Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 10 Apr 2020 at 14:33, Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:
>
> On Fri, Apr 10, 2020 at 01:15:08PM +0200, Ard Biesheuvel wrote:
> > On Fri, 10 Apr 2020 at 11:56, Arnd Bergmann <arnd@arndb.de> wrote:
> > >
> > > On Fri, Apr 10, 2020 at 1:28 AM Jian Cai <caij2003@gmail.com> wrote:
> > > >
> > > > iwmmxt.S contains XScale instructions LLVM ARM backend does not support.
> > > > Skip this file if LLVM integrated assemmbler or LLD is used to build ARM
> > > > kernel.
> > > >
> > > > Signed-off-by: Jian Cai <caij2003@gmail.com>
> > >
> > > It clearly makes sense to limit the Kconfig option to compilers that
> > > can actually build it.
> > > A few questions though:
> > >
> > > - Given that Armada XP with its PJ4B was still marketed until fairly
> > > recently[1],
> > >   wouldn't it make sense to still add support for it? Is it a lot of work?
> > >
> >
> > The part of that file that the assembler chokes on hasn't been touched
> > by anyone since Nico added it 15+ years ago. It can only be built in
> > ARM mode, and it disassembles to the sequence below (the ld/st fe/fp
> > mnemonics are not document in recent versions of the ARM ARM, and
> > aren't understood by Clang either)
>
> For older CPUs, it doesn't matter what the latest ARM ARM says, the
> appropriate version of the ARM ARM is the one relevant for the CPU
> architecture.  This is a mistake frequently made, and it's been pointed
> out by Arm Ltd in the past (before ARMv6 even came on the scene) that
> keeping older revisions is necessary if you want to be interested in
> the older architectures.
>
> However, there's an additional complication here: DEC's license from
> Arm Ltd back in the days of StrongARM allowed them to make changes to
> the architecture - that was passed over to Intel when they bought that
> part of DEC.  Consequently, these "non-Arm vendor" cores contain
> extensions that are not part of the ARM ARM.  iWMMXT is one such
> example, which first appeared in the Intel PXA270 SoC (an ARMv5
> derived CPU).
>
> In fact, several of the features found in later versions of the ARM
> architecture came from DEC and Intel enhancements.
>
> If your compiler/assembler only implements what is in the latest ARM
> ARM, then it is not going to be suitable for these older CPUs and
> alternate vendor "ARM compatible" CPUs.
>

Indeed, and I'm a bit disappointed at the willingness to leave stuff
by the wayside, especially since Clang's integrated assembler has no
other benefit to it than being built into the compiler.

> > Instead of playing all these tricks with Kconfig, couldn't we simply
> > insert the bare opcodes and be done with it?
>
> That gets close to a GPL violation; the GPL requires that source code
> be in the preferred form for making modifications. Encoding raw opcodes
> can in no way be argued to be the preferred form. Arguing that raw
> opcodes is acceptable sets a precedent that makes it acceptable for
> other "works" to do the same, which makes arguments against firmware
> supplied as a hexdump null and void.
>
> Using macros to emulate the instructions and create the appropriate
> opcodes is an alternative; we already have that for some of the VFP
> code as early toolchains had no support for the VFP instructions.
>
> So no, bare opcodes are unacceptable.
>

Fair enough.

The following set of macros appears to emit the opcodes correctly,
assuming we're willing to tweak the source code somewhat, i.e., drop
square brackets and leading # for immediate offsets. (The tmcr/tmrc
instructions are left as an exercise for the reader)


.irp b, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
.set .LwR\b, \b
.set .Lr\b, \b
.endr

.set .LwCSSF, 0x2
.set .LwCASF, 0x3
.set .LwCGR0, 0x8
.set .LwCGR1, 0x9
.set .LwCGR2, 0xa
.set .LwCGR3, 0xb

.macro wldrd, reg:req, base:req, offset:req
.inst 0xedd00100 | (.L\reg << 12) | (.L\base << 16) | (\offset >> 2)
.endm

.macro wldrw, reg:req, base:req, offset:req
.inst 0xfd900100 | (.L\reg << 12) | (.L\base << 16) | (\offset >> 2)
.endm

.macro wstrd, reg:req, base:req, offset:req
.inst 0xedc00100 | (.L\reg << 12) | (.L\base << 16) | (\offset >> 2)
.endm

.macro wstrw, reg:req, base:req, offset:req
.inst 0xfd800100 | (.L\reg << 12) | (.L\base << 16) | (\offset >> 2)
.endm