rpi4/5: no hevc decoding with kernel 6.18.x

Reproduce HEVC playback with Kodi on the Raspberry Pi 4 or 5 using kernel 6.18.20 and inspect the repeated rpi-hevc-dec messages in dmesg; compare the same workload with kernel 6.17. Done means hardware HEVC decoding works on 6.18.x without those errors.

Describe the bug

I’ve only just realised that HEVC hardware decoding no longer works with kernel 6.18.x (currently 6.18.20). Up to and including 6.17, it still works perfectly without me having to make any adjustments.

When attempting to play an HEVC file, dmesg constantly spits out

[   44.523449] rpi-hevc-dec 1000800000.codec: data_byte_offset 62 + bits 384752 (= 48094 bytes) > bytesused 48094
[   44.554123] rpi-hevc-dec 1000800000.codec: data_byte_offset 62 + bits 391280 (= 48910 bytes) > bytesused 48910
[   44.584730] rpi-hevc-dec 1000800000.codec: data_byte_offset 62 + bits 437600 (= 54700 bytes) > bytesused 54700
[   44.615320] rpi-hevc-dec 1000800000.codec: data_byte_offset 62 + bits 442504 (= 55313 bytes) > bytesused 55313
[   44.645926] rpi-hevc-dec 1000800000.codec: data_byte_offset 62 + bits 424192 (= 53024 bytes) > bytesused 53024
[   44.676525] rpi-hevc-dec 1000800000.codec: data_byte_offset 62 + bits 458816 (= 57352 bytes) > bytesused 57352
[   44.707116] rpi-hevc-dec 1000800000.codec: data_byte_offset 62 + bits 465440 (= 58180 bytes) > bytesused 58180
[   44.737775] rpi-hevc-dec 1000800000.codec: data_byte_offset 62 + bits 504136 (= 63017 bytes) > bytesused 63017

.
What could the problem be?
The kernel config is the standard one generated by defconfig with a few minor modifications (the most important being BTRFS in the kernel rather than as a module). So it can’t really be down to the kernel config either.
Does it need another udev rule or a special additional setting in boot?

The firmware should also be up to date; I’ve just checked again:

vcgencmd bootloader_version
08/12/2025 19:29:54
version 2226a853bb9f5fd80392e3a4a89e457aeca88008 (release)
timestamp 1765222194
update-time 1766515549
capabilities 0x0000007f

All that runs under Debian Trixie

Steps to reproduce the behaviour

Playing a HEVC file with Kodi

Device (s)

Raspberry Pi 5 + Pi 4

System

vcgencmd version
2025/12/08 19:29:54
Copyright (c) 2012 Broadcom
version 2226a853 (release) (embedded)

uname -a
Linux kmpifive 6.18.20-16k64+ #1 SMP PREEMPT Sun Apr 5 15:35:10 UTC 2026 aarch64 GNU/Linux

cat /etc/debian_version
13.4

Logs

No response

Additional context

No response