net: airoha: Compute number of descriptors according to reserved memory size

In order to not exceed the reserved memory size for hwfd buffers,
compute the number of hwfd buffers/descriptors according to the
reserved memory size and the size of each hwfd buffer (2KB).

Fixes: 3a1ce9e ("net: airoha: Add the capability to allocate hwfd buffers via reserved-memory")
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250619-airoha-hw-num-desc-v4-1-49600a9b319a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: LorenzoBianconi

drivers/net/ethernet/airoha/airoha_eth.c

@@ -1065,19 +1065,13 @@ static void airoha_qdma_cleanup_tx_queue(struct airoha_queue *q)
 
 static int airoha_qdma_init_hfwd_queues(struct airoha_qdma *qdma)
 {
+	int size, index, num_desc = HW_DSCP_NUM;
 	struct airoha_eth *eth = qdma->eth;
 	int id = qdma - &eth->qdma[0];
 	dma_addr_t dma_addr;
 	const char *name;
-	int size, index;
 	u32 status;
 
-	size = HW_DSCP_NUM * sizeof(struct airoha_qdma_fwd_desc);
-	if (!dmam_alloc_coherent(eth->dev, size, &dma_addr, GFP_KERNEL))
-		return -ENOMEM;
-
-	airoha_qdma_wr(qdma, REG_FWD_DSCP_BASE, dma_addr);
-
 	name = devm_kasprintf(eth->dev, GFP_KERNEL, "qdma%d-buf", id);
 	if (!name)
 		return -ENOMEM;
@@ -1099,15 +1093,24 @@ static int airoha_qdma_init_hfwd_queues(struct airoha_qdma *qdma)
 		rmem = of_reserved_mem_lookup(np);
 		of_node_put(np);
 		dma_addr = rmem->base;
+		/* Compute the number of hw descriptors according to the
+		 * reserved memory size and the payload buffer size
+		 */
+		num_desc = rmem->size / AIROHA_MAX_PACKET_SIZE;
 	} else {
-		size = AIROHA_MAX_PACKET_SIZE * HW_DSCP_NUM;
+		size = AIROHA_MAX_PACKET_SIZE * num_desc;
 		if (!dmam_alloc_coherent(eth->dev, size, &dma_addr,
 					 GFP_KERNEL))
 			return -ENOMEM;
 	}
 
 	airoha_qdma_wr(qdma, REG_FWD_BUF_BASE, dma_addr);
 
+	size = num_desc * sizeof(struct airoha_qdma_fwd_desc);
+	if (!dmam_alloc_coherent(eth->dev, size, &dma_addr, GFP_KERNEL))
+		return -ENOMEM;
+
+	airoha_qdma_wr(qdma, REG_FWD_DSCP_BASE, dma_addr);
 	airoha_qdma_rmw(qdma, REG_HW_FWD_DSCP_CFG,
 			HW_FWD_DSCP_PAYLOAD_SIZE_MASK,
 			FIELD_PREP(HW_FWD_DSCP_PAYLOAD_SIZE_MASK, 0));
@@ -1116,7 +1119,7 @@ static int airoha_qdma_init_hfwd_queues(struct airoha_qdma *qdma)
 	airoha_qdma_rmw(qdma, REG_LMGR_INIT_CFG,
 			LMGR_INIT_START | LMGR_SRAM_MODE_MASK |
 			HW_FWD_DESC_NUM_MASK,
-			FIELD_PREP(HW_FWD_DESC_NUM_MASK, HW_DSCP_NUM) |
+			FIELD_PREP(HW_FWD_DESC_NUM_MASK, num_desc) |
 			LMGR_INIT_START | LMGR_SRAM_MODE_MASK);
 
 	return read_poll_timeout(airoha_qdma_rr, status,