[REGRESSION] BCM4355 Wi-Fi fails with Intel IOMMU on 7.3.0-2-t2-noble

[Peter Kiddier]

Adding the IOMMU maintainers and linux-kernel.

The original report and test results are below for context.

I haven't tested a vanilla upstream kernel, and may have limited time
for further testing, but wanted to share the findings in case they're
useful.

Thank you.


On Tue, 20 Oct 2026 at 22:32, Peter Kiddier <peter@xxxxxxxxxxx> wrote:
>
> Hello,
>
> I am reporting a Wi-Fi regression on a 2018 Apple MacBook Air with a
> T2 chip, running Linux Mint Zena based on Ubuntu 24.04.
>
> The Broadcom BCM4355 Wi-Fi device works with kernel 7.2.11-3-t2-noble
> but fails with 7.3.0-2-t2-noble. Disabling the Intel IOMMU using
> intel_iommu=off restores Wi-Fi on 7.3.0-2.
>
> Hardware:
> - Apple MacBook Air 2018 (MacBookAir8,1)
> - Wi-Fi: Broadcom BCM4355 802.11ac Wireless LAN SoC
> - PCI ID: 14e4:43dc (subsystem 106b:0843)
> - Driver: brcmfmac
> - Interface: wlp2s0
>
> Test results:
>
> - 7.2.11-3-t2-noble, default kernel command line: Wi-Fi works.
> - 7.3.0-2-t2-noble, default kernel command line: Wi-Fi fails.
> - 7.3.0-2-t2-noble with intel_iommu=off: Wi-Fi works.
> - Reloading brcmfmac on the failing configuration does not restore Wi-Fi.
>
> On the failing 7.3.0-2 boot, the driver detects the device, loads its
> firmware and creates wlp2s0:
>
> brcmfmac 0000:02:00.0: enabling device (0000 -> 0002)
> brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac4355c1-pcie for
> chip BCM4355/12
> brcmfmac: brcmf_c_process_txcap_blob: TxCap blob found, loading
> brcmfmac: brcmf_c_preinit_dcmds: Firmware: BCM4355/12 wl0: Jul  8 2023
> 17:49:51 version 9.30.503.0.32.5.87 FWID 01-e74b9cb0
> brcmfmac 0000:02:00.0 wlp2s0: renamed from wlan0
>
> However, Wi-Fi subsequently stops responding. The kernel log reports
> errors including:
>
> brcmf_msgbuf_query_dcmd: Timeout on response for query command
> brcmf_cfg80211_scan: scan error (-5)
> brcmf_run_escan: error (-5)
> brcmf_msgbuf_tx_ioctl: Failed to reserve space in commonring
> brcmf_cfg80211_get_channel: chanspec failed (-5)
>
> There is also an error creating the P2P interface:
>
> brcmf_cfg80211_add_iface: add iface p2p-dev-wlp2s0 type 10 failed: err=-52
>
> I tried reloading the driver using:
>
> sudo modprobe -r brcmfmac_wcc
> sudo modprobe -r brcmfmac
> sudo modprobe brcmfmac
>
> The driver and firmware loaded again, but Wi-Fi remained unusable.
>
> The firmware files are present under /lib/firmware/brcm/, and the
> firmware version reported by the driver is the same on the working and
> failing kernels.
>
> The brcmfmac module metadata differs between the kernels:
>
> Working kernel 7.2.11-3-t2-noble:
> srcversion: 3B6176FDB720DDE8E7C596B
>
> Failing kernel 7.3.0-2-t2-noble:
> srcversion: 3DCA38D52402541701276CF
>
> I did not find obvious PCIe AER or fatal errors in the failing boot's
> kernel log.
>
> It has been suggested testing intel_iommu=off after reloading
> brcmfmac. The driver reload did not help, but booting with
> intel_iommu=off restored Wi-Fi.
>
> This strongly suggests an interaction between Intel IOMMU handling and
> the BCM4355/brcmfmac driver in the 7.3.0-2-t2-noble kernel.
>
> I have not tested a vanilla upstream kernel, so I cannot yet confirm
> whether this reproduces outside the T2 kernel packages. Unfortunately,
> I have limited time available for further testing, so I may not be
> able to investigate additional requests promptly. I wanted to report
> the reproducible behaviour and the intel_iommu=off workaround in case
> it helps identify the regression.
>
> Could you advise on further diagnostics, or whether this should be
> investigated as an IOMMU or brcmfmac regression?
>
> Thank you.