Public bug reported:
[Impact]
RS485 serial ports based on Fintek F81504/508/512 are broken since 24.04
(kernels 6.8, 6.14 and 6.17)
[Fix]
Backport the upstream fix: 27aff0a56b3c (serial: 8250_pci: Fix broken RS485 for
F81504/508/512)
(Merged in 6.19-rc7)
[Test]
With the fix applied, communication with attached devices is again possible.
Also tested with rs485conf:
$ sudo ./rs485conf -e 1 -o 0 -a 1 /dev/ttyS7
= New configuration:
RS485 enabled: true
RTS on send: low
RTS after send: high
= Saved configuration:
RS485 enabled: true
RTS on send: high <- should mirror new configuration, does so
with fix
RTS after send: low <-/
[Where problems could occur]
Because of the small scope of the correction the risk of regressions should be
very low.
ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: linux-image-6.8.0-90-generic 6.8.0-90.91
ProcVersionSignature: Ubuntu 6.8.0-90.91-generic 6.8.12
Uname: Linux 6.8.0-90-generic x86_64
AlsaVersion: Advanced Linux Sound Architecture Driver Version k6.8.0-90-generic.
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.28.1-0ubuntu3.8
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
AudioDevicesInUse: Error: [Errno 2] No such file or directory: 'fuser'
CRDA: N/A
Card0.Amixer.info: Error: [Errno 2] No such file or directory: 'amixer'
Card0.Amixer.values: Error: [Errno 2] No such file or directory: 'amixer'
CasperMD5CheckResult: pass
CurrentDmesg: Error: command ['dmesg'] failed with exit code 1: dmesg: read
kernel buffer failed: Operation not permitted
Date: Tue Jan 27 09:28:28 2026
InstallationDate: Installed on 2026-01-26 (1 days ago)
InstallationMedia: Ubuntu-Server 24.04.3 LTS "Noble Numbat" - Release amd64
(20250805.1)
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig'
Lsusb:
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 046d:c31c Logitech, Inc. Keyboard K120
Bus 001 Device 003: ID 1508:1001 Fibocom Fibocom Modem
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
MachineType: NF841/V NF841/V
ProcFB: 0 i915drmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-6.8.0-90-generic
root=UUID=a8bf37d2-983c-4dfa-a9cb-37eba0d9a990 ro
RelatedPackageVersions:
linux-restricted-modules-6.8.0-90-generic N/A
linux-backports-modules-6.8.0-90-generic N/A
linux-firmware 20240318.git3b128b60-0ubuntu2.22
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
acpidump:
dmi.bios.date: 10/28/2022
dmi.bios.release: 5.12
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: BAT8NA08
dmi.board.asset.tag: Default string
dmi.board.name: NF841/V
dmi.board.vendor: NF841/V
dmi.board.version: 1.0
dmi.chassis.type: 3
dmi.chassis.vendor: NF841/V
dmi.chassis.version: 1.0
dmi.modalias:
dmi:bvnAmericanMegatrendsInc.:bvrBAT8NA08:bd10/28/2022:br5.12:svnNF841/V:pnNF841/V:pvr1.0:rvnNF841/V:rnNF841/V:rvr1.0:cvnNF841/V:ct3:cvr1.0:skuDefaultstring:
dmi.product.family: Default string
dmi.product.name: NF841/V
dmi.product.sku: Default string
dmi.product.version: 1.0
dmi.sys.vendor: NF841/V
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: amd64 apport-bug noble
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https://bugs.launchpad.net/bugs/2139136
Title:
[SRU] Fix broken RS485 for F81504/508/512
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