RE: [net-next v4 4/8] dpaa_eth: add driver's Tx queue selection

[Madalin-Cristian Bucur]

> -----Original Message-----
> From: Wood Scott-B07421
> Sent: Wednesday, December 02, 2015 11:41 PM
> 
> On Mon, 2015-11-02 at 19:31 +0200, Madalin Bucur wrote:
> > Allow the selection of the transmission queue based on the CPU id.
> 
> Explain why.

I'll add more details in the commit log. This is a customer generated
feature. Bypassing the standard XPS can increase performance by making use
of the DPAA HW particularities.

> >
> > Signed-off-by: Madalin Bucur <madalin.bucur@xxxxxxxxxxxxx>
> > ---
> >  drivers/net/ethernet/freescale/dpaa/Kconfig           | 10 ++++++++++
> >  drivers/net/ethernet/freescale/dpaa/dpaa_eth.c        |  3 +++
> >  drivers/net/ethernet/freescale/dpaa/dpaa_eth.h        |  6 ++++++
> >  drivers/net/ethernet/freescale/dpaa/dpaa_eth_common.c |  8 ++++++++
> >  drivers/net/ethernet/freescale/dpaa/dpaa_eth_common.h |  4 ++++
> >  5 files changed, 31 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/freescale/dpaa/Kconfig
> > b/drivers/net/ethernet/freescale/dpaa/Kconfig
> > index 022d5aa..2577aac 100644
> > --- a/drivers/net/ethernet/freescale/dpaa/Kconfig
> > +++ b/drivers/net/ethernet/freescale/dpaa/Kconfig
> > @@ -11,6 +11,16 @@ menuconfig FSL_DPAA_ETH
> >
> >  if FSL_DPAA_ETH
> >
> > +config FSL_DPAA_ETH_USE_NDO_SELECT_QUEUE
> > +	bool "Use driver's Tx queue selection mechanism"
> > +	default y
> > +	---help---
> > +	  The DPAA Ethernet driver defines a ndo_select_queue() callback
> > for optimal selection
> > +	  of the egress FQ. That will override the XPS support for this
> > netdevice.
> > +	  If for whatever reason you want to be in control of the egress FQ
> > -to-CPU selection and mapping,
> > +	  or simply don't want to use the driver's ndo_select_queue()
> > callback, then unselect this
> > +	  and use the standard XPS support instead.
> 
> Is there a use case for needing this to be configurable?

If the standard XPS is desired, the Kconfig option allows the driver user to
select that.

> > diff --git a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
> > b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
> > index 31d55b4..894f1a7 100644
> > --- a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
> > +++ b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
> > @@ -390,6 +390,9 @@ static const struct net_device_ops dpa_private_ops =
> {
> >  	.ndo_get_stats64 = dpa_get_stats64,
> >  	.ndo_set_mac_address = dpa_set_mac_address,
> >  	.ndo_validate_addr = eth_validate_addr,
> > +#ifdef CONFIG_FSL_DPAA_ETH_USE_NDO_SELECT_QUEUE
> > +	.ndo_select_queue = dpa_select_queue,
> > +#endif
> >  	.ndo_change_mtu = dpa_change_mtu,
> >  	.ndo_set_rx_mode = dpa_set_rx_mode,
> >  	.ndo_init = dpa_ndo_init,
> > diff --git a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.h
> > b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.h
> > index 1ba6617..87577cf 100644
> > --- a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.h
> > +++ b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.h
> > @@ -420,9 +420,15 @@ static inline void _dpa_assign_wq(struct dpa_fq
> *fq)
> >  	}
> >  }
> >
> > +#ifdef CONFIG_FSL_DPAA_ETH_USE_NDO_SELECT_QUEUE
> > +/* Use in lieu of skb_get_queue_mapping() */
> > +#define dpa_get_queue_mapping(skb) \
> > +	raw_smp_processor_id()
> > +#else
> >  /* Use the queue selected by XPS */
> >  #define dpa_get_queue_mapping(skb) \
> >  	skb_get_queue_mapping(skb)
> > +#endif
> 
> Why is this necessary?  Shouldn't providing a custom .ndo_select_queue()
> be
> sufficient to ensure that skb_get_queue_mapping() returns the same thing?

dpa_get_queue_mapping() is used in more than one place, the ndo function cannot
be used directly in all places, the current setup is justified.

> And if this goes away, it's just a matter of a function pointer, so if it
> does
> need to be configurable it could be a runtime option.
> 
> -Scott

It's used on the hot path, adding an extra indirection layer to make it selectable
at runtime would defeat the purpose...

Madalin