Arm EABI Port

EABI is the new Extended ABI by Arm ltd. EABI is acually a family of ABI's and one of the "subABIs" is GNU EABI, for Linux. The effective changes for users are:

Gcc view

New ABI is not only a new ABI field, it is also a new GCC target.

Legacy ABI

private flags = 2: [APCS-32] [FPA float format] [has entry point]

Arm EABI:

private flags = 4000002: [Version4 EABI] [has entry point]

Arm floating points

Current debian port creates hardfloat FPA instructions. FPA comes from "Floating Point Accelerator". Since FPA floating point unit was implemented only in very few ARM cores, these days FPA instructions are emulated in kernel via Illegal instruction faults. This is of course very inefficent. FPA unit has the pecularity of having mixed-endian doubles, which is usually the biggest grief for arm porters (along with struct packing issues).

In their great wisdom, ARM has introduced a new floating point unit, VFP (Vector Floating Points), which uses a different instruction set than FPA. VFP is implemented in new ARM11 cores, like in the new TI OMAP2 family. It seems likely that arm cores without VFP will remain popular, as in many places ARM is used floats are unneccesary.

For a generic-purpose distribution like Debian, targetting binary compatability (as opposed to source-based distributions that currently are more popular among Linux systems), EABI lets us have the cake and eat it. We can a soft-float distribution, where -vfp packages (linux-kernel-2.6.x-vfp, libc6-vfp, mediaplayer-vfp, etc) exist where needed.

...And there is still iwmmmx and maverick floating point units, which we will not cover now...

Struct packing

Todo: explain struct packing issues and Arm alignment

Why a new port

In Debian, we want to assure complete binary compatablity. Since the old ABI is not compatible with the new one, we can't allow packages built with old ABI to link against new-abi libs, or the other direction. So the options are:

0. Not an option!

Under no circumstances distribute EABI binaries as .arm.deb depending on current library package names!!!

1. Rename all library packages

This is the ABI transitions affecting all architectures have been done (aout -> elf, c++ ABI)

2. New arch

For the last point, a statically compiled ?ArchUpgrade tool could be created. This would also allow i386->amd64 style migrations.

3. ABI: field in control file

This was suggested as part of Multiarch proposal. It is unknown if it would actually become part of Debian or not

From these choices, we believe a new port is the best compromise.

Let's not make perfect an enemy of good!!

EABI status

Riku Voipio has built a booting EABI rootfilesystem up to X as proof of concept, seems stable. Built with codesourcery gcc 3.4 toolchain.

Naming

Suggested names for the new port