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

-----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:

quoted

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.

quoted

Signed-off-by: Madalin Bucur <redacted>
---
 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.

quoted

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 =

{

quoted

 	.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)

quoted

 	}
 }

+#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