Re: [REGRESSION] intel-iommu/brcmfmac DMAR fault on Broadcom PCIe wifi, v7.3-rc7 -> v7.3 final -- candidate mechanism and proposed fix path

[Aditya Garg]

> On 20 Oct 2026, at 10:40 PM, Arend van Spriel <arend.vanspriel@xxxxxxxxxxxx> wrote:
> 
> Op 20 oktober 2026 18:02:30 schreef Jason Perlow <jperlow@xxxxxxxxx>:
> 
>> NOTE: an earlier report on this bug from us bounced off one of these lists due to HTML/multipart formatting from the mail client used -- apologies, switching clients. This mail includes the full original report plus a follow-up, merged into one self-contained report since the original never reached anyone.
>> 
>> === ORIGINAL REPORT ===
>> 
>> Hardware: Intel CoffeeLake-H platforms, various. The affected device is a Broadcom BCM4364 802.11ac PCIe wifi adapter (14e4:4464) at 0000:05:00.0 on an Apple MacBookPro16,1 (T2 Mac). Aditya, including you directly since this is T2 hardware; copying iommu/pci/wireless because the fault signature and the fact that it reproduces on only one of five tested machines (at the time of the original report -- see follow-up below, that has since changed) looked like it needed eyes on the core IOMMU/PCI path, not the driver.
> 
> [...]
> 
>> THE ONE ANOMALY WE CAN'T YET EXPLAIN
>> 
>> Despite the last two items above being unchanged, the actual generated .config has CONFIG_INTEL_IOMMU_DEFAULT_ON unset after make olddefconfig against the rc7 source tree, but =y after the same process against the final source tree, even though our template explicitly answers "# CONFIG_INTEL_IOMMU_DEFAULT_ON is not set" in both cases. Our working theory is that some other symbol started selecting/implying INTEL_IOMMU_DEFAULT_ON between rc7 and final -- which would override an explicitly-seeded "not set" answer during olddefconfig resolution -- but we have not identified which one yet.

intel_iommu=off has the same effect as disabling this in the kernel config. The real question is WiFi was still working before when intel_iommu was on.

I wonder if PCs with Broadcom chips are also getting affected since we have a confirmed report of a non T2 Mac as well.