointer. The issue is present in the latest firmware
version 15.68.19.p21 of the PCIe+USB card.

Those firmware crashes can be worked around by reading any PCI register
of the card after setting that register, so read the PCI_VENDOR_ID
register here. The reason this works is probably because we keep the bus
from entering an ASPM state for a bit longer, because that's what causes
the cards firmware to crash.

This fixes a bug where during RX/TX traffic and with ASPM L1 substates
enabled (the specific substates where the issue happens appear to be
platform dependent), the firmware crashes and eventually a command
timeout appears in the logs.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=109681
Cc: stable@vger.kernel.org
Signed-off-by: Jonas Dreßler <verdre@v0yd.nl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211011133224.15561-2-verdre@v0yd.nl

drivers/net/wireless/marvell/mwifiex/pcie.c

@@ -1490,6 +1490,14 @@ mwifiex_pcie_send_data(struct mwifiex_adapter *adapter, struct sk_buff *skb,
 			ret = -1;
 			goto done_unmap;
 		}
+
+		/* The firmware (latest version 15.68.19.p21) of the 88W8897 PCIe+USB card
+		 * seems to crash randomly after setting the TX ring write pointer when
+		 * ASPM powersaving is enabled. A workaround seems to be keeping the bus
+		 * busy by reading a random register afterwards.
+		 */
+		mwifiex_read_reg(adapter, PCI_VENDOR_ID, &rx_val);
+
 		if ((mwifiex_pcie_txbd_not_full(card)) &&
 		    tx_param->next_pkt_len) {
 			/* have more packets and TxBD still can hold more */