Because you just stopped at the kernel modules part and haven't spent time reading how HAL in Android actually works, starting at the HIDL link.

There are two HALs in Android.

The old HAL, previous to Treble, which was hardly used by OEMs.

And the new HAL, which is enforced by Treble.

On the new HAL, drivers are implemented as Android services using Android IPC via interfaces defined in HIDL, or by using the new shared memory APIs introduced in Oreo as well.

https://source.android.com/devices/architecture/hidl/service...

https://source.android.com/devices/architecture/hidl/binder-...

https://source.android.com/devices/architecture/hidl/memoryb...

https://source.android.com/devices/architecture/configstore/

I actually looked into the HAL/HIDL docs before posting and found no evidence that they are there to implement drivers in userspace.

As I understand it HALs and HIDL are used to provide a standardized way to implement new device features in a compatible way. So a vendor introduces a odor-sensor and can define an HIDL interface for the userspace to call the device driver provided in a kernel module.

I do not see a requirement to implement drivers in userspace, nor provisions for it, like interrupt handling, i2c/spi access or similar.

HIDL is the basis of Android IPC between processes, known as Binder.

"Binderized HALs. HALs expressed in HAL interface definition language (HIDL). These HALs replace both conventional and legacy HALs used in earlier versions of Android. In a Binderized HAL, the Android framework and HALs communicate with each other using binder inter-process communication (IPC) calls. All devices launching with Android 8.0 or later must support binderized HALs only.

Passthrough HALs. A HIDL-wrapped conventional or legacy HAL. These HALs wrap existing HALs and can serve the HAL in binderized and same-process (passthrough) modes. Devices upgrading to Android 8.0 can use passthrough HALs. "

https://source.android.com/devices/architecture/hal-types

Here are the user space driver APIs for i2c/spi access on Android Things.

https://developer.android.com/reference/com/google/android/t...

Android Things is not included in normal Android as it seems. Android Treble/9 is just not even close to a microkernel.

It if was, there would be documentation on how to write a userspace device driver, but there isn't.

Binder allows for endpoints in kernel space.

Of course, otherwise how would they support passthrough legacy HALs for devices upgrading to Oreo as described on the documentation?

Interprocess RPC is only for Oreo and newer.

The current HAL works via dynamic libraries that get loaded into your process and talk to kernel space with a platform specific API. The passthrough support is just opening both sides in the same process, loading those legacy libraries, and wrapping the libraries in the treble API. So it actually uses IPC where both sides are in user space. That's what they mean when they describe it as 'in process' in the documentation.

Going forward, I expect the vendors to modify their kernel drivers to export the treble API over binder IPC directly while in kernel space.

So I wouldn't be surprised if it ends up that interprocess RPC only ebds up getting used in systems before Oreo.

I'll admit to not being steeped in the terminology, so feel free to educate me.. but some quick Googling suggests nobody else has called Treble a "microkernel." There are hits about Fuchsia (a totally separate OS), and some Android forks on microkernels.

I'm guessing that the Treble modules expose the traditional /dev and /sys interfaces like before, and these new HALs talk to devices through those, right? Is that not still a runtime-linked monolith, with fairly-thin userspace services between the kernel and the framework?

The bulk of driver code logic (e.g. interfacing with a touchscreen controller) is probably still running in the kernel - show me if I'm wrong. I would think this would be how you define a microkernel. One exception may be the GPU -- those drivers are often very thin in the kernel because of GPL, with a fat userspace library that they can legally keep closed-source.

They use Android IPC, which is a kind of RPC between processes.

https://source.android.com/devices/architecture/hal-types

This is exactly how I understand it.