From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 42C5272603 for ; Sat, 30 May 2026 15:18:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780154291; cv=none; b=hc3pK7lLBsif6byidF6pCuh5uFagSCsyq0FBJHdlyjlKPkLhV7vvJsTD6tjWtVgHfvJ5nuwcXYeMT5X2Jv/N15il6DSgqPBHtC7n1B9p2m1OyYvKgKdIle/UuecAt6LBgMHnX5OdQVSGPfSHFNC9eQHJcX3kICAuT3SOKG1DVOY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780154291; c=relaxed/simple; bh=srkXpMtC7TI+65DqN97fZmDaIf0xOdiLkXAqYUsdmzU=; h=From:To:Subject:Date:Message-ID:Content-Type:MIME-Version; b=LsImOgglf2t0zM9wCuhaNDiSHzDmf5zWv0aflxZb8P8KLGz5GRKPKk2T+OGM+aFFPI4Rgm27WFnJdEp+wtFTQ19VtSdnhMETdzh4kEo73rmgbSVHb/2hE0wW28m4vjF/NWWozGpDMqDFfputt0s/7kA8J3CQvjliMRbxlmB4Un8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=I+lMzROR; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="I+lMzROR" Received: by smtp.kernel.org (Postfix) with ESMTPS id CC00CC19425 for ; Sat, 30 May 2026 15:18:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1780154290; bh=srkXpMtC7TI+65DqN97fZmDaIf0xOdiLkXAqYUsdmzU=; h=From:To:Subject:Date:From; b=I+lMzRORO8P/qBKLjUX7oeYR0Dp4B6Ay37vjzEPagS7y593tBpozwKVcIzzWKnCTK GNofnN30V8U1/zpv/6o65uwGv6duW0rTDDAPGLNhSNk3/uT5mdpIioMQJtiyGpgkvZ t3fGHwiNWTMIPRWwUFFsUmIThP6lQHeVDeQKrrC89fMYGSQfx3E1XjhepiEfJUCOau 6O78WRilNAS1rTAwR3/XZh7L/hjYPYJEAz08iZfWBHyfJQnevYGbTiBGN3WbknKdjN RVDNZuI7JZRmmUsiWE9+kYAnL96CpmYNlqeejzHvv99xssKIyskdYRQre30Mn9GMxl 1THtYlwt0nikA== Received: by aws-us-west-2-korg-bugzilla-1.web.codeaurora.org (Postfix, from userid 48) id AFCEBC3279F; Sat, 30 May 2026 15:18:10 +0000 (UTC) From: bugzilla-daemon@kernel.org To: linux-bluetooth@vger.kernel.org Subject: [Bug 221598] New: Bluetooth: btusb/btrtl RTL8821C 0bda:c821 HFP/mSBC microphone stutters with bursty SCO RX delivery Date: Sat, 30 May 2026 15:18:10 +0000 X-Bugzilla-Reason: AssignedTo X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: Drivers X-Bugzilla-Component: Bluetooth X-Bugzilla-Version: 2.5 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: apocarteres@gmail.com X-Bugzilla-Status: NEW X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: linux-bluetooth@vger.kernel.org X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: bug_id short_desc product version cf_kernel_version rep_platform op_sys bug_status bug_severity priority component assigned_to reporter cf_regression attachments.created Message-ID: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: https://bugzilla.kernel.org/ Auto-Submitted: auto-generated Precedence: bulk X-Mailing-List: linux-bluetooth@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 https://bugzilla.kernel.org/show_bug.cgi?id=3D221598 Bug ID: 221598 Summary: Bluetooth: btusb/btrtl RTL8821C 0bda:c821 HFP/mSBC microphone stutters with bursty SCO RX delivery Product: Drivers Version: 2.5 Kernel Version: 6.12.90+deb13.1-rt-amd64, 6.12.90+deb13-amd64, 6.19.10-300.fc44.x86_64 Hardware: All OS: Linux Status: NEW Severity: normal Priority: P3 Component: Bluetooth Assignee: linux-bluetooth@vger.kernel.org Reporter: apocarteres@gmail.com Regression: No Created attachment 310210 --> https://bugzilla.kernel.org/attachment.cgi?id=3D310210&action=3Dedit btsnoop captures and report draft I am seeing severe HFP/mSBC microphone stutter with a Realtek RTL8821C Bluetooth USB controller on Linux. The same PC and the same Bluetooth heads= ets work correctly under Windows 11. The problem reproduces on both Debian and Fedora live systems, so it does n= ot look specific to one distribution audio stack. ### Hardware Bluetooth controller: ``` Bus 003 Device 007: ID 0bda:c821 Realtek Semiconductor