[PATCH net-next 5/6] net: stmmac: use first_desc for TBS

[Russell King (Oracle)]

Rather than freshly getting a pointer for the TBS descriptor (because
we want to access its enhanced fields) convert the existing first_desc
basic descriptor to a pointer to the enhanced descriptor.

Add a comment explaining why it is safe to convert from the basic
descriptor pointer to the enhanced descriptor pointer.

Signed-off-by: Russell King (Oracle) <rmk+kernel at armlinux.org.uk>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 95da2940cc3d..a733249c3f9b 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -4801,10 +4801,15 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (priv->sarc_type)
 		stmmac_set_desc_sarc(priv, first_desc, priv->sarc_type);
 
+	/* STMMAC_TBS_EN can only be set if SYMMC_TBS_AVAIL has already
+	 * been set, which means the underlying type of the descriptors
+	 * will be struct stmmac_edesc. Therefore, it is safe to convert
+	 * the basic descriptor to the enhanced descriptor here.
+	 */
 	if (tx_q->tbs & STMMAC_TBS_EN) {
 		struct timespec64 ts = ns_to_timespec64(skb->tstamp);
 
-		tbs_desc = &tx_q->dma_entx[first_entry];
+		tbs_desc = dma_desc_to_edesc(first_desc);
 		stmmac_set_desc_tbs(priv, tbs_desc, ts.tv_sec, ts.tv_nsec);
 	}
 
-- 
2.47.3