Public bug reported: [Impact] On Raspberry Pi 3B, USB audio playback at 44100 Hz through a full-speed DAC (SMSL Q5, 0451:0003, behind the on-board LAN9514 high-speed hub) is audibly distorted on linux-raspi 7.0.0-1011/7.0.0-1014 (Ubuntu 26.04). 48000 Hz playback is clean. The same hardware works correctly on linux-raspi 6.8 (Ubuntu 24.04), last verified good: 6.8.0-1050-raspi.
[Test case] aplay -D plughw:smsl <44.1kHz wav> -> distorted audio + kernel WARNING (once per boot) aplay -D plughw:smsl <48kHz wav> -> clean audio, no WARNING [Kernel log] WARNING: drivers/usb/dwc2/hcd.c:2684 at dwc2_assign_and_init_hc — via dwc2_hcd_select_transactions / dwc2_handle_hcd_intr (full trace in the attached apport report and dwc2-warning-trace.txt). [Analysis / suspected cause] The WARN is the unaligned-xfer_dma check on isoc OUT split transfers. 44.1 kHz full-speed audio alternates 264/270-byte packets (S24_3LE stereo); 270 % 4 = 2, so every other packet's DMA address is word-unaligned, whereas 48 kHz packets are a constant 288 bytes (aligned) — matching the good/bad pattern. The suspect is the downstream raspberrypi.com dwc2 split-transfer series merged June 2025 (present in the 7.0-raspi tree, absent from 6.8-raspi), in particular raspberrypi/linux commit 279fbca16aed "drivers: dwc2: better handle hardware length & alignment issues", which makes URB_NO_TRANSFER_DMA_MAP URBs (used by snd-usb-audio) skip the DMA realignment/bounce path. Verified the running 7.0.0-1014-raspi vmlinuz contains this commit's "Unaligned DMA-mapped buffer" string. Upstream report with full analysis: https://github.com/raspberrypi/linux/issues/7475 Note: apport report is attached manually (linux-raspi-dwc2.apport) because this machine has no direct internet access, so apport-collect cannot run from it. ** Affects: linux-raspi (Ubuntu) Importance: Undecided Status: New ** Tags: regression-release ** Attachment added: "apport report collected on the affected Pi (offline device, uploaded manually)" https://bugs.launchpad.net/bugs/2159597/+attachment/5980287/+files/linux-raspi-dwc2.apport -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2159597 Title: 44.1 kHz USB audio distorted on RPi 3B (dwc2 isoc OUT split), regression from 6.8-raspi to 7.0-raspi To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux-raspi/+bug/2159597/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