Corp. Bluetooth Radio Negotiated speed: Full Speed (12Mbps) ``` Controller details from Linux: ``` hci0: Type: Primary Bus: USB BD Address: 64:57:25:4C:4E:BA HCI Version: 4.2 (0x8) Revision: 0x75b8 LMP Version: 4.2 (0x8) Subversion: 0xf098 Manufacturer: Realtek Semiconductor Corporation (93) ``` Kernel log during initialization: ``` Bluetooth: hci0: RTL: examining hci_ver=3D08 hci_rev=3D000c lmp_ver=3D08 lmp_subver=3D8821 Bluetooth: hci0: RTL: rom_version status=3D0 version=3D1 Bluetooth: hci0: RTL: loading rtl_bt/rtl8821c_fw.bin Bluetooth: hci0: RTL: loading rtl_bt/rtl8821c_config.bin Bluetooth: hci0: RTL: cfg_sz 10, total sz 34926 Bluetooth: hci0: RTL: fw version 0x75b8f098 ``` Headsets tested: - JBL TOUR ONE M2 - Jabra headset, exact model unknown Both headsets show the same problem on Linux and work normally on Windows 1= 1 on the same machine. ### Software tested Debian: ``` Linux dev 6.12.90+deb13.1-rt-amd64 #1 SMP PREEMPT_RT Debian 6.12.90-2 (2026-05-27) x86_64 GNU/Linux bluez 5.82-1.1 pipewire 1.4.2-1 wireplumber 0.5.8-2 firmware-realtek 20250410-2 ``` The same issue was also reproduced on the non-RT Debian 6.12.90 kernel. Fedora live: ``` Linux version 6.19.10-300.fc44.x86_64 Bluetooth subsystem version 2.22 ``` The Fedora capture was made from a live Fedora Workstation 44 system. ### Actual behavior In HFP/mSBC mode the microphone audio has severe stutter, dropouts and "jer= ky" timing. Applications receive very poor audio. PipeWire/WirePlumber logs repeated mSBC decode failures: ``` wireplumber: spa.bluez5.source.sco: decode failed: -3 ``` Switching to CVSD avoids the mSBC decoder errors but the microphone still stutters and the audio quality is too low to be useful. ### Expected behavior The HFP microphone should work without dropouts, as it does on the same mac= hine and the same headsets under Windows 11. ### eSCO / mSBC negotiation The btsnoop captures show eSCO with transparent air coding and 60 byte RX/TX packet lengths. Example from the Fedora capture: ``` Setup Synchronous Connection: Tx/Rx bandwidth: 8000 Max latency: 13 Air Coding Format: Transparent Data Retransmission effort: Optimize for link quality Synchronous Connect Complete: Link type: eSCO Transmission interval: 0x0c Retransmission window: 0x06 RX packet length: 60 TX packet length: 60 Air mode: Transparent ``` The actual SCO packets in the captures are 72 byte packets: ``` SCO Data RX: Handle 2 flags 0x00 dlen 72 SCO Data TX: Handle 2 flags 0x00 dlen 72 ``` ### Capture timing summary The relevant pattern is that incoming SCO packets are delivered in bursts: about 20% of RX inter-arrival times are below 1 ms, followed by regular long gaps above 15 ms. This is visible on both Debian and Fedora. TX timing is m= uch smoother and does not show the same sub-1 ms burst pattern in most captures. Timing was computed from `btmon -r ` timestamps for `SCO Data RX/T= X` lines. ``` Debian, bt-sco-bad.snoop: RX n=3D1765 dlen=3D72 avg=3D9.003 ms min=3D0.001 max=3D20.154 lt1ms=3D20.= 0% gt15ms=3D10.0% gt20ms=3D5.2% TX n=3D1765 dlen=3D72 avg=3D8.998 ms min=3D0.022 max=3D21.657 lt1ms=3D9.2= % gt15ms=3D8.1%=20 gt20ms=3D4.9% Debian, bt-sco-bad1.snoop: RX n=3D3069 dlen=3D72 avg=3D9.000 ms min=3D0.001 max=3D20.166 lt1ms=3D20.= 0% gt15ms=3D10.0% gt20ms=3D4.2% TX n=3D3066 dlen=3D72 avg=3D9.000 ms min=3D0.024 max=3D21.590 lt1ms=3D3.8= % gt15ms=3D12.7% gt20ms=3D2.2% Debian, bt-sco-bad2.snoop: RX n=3D1943 dlen=3D72 avg=3D8.996 ms min=3D0.002 max=3D20.136 lt1ms=3D20.