kernel | planet

March 28, 2025

Linux 6.14, an almost forgotten release

Linux 6.14 is the second release of 2025, and as usual Igalia took part on it. It’s a very normal release, except that it was release on Monday, instead of the usual Sunday release that has been going on for years now. The reason behind this? Well, quoting Linus himself:

I’d like to say that some important last-minute thing came up and delayed things.

But no. It’s just pure incompetence.

But we did not forget about it, so here’s our Linux 6.14 blog post!

A part of the development cycle for this release happened during late December, when a lot of maintainers and developers were taking their deserved breaks. As a result of this, this release contains less changes than usual as stated by LWN as the “lowest level of merge-window activity seen in years”. Nevertheless, some cool features made through this release:

  • NT synchronization primitives: Elizabeth Figura, from Codeweavers, is know from her work around improving Wine sync functions, like mutexes and semaphores. She was one the main collaborators behind the futex_waitv() work and now developed a virtual driver that is more compliant with the precise semantics that the NT kernel exposes. This allows Wine to behave closer to Windows without the need to create new syscalls, since this driver uses ioctl() as the front-end uAPI.
  • RWF_UNCACHED: Linux has two ways of dealing with storage I/O: buffered I/O (usually the preferred one) that stores data in a temporary buffer and regularly syncs the cache data with the device; and direct I/O that doesn’t use cache and always writes/reads synchronously with the storage device. Now a new mixed approach is available: uncached buffered I/O. This method is aimed to have a fast way to write or read data that will not be needed again in the short term. For reading, the device writes data in the buffer and as soon as the user finished reading the buffer, it’s cleared from the cache. For writing, as soon as userspace fills the cache, the device reads it and removes it from the cache. In this way we still have the advantage of using a fast cache but reducing the cache pressure.
  • amdgpu panic support: AMD developers added kernel panic support for amdgpu driver, “which displays a pretty user friendly message on the screen when a Linux kernel panic occurs” instead of just a black screen or a partial dmesg log.

As usual Kernel Newbies provides a very good summary, you should check it for more details: Linux 6.14 changelog. Now let’s jump to see what were the merged contributions by Igalia for this release!

DRM

For the DRM common infrastructure, we helped to land a standardization for DRM client memory usage reporting. Additionally, we contributed to improve and fix bugs found in drivers of AMD, Intel, Broadcom, and Vivante.

AMDGPU

For the AMD driver, we fixed bugs experienced by users of Cosmic Desktop Environment on several AMD hardware versions. One was uncovered with the introduction of overlay cursor mode, and a definition mismatch across the display driver caused a page-fault in the usage of multiple overlay planes. Another bug was related to division by zero on plane scaling. Also, we fixed regressions on VRR and MST generated by the series of changes to migrate AMD display driver from open-coded EDID handling to drm_edid struct.

Intel

For the Intel drivers, we fixed a bug in the xe GPU driver which prevented certain type of workarounds from being applied, helped with the maintainership of the i915 driver, handled external code contributions, maintained the development branch and sent several pull requests.

Raspberry Pi (V3D)

We fixed the GPU resets for the Raspberry Pi 4 as we found out to be broken as per a user bug report.

Also in the V3D driver, the active performance monitor is now properly stopped before being destroyed, addressing a potential use-after-free issue. Additionally, support for a global performance monitor has been added via a new DRM_IOCTL_V3D_PERFMON_SET_GLOBAL ioctl. This allows all jobs to share a single, globally configured perfmon, enabling more consistent performance tracking and paving the way for integration with user-space tools such as perfetto.

A small video demo of perfetto integration with V3D

etnaviv

On the etnaviv side, fdinfo support has been implemented to expose memory usage statistics per file descriptor, enhancing observability and debugging capabilities for memory-related behavior.

sched_ext

Many BPF schedulers (e.g., scx_lavd) frequently call bpf_ktime_get_ns() for tracking tasks’ runtime properties. bpf_ktime_get_ns() eventually reads a hardware timestamp counter (TSC). However, reading a hardware TSC is not performant in some hardware platforms, degrading instructions per cycyle (IPC).

We addressed the performance problem of reading hardware TSC by leveraging the rq clock in the scheduler core, introducing a scx_bpf_now() function for BPF schedulers. Whenever the rq clock is fresh and valid, scx_bpf_now() provides the rq clock, which is already updated by the scheduler core, so it can reduce reading the hardware TSC. Using scx_bpf_now() reduces the number of reading hardware TSC by 50-80% (e.g., 76% for scx_lavd).

Assorted kernel fixes

Continuing our efforts on cleaning up kernel bugs, we provided a few fixes that address issues reported by syzbot with the goal of increasing stability and security, leveraging the fuzzing capabilities of syzkaller to bring to the surface certain bugs that are hard to notice otherwise. We’re addressing bug reports from different kernel areas, including drivers and core subsystems such as the memory manager. As part of this effort, several fixes were done for the probe path of the rtlwifi driver.

Check the complete list of Igalia’s contributions for the 6.14 release

Authored (38)

Changwoo Min

Christian Gmeiner

Luis Henriques

Maíra Canal

Melissa Wen

Ricardo Cañuelo Navarro

Rodrigo Siqueira

Thadeu Lima de Souza Cascardo

Tvrtko Ursulin

Reviewed (36)

André Almeida

Christian Gmeiner

Iago Toral Quiroga

Jose Maria Casanova Crespo

Luis Henriques

Maíra Canal

Tvrtko Ursulin

Tested (2)

André Almeida

Christian Gmeiner

Acked (1)

Iago Toral Quiroga

Maintainer SoB (6)

Maíra Canal

Tvrtko Ursulin

© André Almeida 2022
Licensed as CC BY 4.0

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