[PATCH net-next v5 2/5] net: add dev->bql flag to allow BQL sysfs for IFF_NO_QUEUE devices

[hawk@kernel.org]

From: Jesper Dangaard Brouer <hawk@kernel.org>

Virtual devices with IFF_NO_QUEUE or lltx are excluded from BQL sysfs
by netdev_uses_bql(), since they traditionally lack real hardware
queues. However, some virtual devices like veth implement a real
ptr_ring FIFO with NAPI processing and benefit from BQL to limit
in-flight bytes and reduce latency.

Add a per-device 'bql' bitfield boolean in the priv_flags_slow section
of struct net_device. When set, it overrides the IFF_NO_QUEUE/lltx
exclusion and exposes BQL sysfs entries (/sys/class/net/<dev>/queues/
tx-<n>/byte_queue_limits/). The flag is still gated on CONFIG_BQL.

This allows drivers that use BQL despite being IFF_NO_QUEUE to opt in
to sysfs visibility for monitoring and debugging.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Tested-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
---
 Documentation/networking/net_cachelines/net_device.rst | 1 +
 include/linux/netdevice.h                              | 2 ++
 net/core/net-sysfs.c                                   | 8 +++++++-
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/net_cachelines/net_device.rst b/Documentation/networking/net_cachelines/net_device.rst
index 1c19bb7705df..b775d3235a2d 100644
--- a/Documentation/networking/net_cachelines/net_device.rst
+++ b/Documentation/networking/net_cachelines/net_device.rst
@@ -170,6 +170,7 @@ unsigned_long:1                     see_all_hwtstamp_requests
 unsigned_long:1                     change_proto_down
 unsigned_long:1                     netns_immutable
 unsigned_long:1                     fcoe_mtu
+unsigned_long:1                     bql                                                                 netdev_uses_bql(net-sysfs.c)
 struct list_head                    net_notifier_list
 struct macsec_ops*                  macsec_ops
 struct udp_tunnel_nic_info*         udp_tunnel_nic_info
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 744ffa243501..d4c7b020b6a7 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2065,6 +2065,7 @@ enum netdev_reg_state {
  *	@change_proto_down: device supports setting carrier via IFLA_PROTO_DOWN
  *	@netns_immutable: interface can't change network namespaces
  *	@fcoe_mtu:	device supports maximum FCoE MTU, 2158 bytes
+ *	@bql:		device uses BQL (DQL sysfs) despite having IFF_NO_QUEUE
  *
  *	@net_notifier_list:	List of per-net netdev notifier block
  *				that follow this device when it is moved
@@ -2479,6 +2480,7 @@ struct net_device {
 	unsigned long		change_proto_down:1;
 	unsigned long		netns_immutable:1;
 	unsigned long		fcoe_mtu:1;
+	unsigned long		bql:1;
 
 	struct list_head	net_notifier_list;
 
diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 3318b5666e43..82833e5dae03 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -1945,10 +1945,16 @@ static const struct kobj_type netdev_queue_ktype = {
 
 static bool netdev_uses_bql(const struct net_device *dev)
 {
+	if (!IS_ENABLED(CONFIG_BQL))
+		return false;
+
+	if (dev->bql)
+		return true;
+
 	if (dev->lltx || (dev->priv_flags & IFF_NO_QUEUE))
 		return false;
 
-	return IS_ENABLED(CONFIG_BQL);
+	return true;
 }
 
 static int netdev_queue_add_kobject(struct net_device *dev, int index)
-- 
2.43.0