Re: [dpdk-dev] [PATCH] net/ixgbe: fix TDH register setting issue

["Zhang, Qi Z" <qi.z.zhang@intel.com>]

> -----Original Message-----
> From: Wu, Yanglong
> Sent: Monday, November 19, 2018 6:12 PM
> To: Zhang, Qi Z <qi.z.zhang@intel.com>; dev@dpdk.org
> Cc: Wu, Jingjing <jingjing.wu@intel.com>; Byrne, Stephen1
> <stephen1.byrne@intel.com>
> Subject: RE: [PATCH] net/ixgbe: fix TDH register setting issue
> 
> Hi, qi
> I don't understand why dose this patch not change register  write order.
> THD is write before  TXDCTL.ENABL.
> 
> The Intel 82599 data sheet
> (https://www.intel.com/content/dam/www/public/us/en/documents/datashe
> ets/82599-10-gbe-controller-datasheet.pdf, §8.2.3.9.8) states that "The only
> time that software should write to [the TDH] register is after a reset (hardware
> reset or CTRL.RST) and before enabling the transmit function (TXDCTL.ENABLE).
> If software were to write to this register while the transmit function was
> enabled, the on-chip descriptor buffers might be invalidated and the hardware
> could become confused".

OK got it, I think you can use below commit as to fix.

commit 029fd06d40fa8cff7f562dc85d7cc4609d5c91fb
Author: Ouyang Changchun <changchun.ouyang@intel.com>
Date:   Wed May 28 16:06:37 2014 +0800

    ixgbe: queue start and stop

    This patch implements queue start and stop functionality in IXGBE PMD;
    it also enable hardware loopback for VMDQ mode in IXGBE PMD.

    Signed-off-by: Ouyang Changchun <changchun.ouyang@intel.com>
    Tested-by: Waterman Cao <waterman.cao@intel.com>
    Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>

> 
> Regard
> yanglong
> 
> -----Original Message-----
> From: Zhang, Qi Z
> Sent: Saturday, November 17, 2018 3:52 AM
> To: Wu, Yanglong <yanglong.wu@intel.com>; dev@dpdk.org
> Cc: Wu, Jingjing <jingjing.wu@intel.com>; Byrne, Stephen1
> <stephen1.byrne@intel.com>
> Subject: RE: [PATCH] net/ixgbe: fix TDH register setting issue
> 
> Hi Yanglong:
> 
> > -----Original Message-----
> > From: Wu, Yanglong
> > Sent: Thursday, November 15, 2018 6:32 PM
> > To: dev@dpdk.org
> > Cc: Zhang, Qi Z <qi.z.zhang@intel.com>; Wu, Jingjing
> > <jingjing.wu@intel.com>; Byrne, Stephen1 <stephen1.byrne@intel.com>;
> > Wu, Yanglong <yanglong.wu@intel.com>
> > Subject: [PATCH] net/ixgbe: fix TDH register setting issue
> >
> > The only time that software should write to the TDH register is after
> > a reset (hardware reset or CTRL.RST) and before enabling the transmit
> > function (TXDCTL.ENABLE).
> > If software were to write to this register while the transmit function
> > was enabled, the on-chip descriptor buffers might be invalidated and
> > the hardware could become confused.
> >
> > Fixes: a8cdaf0964f7 ("net/ixgbe: remove redundant queue id checks")
> 
> I try to understand why this patch is going to fix above commit.
> seems the commit didn't change the register write order, and even if we go
> much early I saw the TDH register is always be written after TXDCTL.ENABL.
> Did I missed something?
> btw, I'm also curious why only TDH matters but TDT does not.
> 
> > Signed-off-by: Yanglong Wu <yanglong.wu@intel.com>
> > ---
> >  drivers/net/ixgbe/ixgbe_rxtx.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c
> > b/drivers/net/ixgbe/ixgbe_rxtx.c index 2f0262ae1..ddc7efa87 100644
> > --- a/drivers/net/ixgbe/ixgbe_rxtx.c
> > +++ b/drivers/net/ixgbe/ixgbe_rxtx.c
> > @@ -5264,6 +5264,7 @@ ixgbe_dev_tx_queue_start(struct rte_eth_dev
> > *dev, uint16_t tx_queue_id)
> >  	hw = IXGBE_DEV_PRIVATE_TO_HW(dev->data->dev_private);
> >
> >  	txq = dev->data->tx_queues[tx_queue_id];
> > +	IXGBE_WRITE_REG(hw, IXGBE_TDH(txq->reg_idx), 0);
> >  	txdctl = IXGBE_READ_REG(hw, IXGBE_TXDCTL(txq->reg_idx));
> >  	txdctl |= IXGBE_TXDCTL_ENABLE;
> >  	IXGBE_WRITE_REG(hw, IXGBE_TXDCTL(txq->reg_idx), txdctl); @@
> > -5281,7 +5282,6 @@ ixgbe_dev_tx_queue_start(struct rte_eth_dev *dev,
> > uint16_t tx_queue_id)
> >  				tx_queue_id);
> >  	}
> >  	rte_wmb();
> > -	IXGBE_WRITE_REG(hw, IXGBE_TDH(txq->reg_idx), 0);
> >  	IXGBE_WRITE_REG(hw, IXGBE_TDT(txq->reg_idx), 0);
> >  	dev->data->tx_queue_state[tx_queue_id] =
> > RTE_ETH_QUEUE_STATE_STARTED;
> >
> > --
> > 2.11.0