DualPI2 drops packets during dequeue but was using kfree_skb_reason()
directly, bypassing trace_qdisc_drop. Convert to qdisc_dequeue_drop()
and add QDISC_DROP_L4S_STEP_NON_ECN to the qdisc drop reason enum.

- Set TCQ_F_DEQUEUE_DROPS flag in dualpi2_init()
- Use enum qdisc_drop_reason in drop_and_retry()
- Replace kfree_skb_reason() with qdisc_dequeue_drop()

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/177211351978.3011628.11267023360997620069.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Rename QDISC_DROP_CAKE_FLOOD to QDISC_DROP_FLOOD_PROTECTION to use a
generic name without embedding the qdisc name. This follows the
principle that drop reasons should describe the drop mechanism rather
than being tied to a specific qdisc implementation.

The flood protection drop reason is used by qdiscs implementing
probabilistic drop algorithms (like BLUE) that detect unresponsive
flows indicating potential DoS or flood attacks. CAKE uses this via
its Cobalt AQM component.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/177211347537.3011628.13759059534638729639.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Rename FQ-specific drop reasons to generic names:
- QDISC_DROP_FQ_BAND_LIMIT -> QDISC_DROP_BAND_LIMIT
- QDISC_DROP_FQ_HORIZON_LIMIT -> QDISC_DROP_HORIZON_LIMIT

This follows the principle that drop reasons should describe the drop
mechanism rather than being tied to a specific qdisc implementation.
These concepts (priority band limits, timestamp horizon) could apply
to other qdiscs as well.

Remove the local macro define FQDR() and instead use the
full QDISC_DROP_* name to make it easier to navigate code.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/177211346902.3011628.12523261489552097455.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Convert SFQ to use the new qdisc-specific drop reason infrastructure.

This patch demonstrates how to convert a flow-based qdisc to use the
new enum qdisc_drop_reason. As part of this conversion:

- Add QDISC_DROP_MAXFLOWS for flow table exhaustion
- Rename FQ_FLOW_LIMIT to generic FLOW_LIMIT, now shared by FQ and SFQ
- Use QDISC_DROP_OVERLIMIT for sfq_drop() when overall limit exceeded
- Use QDISC_DROP_FLOW_LIMIT for per-flow depth limit exceeded

The FLOW_LIMIT reason is now a common drop reason for per-flow limits,
applicable to both FQ and SFQ qdiscs.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/177211345946.3011628.12770616071857185664.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Create new enum qdisc_drop_reason and trace_qdisc_drop tracepoint
for qdisc layer drop diagnostics with direct qdisc context visibility.

The new tracepoint includes qdisc handle, parent, kind (name), and
device information. Existing SKB_DROP_REASON_QDISC_DROP is retained
for backwards compatibility via kfree_skb_reason().

Convert qdiscs with drop reasons to use the new infrastructure.

Change CAKE's cobalt_should_drop() return type from enum skb_drop_reason
to enum qdisc_drop_reason to fix implicit enum conversion warnings.
Use QDISC_DROP_UNSPEC as the 'not dropped' sentinel instead of
SKB_NOT_DROPPED_YET. Both have the same compiled value (0), so the
comparison logic remains semantically equivalent.

Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/177211345275.3011628.1974310302645218067.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>