[jeroenhd]

The kernel is usually frozen but sometimes projects like PostmarketOS can use the changes to upstream the changes and add general Linux support.

Anyone can make a diff between the upstream kernel and the Qualcomm kernel. Maintaining these changes into later versions of the kernel will be quite challenging, but the base is already there.

That said, phones also come with plenty of binary drivers and those cannot be ported. That's an important reason not to bother with later kernel versions in custom ROMs: after all of your hard work, the end result will be missing important features such as GPU acceleration.

[donkeybeer]

What do you think are the reasons pc's don't need Dell xps 13 9350 Windows and Lenovo ThinkPad T14s Gen 6 Linux and so on but phones need Galaxy S26 Linux, Xiaomi 16 Linux and so on?

[jeroenhd]

Because ARM lacks some of the device auto-discovery features that amd64 provides for free, unless you're lucky and use a device with ACPI+DSDTs on ARM. You need a special build for the hardware, but you don't need to alter the source code.

Custom kernels also exist for amd64 devices, often including workarounds and patches that are not in mainline to improve performance or compatibility.

As a vendor, that requires practically zero extra effort.

https://wiki.postmarketos.org/wiki/Devices has a list of devices that run either mainline or almost-mainline Linux. Only the "downstream" devices require vendor Linux kernels. Of course, hardware support is partial for most of these devices because vendors haven't contributed proper upstreamable drivers and volunteers haven't had the time to write them yet, but it's not like every ARM device needs a special kernel fork, that's just something ARM vendors do out of laziness.