Merge branch 'net-ravb-fix-soc-specific-configuration-and-descriptor-handling-issues'

Lad Prabhakar says:

====================
net: ravb: Fix SoC-specific configuration and descriptor handling issues [part]

This series addresses several issues in the Renesas Ethernet AVB (ravb)
driver related descriptor ordering.

A potential ordering hazard in descriptor setup could cause
the DMA engine to start prematurely, leading to TX stalls on some
platforms.

The series includes the following changes:

Enforce descriptor type ordering to prevent early DMA start
Ensure proper write ordering of TX descriptor type fields to prevent the
DMA engine from observing an incomplete descriptor chain. This fixes
observed TX stalls on RZ/G2L platforms running RT kernels.

Tested on R/G1x Gen2, RZ/G2x Gen3 and RZ/G2L family hardware.
====================

Link: https://patch.msgid.link/20251017151830.171062-1-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

drivers/net/ethernet/renesas/ravb_main.c

@@ -2211,15 +2211,35 @@ static netdev_tx_t ravb_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 
 		skb_tx_timestamp(skb);
 	}
-	/* Descriptor type must be set after all the above writes */
-	dma_wmb();
+
 	if (num_tx_desc > 1) {
 		desc->die_dt = DT_FEND;
 		desc--;
+		/* When using multi-descriptors, DT_FEND needs to get written
+		 * before DT_FSTART, but the compiler may reorder the memory
+		 * writes in an attempt to optimize the code.
+		 * Use a dma_wmb() barrier to make sure DT_FEND and DT_FSTART
+		 * are written exactly in the order shown in the code.
+		 * This is particularly important for cases where the DMA engine
+		 * is already running when we are running this code. If the DMA
+		 * sees DT_FSTART without the corresponding DT_FEND it will enter
+		 * an error condition.
+		 */
+		dma_wmb();
 		desc->die_dt = DT_FSTART;
 	} else {
+		/* Descriptor type must be set after all the above writes */
+		dma_wmb();
 		desc->die_dt = DT_FSINGLE;
 	}
+
+	/* Before ringing the doorbell we need to make sure that the latest
+	 * writes have been committed to memory, otherwise it could delay
+	 * things until the doorbell is rang again.
+	 * This is in replacement of the read operation mentioned in the HW
+	 * manuals.
+	 */
+	dma_wmb();
 	ravb_modify(ndev, TCCR, TCCR_TSRQ0 << q, TCCR_TSRQ0 << q);
 
 	priv->cur_tx[q] += num_tx_desc;