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Date:	Sun, 22 Jun 2014 07:49:11 +0000
From:	"fugang.duan@...escale.com" <fugang.duan@...escale.com>
To:	Russell King <rmk+kernel@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH RFC 24/30] net: fec: better implementation of iMX6
 ERR006358 quirk

From: Russell King <rmk@....linux.org.uk> Data: Friday, June 20, 2014 8:14 PM
>To: linux-arm-kernel@...ts.infradead.org
>Cc: Duan Fugang-B38611; netdev@...r.kernel.org
>Subject: [PATCH RFC 24/30] net: fec: better implementation of iMX6
>ERR006358 quirk
>
>Using a (delayed) workqueue for ERR006358 is not correct - a work queue is
>a single-trigger device.  Once the work queue has been scheduled, it can't
>be re-scheduled until it has been run.  This can cause problems - with an
>appropriate packet timing, we can end up with packets queued, but not sent
>by the hardware, resulting in the transmit timeout firing.
>
>Re-implement this as per the workaround detailed in the ERR006358
>documentation - if there are packets waiting to be sent when we service
>the transmit ring, and we see that the transmitter is not running, kick
>the transmitter to run the pending entries in the ring.
>
>Testing here with a 10Mbit half duplex link sees the resulting iperf TCP
>bandwidth increase from between 1 to 2Mbps to between 8 to 9Mbps.
>
>Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
>---
> drivers/net/ethernet/freescale/fec.h      |  1 -
> drivers/net/ethernet/freescale/fec_main.c | 31 +++++---------------------
>-----
> 2 files changed, 5 insertions(+), 27 deletions(-)
>
>diff --git a/drivers/net/ethernet/freescale/fec.h
>b/drivers/net/ethernet/freescale/fec.h
>index 96d2a18f1b99..17e294970207 100644
>--- a/drivers/net/ethernet/freescale/fec.h
>+++ b/drivers/net/ethernet/freescale/fec.h
>@@ -259,7 +259,6 @@ struct bufdesc_ex {
> struct fec_enet_delayed_work {
> 	struct delayed_work delay_work;
> 	bool timeout;
>-	bool trig_tx;
> };
>
> /* The FEC buffer descriptors track the ring buffers.  The rx_bd_base and
>diff --git a/drivers/net/ethernet/freescale/fec_main.c
>b/drivers/net/ethernet/freescale/fec_main.c
>index 62120e47ef5a..957bb98add5a 100644
>--- a/drivers/net/ethernet/freescale/fec_main.c
>+++ b/drivers/net/ethernet/freescale/fec_main.c
>@@ -342,22 +342,6 @@ fec_enet_clear_csum(struct sk_buff *skb, struct
>net_device *ndev)
> 	return 0;
> }
>
>-static void
>-fec_enet_submit_work(struct bufdesc *bdp, struct fec_enet_private *fep) -
>{
>-	const struct platform_device_id *id_entry =
>-				platform_get_device_id(fep->pdev);
>-	struct bufdesc *bdp_pre;
>-
>-	bdp_pre = fec_enet_get_prevdesc(bdp, fep);
>-	if ((id_entry->driver_data & FEC_QUIRK_ERR006358) &&
>-	    !(bdp_pre->cbd_sc & BD_ENET_TX_READY)) {
>-		fep->delay_work.trig_tx = true;
>-		schedule_delayed_work(&(fep->delay_work.delay_work),
>-					msecs_to_jiffies(1));
>-	}
>-}
>-
> static int
> fec_enet_txq_submit_frag_skb(struct sk_buff *skb, struct net_device *ndev)
>{ @@ -545,8 +529,6 @@ static int fec_enet_txq_submit_skb(struct sk_buff
>*skb, struct net_device *ndev)
> 	status |= (BD_ENET_TX_READY | BD_ENET_TX_TC);
> 	bdp->cbd_sc = status;
>
>-	fec_enet_submit_work(bdp, fep);
>-
> 	/* If this was the last BD in the ring, start at the beginning again.
>*/
> 	bdp = fec_enet_get_nextdesc(last_bdp, fep);
>
>@@ -735,8 +717,6 @@ static int fec_enet_txq_submit_tso(struct sk_buff *skb,
>struct net_device *ndev)
> 	/* Save skb pointer */
> 	fep->tx_skbuff[index] = skb;
>
>-	fec_enet_submit_work(bdp, fep);
>-
> 	skb_tx_timestamp(skb);
> 	fep->cur_tx = bdp;
>
>@@ -1065,11 +1045,6 @@ static void fec_enet_work(struct work_struct *work)
> 		}
> 		rtnl_unlock();
> 	}
>-
>-	if (fep->delay_work.trig_tx) {
>-		fep->delay_work.trig_tx = false;
>-		writel(0, fep->hwp + FEC_X_DES_ACTIVE);
>-	}
> }
>
> static void
>@@ -1166,7 +1141,11 @@ fec_enet_tx(struct net_device *ndev)
> 				netif_wake_queue(ndev);
> 		}
> 	}
>-	return;
>+
>+	/* ERR006538: Keep the transmitter going */
>+	if (fep->dirty_tx != fep->cur_tx &&
>+	    readl(fep->hwp + FEC_X_DES_ACTIVE) == 0)
>+		writel(0, fep->hwp + FEC_X_DES_ACTIVE);
> }
>
If fep->dirty_tx == fep->cur_tx, which means tx bd entry is empty, it doesn't need to trigger TDAR.
The issue is the last tx frame cannot be trasmited by uDMA since uDMA had cleared the TDAR after user write TDAR.
User					uDMA
					Process the previous frame, detect the next BD ready bit,
					The next BD ready bit is not set
Set the next BD ready bit
Set TDAR
					Since the next BD ready bit detected as not set, start to clear TDAR.

=> So, the next frame cannot be tramsmit until the next next frame is filled by user.

The issue has happened customer product, and was fixed by previous workaround.
The issue only happened at little packets transmission. Special for command communicate protocol to the other end.

If you move the extra trigger to tx clean function, you must check:
bdp_pre = fec_enet_get_prevdesc(fep->cur_tx, fep);
bdp_next = fec_enet_get_nextdesc(fep->dirty_tx, fep); 
if (bdp_next == bdp_pre &&
	bdp_pre->cbd_sc & BD_ENET_TX_READY &&
	readl(fep->hwp + FEC_X_DES_ACTIVE) == 0) 
	writel(0, fep->hwp + FEC_X_DES_ACTIVE);

Thanks,
Andy
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