netdev - RE: [PATCH v2] net: fec_main: dma_map() only the length of the skb

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Date:	Mon, 2 Dec 2013 09:58:02 +0000
From:	Fugang Duan <fugang.duan@...escale.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Fabio Estevam <Fabio.Estevam@...escale.com>,
	Frank Li <frank.li@...escale.net>,
	Jim Baxter <jim_baxter@...tor.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: RE: [PATCH v2] net: fec_main: dma_map() only the length of the skb

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Data: Monday, December 02, 2013 5:53 PM

>To: Duan Fugang-B38611
>Cc: netdev@...r.kernel.org; David S. Miller; Estevam Fabio-R49496; Frank Li;
>Jim Baxter; Marek Szyprowski
>Subject: [PATCH v2] net: fec_main: dma_map() only the length of the skb
>
>On tx submit the driver always dma_map_single() FEC_ENET_TX_FRSIZE (=2048)
>bytes. This works because we don't overwrite any memory after the data buffer,
>we remove it from cache if it was there. So we hurt performace in case the
>mapping of a smaller area makes a difference.
>There is also a bug: If the data area starts shortly before the end of RAM say
>0xc7fffa10 and the RAM ends at 0xc8000000 then we have enough space to fit the
>data area (according to skb->len) but we would map beyond end of ram if we are
>using 2048. In v2.6.31 (against which kernel this patch
>made) there is the following check in dma_cache_maint():
>
>|BUG_ON(!virt_addr_valid(start) || !virt_addr_valid(start + size - 1));
>
>Since the area starting at 0xc8000000 is no longer virt_addr_valid() we
>BUG() during dma_map_single(). The BUG() statement was removed in v3.5-rc1 as
>per 2dc6a016 ("ARM: dma-mapping: use asm-generic/dma-mapping-common.h").
>
>This patch was tested on v2.6.31 and then forward-ported and compile tested
>only against the net tree. I think it is still worth fixing mainline even after
>the BUG() statement is gone.
>
>Tested-by: Fugang Duan <B38611@...escale.com>
>Cc: Marek Szyprowski <m.szyprowski@...sung.com>
>Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>---
>v1..v2: removed a blank line.
>
> drivers/net/ethernet/freescale/fec_main.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
>diff --git a/drivers/net/ethernet/freescale/fec_main.c
>b/drivers/net/ethernet/freescale/fec_main.c
>index 4cbebf3..73b000d 100644
>--- a/drivers/net/ethernet/freescale/fec_main.c
>+++ b/drivers/net/ethernet/freescale/fec_main.c
>@@ -385,7 +385,7 @@ fec_enet_start_xmit(struct sk_buff *skb, struct net_device
>*ndev)
> 	 * data.
> 	 */
> 	bdp->cbd_bufaddr = dma_map_single(&fep->pdev->dev, bufaddr,
>-			FEC_ENET_TX_FRSIZE, DMA_TO_DEVICE);
>+			skb->len, DMA_TO_DEVICE);
> 	if (dma_mapping_error(&fep->pdev->dev, bdp->cbd_bufaddr)) {
> 		bdp->cbd_bufaddr = 0;
> 		fep->tx_skbuff[index] = NULL;
>@@ -779,11 +779,10 @@ fec_enet_tx(struct net_device *ndev)
> 		else
> 			index = bdp - fep->tx_bd_base;
>
>-		dma_unmap_single(&fep->pdev->dev, bdp->cbd_bufaddr,
>-				FEC_ENET_TX_FRSIZE, DMA_TO_DEVICE);
>-		bdp->cbd_bufaddr = 0;
>-
> 		skb = fep->tx_skbuff[index];
>+		dma_unmap_single(&fep->pdev->dev, bdp->cbd_bufaddr, skb->len,
>+				DMA_TO_DEVICE);
>+		bdp->cbd_bufaddr = 0;
>
> 		/* Check for errors. */
> 		if (status & (BD_ENET_TX_HB | BD_ENET_TX_LC |
>--
>1.8.4.4
>

Acked-by: Fugang Duan <B38611@...escale.com>