| > these PC systems sort of lack a bios, each one needing a hand built DeviceTree: it makes supporting them kind of a nightmare. Modern PC ARM systems like Snapdragon Elite X use UEFI and ACPI. This is actually what makes them difficult, because they're trying to operate in a "new world" while most ARM SOC IP and peripheral drivers work in the "old world." The issue with ARM has never _really_ been early boot; yes, it's arcane and a pain in the butt on some platforms, but it really only needs to be done once - once your DRAM is trained and running (this is usually the hardest part) and you can load and jump into a kernel, you're set. Hypervisor / security processor driven systems like Qualcomm (and for that matter, Intel and AMD) actually make this even easier at the expense of openness, because the vendor blob usually brought everything up for you already. The issue has always been hardware discovery and mutable device configuration. When ARM devices were first supported by Linux, they were mostly embedded devices with one configuration, ever. So, they used devicetree, which is a fixed structure for each board, defined before boot and provided by the bootloader. Because of this, most SOC / platform / IP soft-core drivers were built to work with fixed, proprietary configurations and usually only tested against a single platform to start. On the other hand, x86 devices have been forced to work as highly mutable, arbitrary combinations of hardware (Plug n Play) with dynamic reconfiguration using ACPI since the start, so the drivers for x86 peripherals have always had to cope with a completely unpredictable environment. What this means is that there's a ton of effort required to transition ARM _peripheral drivers_ from the "devicetree" world where drivers took fixed arbitrary, proprietary key=value parameters provided by a magic blob at boot to the ACPI world, where everything is dynamic, scripted, and abstract. I'd actually argue that Pi have the most hacked tooling on top of the "old devicetree way," which means they're the most set on it. Pi peripherals are usually configured at pre-boot time using devicetree overlays and their drivers usually don't support any kind of probing/autodiscovery. As far as I know there's no real plan to change this (and maybe there doesn't have to be; it seems to work for them). Anyway, this is all to say: I don't think the issue with either system is the "boot situation," it's the "peripheral configuration situation." In this sense, Asahi are actually in a fine situation to use devicetrees, which they do, because basically all of the SOC peripherals are proprietary and there are a fixed number of Apple devices to target and the only external interfaces are existing hot plug standards (USB/Thunderbolt/HDMI/DP). Qualcomm are smart to have started to try to use ACPI, because their SoCs could be hosted on boards with standard peripherals configured in thousands of different ways, like all PCs. But, they're playing on hard mode because most of the existing ARM peripheral drivers weren't made to support this model. |
Originally, embedded Linux ARM devices used a board file with a platform bus and hard-coded device metadata. The bootloader had to pass a machine id which told the kernel which hardware you were running on and which board file to use.
You can see remenants of this in the kernel still, though it's quickly being removed. I'm actually working on a hybrid kernel with the goal of bringing modern Linux support (on an lts branch) to old MSM7x300 devices, like the Evo 4G Shift I intent to use a tmux console/cyberdeck.
On another note, ACPI/UEFI doesn't always give you a clean abstract surface to work with either. ACPI is notorious for building in OS checks into it's compiled bytecode to the point that Linux often lies to it about what OS is running.