Issue #63: standard `btusb` ACL ZLP support

The aic_zlp_quirk companion module supplies the Bluetooth ACL bulk TX zero-length-packet behavior confirmed in issue #63. It is now part of the normal aic8800 DKMS build rather than a replacement for the distribution's Bluetooth driver.

The validated failure was specific and reproducible: AAC playback through USB device 368b:8d81 stalled after about one minute, while SBC remained stable. Adding URB_ZERO_PACKET to the ACL bulk OUT URBs kept AAC connected for more than one hour. The standalone module was then verified with the system btusb, no loaded aic_btusb, and 2,113 observed ZLP injections.

Architecture and scope

The module first tries to attach a kretprobe to the standard btusb function that allocates ACL bulk OUT URBs. The target is module-qualified as btusb:alloc_bulk_urb, avoiding similarly named symbols in unrelated USB drivers.

Some distribution builds inline that private function. When the preferred hook is unavailable, the module falls back to the stable usb_submit_urb entry point. The fallback changes an URB only when all of these checks pass:

USB device is exactly 368b:8d81;
the transfer is bulk OUT;
the endpoint belongs to Bluetooth interface 0 (e0/01/01).

Both paths add only URB_ZERO_PACKET. Wi-Fi interfaces and unrelated USB devices are unchanged. If neither probe can be installed or kprobes are disabled, the module refuses to load and system btusb remains unchanged.

The module carries a USB modalias for 368b:8d81, so it is inactive on other hardware and normally autoloads only when the validated device appears.

Install from the unified branch

git fetch origin
git switch test/unified-wifi-bt-zlp
git pull --ff-only
sudo ./install.sh
sudo reboot

The installer removes the earlier btusb-aic-zlp/0.1 and aic-zlp-quirk/0.1 diagnostic DKMS packages when present. To remove them manually before installation, use:

sudo dkms remove btusb-aic-zlp/0.1 --all
sudo dkms remove aic-zlp-quirk/0.1 --all
sudo depmod -a

The unified installer also removes active configuration left by the obsolete custom aic_btusb transport.

Verify

Confirm that system btusb owns Bluetooth interfaces 0/1 and that the quirk is loaded:

lsusb -t
lsmod | grep -E '^(btusb|aic_zlp_quirk|aic_btusb)\b'
modinfo -n btusb
modinfo -n aic_zlp_quirk
cat /sys/module/aic_zlp_quirk/parameters/hook
cat /sys/module/aic_zlp_quirk/parameters/injections
sudo ./diagnose_bt.sh

Expected results include:

Bluetooth interfaces 0/1: btusb
aic_btusb: not loaded
aic_zlp_quirk: loaded
active hook: btusb:alloc_bulk_urb

active hook: usb_submit_urb is also valid when the compiler inlined the preferred function. During Bluetooth traffic, the injections value must increase.

For the promotion test, select AAC and play audio for at least one hour while also confirming that Wi-Fi scan, association, DHCP, and real traffic remain working.

Isolated retest helper

aic-zlp-quirk-test.sh remains available only for isolated issue #63 retests. It builds the same canonical source from drivers/aic8800/aic_zlp_quirk/aic_zlp_quirk.c as a separate DKMS package. A normal installation should use the repository-level install.sh instead.

Kernel requirements

matching kernel headers;
CONFIG_KPROBES=y and kretprobe support;
standard btusb plus either its alloc_bulk_urb symbol or the USB core usb_submit_urb symbol;
a valid module signature when Secure Boot policy requires one.

The preferred function has the same signature in Linux 5.15, 6.1, 6.6, 6.12, 6.18, and 7.1.3, although compilers may inline it. DKMS builds one module for each installed kernel from the single canonical source file.

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Issue #63: standard btusb ACL ZLP support