Subject: [PATCH] net: ethernet: mtk_eth_soc: cache hardware pointer of last freed tx descriptor
From: Felix Fietkau <[email-masked]>
Date: Thu, 27 Aug 2020 06:32:03 +0200

The value is only updated by the CPU, so it is cheaper to access from the ring
data structure than from a hardware register

Signed-off-by: Felix Fietkau <[email-masked]>
---

--- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
+++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
@@ -1358,7 +1358,7 @@ static int mtk_poll_tx_qdma(struct mtk_e
 	struct mtk_tx_buf *tx_buf;
 	u32 cpu, dma;
 
-	cpu = mtk_r32(eth, MTK_QTX_CRX_PTR);
+	cpu = ring->last_free_ptr;
 	dma = mtk_r32(eth, MTK_QTX_DRX_PTR);
 
 	desc = mtk_qdma_phys_to_virt(ring, cpu);
@@ -1392,6 +1392,7 @@ static int mtk_poll_tx_qdma(struct mtk_e
 		cpu = next_cpu;
 	}
 
+	ring->last_free_ptr = cpu;
 	mtk_w32(eth, cpu, MTK_QTX_CRX_PTR);
 
 	return budget;
@@ -1592,6 +1593,7 @@ static int mtk_tx_alloc(struct mtk_eth *
 	atomic_set(&ring->free_count, MTK_DMA_SIZE - 2);
 	ring->next_free = &ring->dma[0];
 	ring->last_free = &ring->dma[MTK_DMA_SIZE - 1];
+	ring->last_free_ptr = (u32)(ring->phys + ((MTK_DMA_SIZE - 1) * sz));
 	ring->thresh = MAX_SKB_FRAGS;
 
 	/* make sure that all changes to the dma ring are flushed before we
@@ -1605,9 +1607,7 @@ static int mtk_tx_alloc(struct mtk_eth *
 		mtk_w32(eth,
 			ring->phys + ((MTK_DMA_SIZE - 1) * sz),
 			MTK_QTX_CRX_PTR);
-		mtk_w32(eth,
-			ring->phys + ((MTK_DMA_SIZE - 1) * sz),
-			MTK_QTX_DRX_PTR);
+		mtk_w32(eth, ring->last_free_ptr, MTK_QTX_DRX_PTR);
 		mtk_w32(eth, (QDMA_RES_THRES << 8) | QDMA_RES_THRES,
 			MTK_QTX_CFG(0));
 	} else {
--- a/drivers/net/ethernet/mediatek/mtk_eth_soc.h
+++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.h
@@ -624,6 +624,7 @@ struct mtk_tx_buf {
  * @phys:		The physical addr of tx_buf
  * @next_free:		Pointer to the next free descriptor
  * @last_free:		Pointer to the last free descriptor
+ * @last_free_ptr:	Hardware pointer value of the last free descriptor
  * @thresh:		The threshold of minimum amount of free descriptors
  * @free_count:		QDMA uses a linked list. Track how many free descriptors
  *			are present
@@ -634,6 +635,7 @@ struct mtk_tx_ring {
 	dma_addr_t phys;
 	struct mtk_tx_dma *next_free;
 	struct mtk_tx_dma *last_free;
+	u32 last_free_ptr;
 	u16 thresh;
 	atomic_t free_count;
 	int dma_size;