stmmac: fix and better tune the default buffer sizes

This patch is to fix and tune the default buffer sizes.
It reduces the default bufsize used by the driver from
4KiB to 1536 bytes.

Patch has been tested on both ARM and SH4 platform based.
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -92,8 +92,8 @@ static int tc = TC_DEFAULT;
 module_param(tc, int, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(tc, "DMA threshold control value");
 
-#define DMA_BUFFER_SIZE	BUF_SIZE_4KiB
-static int buf_sz = DMA_BUFFER_SIZE;
+#define	DEFAULT_BUFSIZE	1536
+static int buf_sz = DEFAULT_BUFSIZE;
 module_param(buf_sz, int, S_IRUGO | S_IWUSR);
 MODULE_PARM_DESC(buf_sz, "DMA buffer size");
 
@@ -136,8 +136,8 @@ static void stmmac_verify_args(void)
 		dma_rxsize = DMA_RX_SIZE;
 	if (unlikely(dma_txsize < 0))
 		dma_txsize = DMA_TX_SIZE;
-	if (unlikely((buf_sz < DMA_BUFFER_SIZE) || (buf_sz > BUF_SIZE_16KiB)))
-		buf_sz = DMA_BUFFER_SIZE;
+	if (unlikely((buf_sz < DEFAULT_BUFSIZE) || (buf_sz > BUF_SIZE_16KiB)))
+		buf_sz = DEFAULT_BUFSIZE;
 	if (unlikely(flow_ctrl > 1))
 		flow_ctrl = FLOW_AUTO;
 	else if (likely(flow_ctrl < 0))
@@ -901,10 +901,10 @@ static int stmmac_set_bfsize(int mtu, int bufsize)
 		ret = BUF_SIZE_8KiB;
 	else if (mtu >= BUF_SIZE_2KiB)
 		ret = BUF_SIZE_4KiB;
-	else if (mtu >= DMA_BUFFER_SIZE)
+	else if (mtu > DEFAULT_BUFSIZE)
 		ret = BUF_SIZE_2KiB;
 	else
-		ret = DMA_BUFFER_SIZE;
+		ret = DEFAULT_BUFSIZE;
 
 	return ret;
 }