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

From: Scott Wood <[email-masked]>
Date: Wed, 2 Dec 2015 15:40:31 -0600

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.

> 
> Signed-off-by: Madalin Bucur <[email-masked]>
> ---
>  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?

> 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?

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