linux-kernel - Re: [PATCH] ARM: OMAP2+: drop unnecessary adrl

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Date:   Thu, 2 Apr 2020 11:48:19 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>, Peter.Smith@....com
Cc:     Stefan Agner <stefan@...er.ch>, Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-omap@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: OMAP2+: drop unnecessary adrl

On Wed, 1 Apr 2020 at 20:02, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Sun, Mar 29, 2020 at 1:33 PM Stefan Agner <stefan@...er.ch> wrote:
> >
> > The adrl instruction has been introduced with commit dd31394779aa ("ARM:
> > omap3: Thumb-2 compatibility for sleep34xx.S"), back when this assembly
> > file was considerably longer. Today adr seems to have enough reach, even
> > when inserting about 60 instructions between the use site and the label.
> > Replace adrl with conventional adr instruction.
> >
> > This allows to build this file using Clang's integrated assembler (which
> > does not support the adrl pseudo instruction).
>
> Context: https://github.com/ClangBuiltLinux/linux/issues/430#issuecomment-476124724
> If Peter says it's difficult to implement, I trust him.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>

I take it this implies that the LLVM linker does not support the
R_ARM_ALU_PC_Gn relocations? Since otherwise, adrl could simply be
expanded to a pair of adds with the appropriate relocations, letting
the linker fix up the immediates (and the ADD vs SUB bits)

> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/430
> > Signed-off-by: Stefan Agner <stefan@...er.ch>
> > ---
> >  arch/arm/mach-omap2/sleep34xx.S | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/mach-omap2/sleep34xx.S b/arch/arm/mach-omap2/sleep34xx.S
> > index ac1324c6453b..c4e97d35c310 100644
> > --- a/arch/arm/mach-omap2/sleep34xx.S
> > +++ b/arch/arm/mach-omap2/sleep34xx.S
> > @@ -72,7 +72,7 @@ ENTRY(enable_omap3630_toggle_l2_on_restore)
> >         stmfd   sp!, {lr}       @ save registers on stack
> >         /* Setup so that we will disable and enable l2 */
> >         mov     r1, #0x1
> > -       adrl    r3, l2dis_3630_offset   @ may be too distant for plain adr
> > +       adr     r3, l2dis_3630_offset
> >         ldr     r2, [r3]                @ value for offset
> >         str     r1, [r2, r3]            @ write to l2dis_3630
> >         ldmfd   sp!, {pc}       @ restore regs and return
> > --
> > 2.25.1
> >
> > --
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> > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/5a6807f19fd69f2de6622c794639cc5d70b9563a.1585513949.git.stefan%40agner.ch.
>
>
>
> --
> Thanks,
> ~Nick Desaulniers
>
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