linux-kernel - Re: [RFC/PATCH 0/3] ARM: Use udiv/sdiv for __aeabi

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Date:	Mon, 23 Nov 2015 09:15:09 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Peter Maydell <peter.maydell@...aro.org>,
	Måns Rullgård <mans@...sr.com>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	arm-mail-list <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC/PATCH 0/3] ARM: Use udiv/sdiv for __aeabi_{u}idiv library functions

On Sunday 22 November 2015 21:36:45 Nicolas Pitre wrote:
> On Sun, 22 Nov 2015, Arnd Bergmann wrote:
> 
> > I've also found some /proc/cpuinfo output to cross-reference SoCs
> > to their core names.
> > 
> >               variant part    revision        name            features
> > dove:         0       0x581   5               PJ4             idivt
> 
> I just managed to boot my dusty Dove DB and ran a quick test programon 
> it. Its cpuinfo corresponds to the above.
> 
> $ cat m.c
> #include <stdio.h>
> int mydiv(int, int);
> int main()
> {
>         printf("div test\n");
>         printf("%d\n", mydiv(12345678, 37));
>         return 0;
> }
> $ cat d.c
> int mydiv(int x, int y)
> {
>         return x/y;
> }
> $ gcc -o test m.c d.c
> $ ./test
> div test
> 333666
> $ gcc -o test m.c d.c -march=armv7ve -mthumb
> $ ./test
> div test
> 333666
> $ gcc -o test m.c d.c -march=armv7ve -marm
> $ ./test
> div test
> Illegal instruction (core dumped)
> $

Ok, thanks a lot! So the reporting in /proc/cpuinfo clearly matches
the actual features, and we can just treat this as no LPAE / no IDIV
for kernel compilation, as nobody ever seems to use THUMB2_KERNEL
in practice.

PJ4-MP is like Cortex-A15/A7/A12/A17 and supports both IDIV and LPAE,
which leaves the question whether Scorpion or Krait do the same as
well, or whether they are outliers and need a special configuration.

	Arnd
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