PROBLEM: Wireless networking goes down on Acer C720P Chromebook (bisected)

[Stephen Oberholtzer]

Wireless networking goes down on Acer C720P Chromebook (bisected)

Culprit: 7a89233a ("mac80211: Use Airtime-based Queue Limits (AQL) on
packet dequeue")

I found that the newest kernel (5.4) displayed a curious issue on my
Acer C720P Chromebook: shortly after bringing networking up, all
connections would suddenly fail.  I discovered that I could
consistently reproduce the issue by ssh'ing into the machine and
running 'dmesg' -- on a non-working kernel; I would get partial
output, and then the connection would completely hang. This was so
consistent, in fact, that I was able to leverage it to automate the
process from 'git bisect run'.

KEYWORDS: c720p, chromebook, wireless, networking, mac80211

KERNEL: any kernel containing commit 7a89233a ("mac80211: Use
Airtime-based Queue Limits (AQL) on packet dequeue")

I find this bit in the offending commit's message suspicious:

> This patch does *not* include any mechanism to wake a throttled TXQ again,
> on the assumption that this will happen anyway as a side effect of whatever
>  freed the skb (most commonly a TX completion).

Methinks this assumption is not a fully valid one.

I'll be happy to test any patches. If you need some printk calls, just
tell me where to put 'em.

This is the card:

01:00.0 Network controller: Qualcomm Atheros AR9462 Wireless Network
Adapter (rev 01)
        Subsystem: Foxconn International, Inc. AR9462 Wireless Network Adapter
        Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop-
ParErr- Stepping- SERR- FastB2B- DisINTx-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort-
<TAbort- <MAbort- >SERR- <PERR- INTx-
        Latency: 0, Cache Line Size: 64 bytes
        Interrupt: pin A routed to IRQ 16
        Region 0: Memory at e0400000 (64-bit, non-prefetchable) [size=512K]
        Expansion ROM at e0480000 [disabled] [size=64K]
        Capabilities: [40] Power Management version 2
                Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=375mA
PME(D0+,D1+,D2+,D3hot+,D3cold+)
                Status: D0 NoSoftRst- PME-Enable- DSel=0 DScale=0 PME-
        Capabilities: [50] MSI: Enable- Count=1/4 Maskable+ 64bit+
                Address: 0000000000000000  Data: 0000
                Masking: 00000000  Pending: 00000000
        Capabilities: [70] Express (v2) Endpoint, MSI 00
                DevCap: MaxPayload 128 bytes, PhantFunc 0, Latency L0s
unlimited, L1 <64us
                        ExtTag- AttnBtn- AttnInd- PwrInd- RBE+
FLReset- SlotPowerLimit 0.000W
                DevCtl: CorrErr- NonFatalErr- FatalErr- UnsupReq-
                        RlxdOrd+ ExtTag- PhantFunc- AuxPwr- NoSnoop-
                        MaxPayload 128 bytes, MaxReadReq 512 bytes
                DevSta: CorrErr- NonFatalErr- FatalErr- UnsupReq-
AuxPwr+ TransPend-
                LnkCap: Port #0, Speed 2.5GT/s, Width x1, ASPM L0s L1,
Exit Latency L0s <4us, L1 <64us
                        ClockPM- Surprise- LLActRep- BwNot- ASPMOptComp-
                LnkCtl: ASPM L0s L1 Enabled; RCB 64 bytes Disabled- CommClk+
                        ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-
                LnkSta: Speed 2.5GT/s (ok), Width x1 (ok)
                        TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-
                DevCap2: Completion Timeout: Not Supported,
TimeoutDis+, LTR-, OBFF Not Supported
                         AtomicOpsCap: 32bit- 64bit- 128bitCAS-
                DevCtl2: Completion Timeout: 50us to 50ms,
TimeoutDis-, LTR-, OBFF Disabled
                         AtomicOpsCtl: ReqEn-
                LnkCtl2: Target Link Speed: 2.5GT/s, EnterCompliance- SpeedDis-
                         Transmit Margin: Normal Operating Range,
EnterModifiedCompliance- ComplianceSOS-
                         Compliance De-emphasis: -6dB
                LnkSta2: Current De-emphasis Level: -6dB,
EqualizationComplete-, EqualizationPhase1-
                         EqualizationPhase2-, EqualizationPhase3-,
LinkEqualizationRequest-
        Capabilities: [100 v1] Advanced Error Reporting
                UESta:  DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt-
UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
                UEMsk:  DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt-
UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
                UESvrt: DLP+ SDES+ TLP- FCP+ CmpltTO- CmpltAbrt-
UnxCmplt- RxOF+ MalfTLP+ ECRC- UnsupReq- ACSViol-
                CESta:  RxErr- BadTLP- BadDLLP- Rollover- Timeout-
AdvNonFatalErr-
                CEMsk:  RxErr- BadTLP- BadDLLP- Rollover- Timeout-
AdvNonFatalErr+
                AERCap: First Error Pointer: 00, ECRCGenCap-
ECRCGenEn- ECRCChkCap- ECRCChkEn-
                        MultHdrRecCap- MultHdrRecEn- TLPPfxPres- HdrLogCap-
                HeaderLog: 00000000 00000000 00000000 00000000
        Capabilities: [140 v1] Virtual Channel
                Caps:   LPEVC=0 RefClk=100ns PATEntryBits=1
                Arb:    Fixed- WRR32- WRR64- WRR128-
                Ctrl:   ArbSelect=Fixed
                Status: InProgress-
                VC0:    Caps:   PATOffset=00 MaxTimeSlots=1 RejSnoopTrans-
                        Arb:    Fixed- WRR32- WRR64- WRR128- TWRR128- WRR256-
                        Ctrl:   Enable+ ID=0 ArbSelect=Fixed TC/VC=ff
                        Status: NegoPending- InProgress-
        Capabilities: [160 v1] Device Serial Number 00-00-00-00-00-00-00-00
        Kernel driver in use: ath9k
        Kernel modules: ath9k



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-- Stevie-O
Real programmers use COPY CON PROGRAM.EXE