= 2% gt15ms=3D10.2% gt20ms=3D6.0% TX n=3D1942 dlen=3D72 avg=3D8.999 ms min=3D5.016 max=3D16.256 lt1ms=3D0.0= % gt15ms=3D16.2% gt20ms=3D0.0% Debian, bt-sco-bad3.snoop: RX n=3D1850 dlen=3D72 avg=3D9.000 ms min=3D0.001 max=3D20.102 lt1ms=3D20.= 3% gt15ms=3D10.3% gt20ms=3D1.8% TX n=3D1850 dlen=3D72 avg=3D8.997 ms min=3D3.788 max=3D16.241 lt1ms=3D0.0= % gt15ms=3D16.2% gt20ms=3D0.0% Debian, bt-sco-bad4.snoop: RX n=3D5600 dlen=3D72 avg=3D9.000 ms min=3D0.001 max=3D20.159 lt1ms=3D20.= 0% gt15ms=3D10.0% gt20ms=3D5.3% TX n=3D5597 dlen=3D72 avg=3D9.000 ms min=3D4.876 max=3D16.386 lt1ms=3D0.0= % gt15ms=3D16.2% gt20ms=3D0.0% Fedora live, fedora.cap: RX n=3D2089 dlen=3D72 avg=3D8.249 ms min=3D0.001 max=3D18.447 lt1ms=3D20.= 0% gt15ms=3D10.0% gt18ms=3D10.0% TX n=3D1914 dlen=3D72 avg=3D8.998 ms min=3D6.215 max=3D15.234 lt1ms=3D0.0= % gt15ms=3D10.6% ``` ### Things already tested - Same Bluetooth adapter and headsets work correctly on Windows 11. - Reproduced with two different Bluetooth headsets. - Reproduced on Debian 6.12 and Fedora live 6.19. - Disabled USB autosuspend for `btusb`: ``` /sys/module/btusb/parameters/enable_autosuspend =3D N ``` - Unloaded/blacklisted the Wi-Fi side of the combo device (`rtw88_8821ce`):= no fix. - Disabled nearby USB audio/camera devices on the same USB bus: no fix. - Tried Debian PREEMPT_RT kernel: no fix. - Tried `btusb force_scofix=3D1`: worse. - Tried `btusb disable_scofix=3D1`: worse. - Tried `btusb reset=3D0`: no durable fix. - Tried disabling eSCO with `bluetooth.disable_esco=3D1`: microphone became unavailable. - Tried PipeWire/WirePlumber SCO/mSBC workarounds, including CVSD and hardw= are offload experiments: no useful fix. Current relevant module parameters: ``` /sys/module/bluetooth/parameters/disable_ertm =3D N /sys/module/bluetooth/parameters/disable_esco =3D N /sys/module/bluetooth/parameters/enable_ecred =3D Y /sys/module/btusb/parameters/reset =3D Y /sys/module/btusb/parameters/force_scofix =3D N /sys/module/btusb/parameters/disable_scofix =3D N /sys/module/btusb/parameters/enable_autosuspend =3D N ``` ### Possibly related upstream reports / patches This looks related to the Realtek SCO/WBS path in `btusb`/`btrtl`. A very similar Realtek patch was posted but does not appear to be merged: ``` [PATCH v5] Bluetooth: btusb: Work around spotty SCO quality https://patchew.org/linux/20221226074829.8682-1-hildawu%40realtek.com/ ``` That patch targeted other Realtek USB IDs (`0bda:8771`, `0bda:8773`, `0bda:8871`, `0bda:8873`), not `0bda:c821`, but the symptom family is close: Realtek USB Bluetooth, WBS/SCO, spotty SCO quality, and packet handling aro= und WBS. Possibly related Bugzilla entries: ``` Bug 219997 - [rtw89] Headset unusable because of delays and stutter https://bugzilla.kernel.org/show_bug.cgi?id=3D219997 Bug 219992 - Linux logs warning `Bluetooth: hci0: SCO packet for unknown connection handle 3` https://bugzilla.kernel.org/show_bug.cgi?id=3D219992 ``` These do not seem to be exact matches for this controller (`0bda:c821` / RTL8821C), but they may be related to the same SCO/WBS area. ### Attachments available The following captures are available and should be attached: ``` /home/apocarteres/bt-sco-bad.snoop /home/apocarteres/bt-sco-bad1.snoop /home/apocarteres/bt-sco-bad2.snoop /home/apocarteres/bt-sco-bad3.snoop /home/apocarteres/bt-sco-bad4.snoop /opt/fedora.cap ``` Recommended minimum attachments: ``` /home/apocarteres/bt-sco-bad4.snoop /opt/fedora.cap ``` `bt-sco-bad4.snoop` is a longer Debian capture. `fedora.cap` demonstrates t= hat the same bursty SCO RX pattern also happens on Fedora 6.19. --=20 You may reply to this email to add a comment. 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