[Buildroot] [PATCH 04/16] uclibc: Enable compile in thumb mode when selected
Thomas Petazzoni
thomas.petazzoni at free-electrons.com
Thu Mar 17 22:03:10 UTC 2016
Hello,
On Thu, 17 Mar 2016 22:44:16 +0100, Peter Korsgaard wrote:
> > USE_LDREXSTREX is mandatory for uClibc to build on Thumb2 (see commit
> > https://lists.uclibc.org/pipermail/uclibc/2014-November/048704.html).
> > Otherwise, the build fails with:
>
> Ahh, ok - Good to know. So this is only really needed for thumb2-only
> CPUs (cortex-M).
Mandatory for Thumb2-only CPUs, but perfectly usable for ARMv7-A as
well.
> Completely unrelated, but notice that my browser wouldn't let
> me visit https://lists.uclibc.org because of the HSTS header on
> uclibc.org and the lists.uclibc.org vhost using a certificate for
> *.osuosl.org.
Same here, I have to use Firefox to be able to bypass the "security
checks". Chromium doesn't allow to bypass such security checks (at
least by default).
> > So I think we should:
>
> > 1/ Enable COMPILE_IN_THUMB_MODE whenever ARM_INSTRUCTIONS_THUMB(2) is
> > enabled.
>
> Ok.
>
> > 2/ Enable USE_BX whenever ARM_INSTRUCTIONS_THUMB(2) is enabled. I
> > don't see why USE_BX for later cores can cause performance problems,
> > it's just about using a simple bx instruction to switch between ARM
> > and Thumb modes.
>
> > 3/ Enable USE_LDREXSTREX whenver ARM_INSTRUCTIONS_THUMB(2) is enabled
> > *and* we're on ARMv7. Otherwise, disable it.
>
> With ARMv7 I guess you mean thumb2-only CPUs (cortex-m)?
>
> But COMPILE_IN_THUMB_MODE selects USE_BX / USE_LDREXTREX, so we cannot
> really do that.
>
> I guess we should leave COMPILE_IN_THUMB_MODE alone (it just passes
> -mthumb in CFLAGS which we don't need), and just enable USE_BX/LDREXTREX
> when needed.
USE_LDREXSTREX depends on COMPILE_IN_THUMB_MODE, so you can't have
USE_LDREXSTREX without having COMPILE_IN_THUMB_MODE.
So, instead what I've done is cook a set of patches for uClibc that
simply remove USE_BX, USE_LDREXSTREX and COMPILE_IN_THUMB. None of
those options are needed, everything can be guessed by looking at the
selected ARM variant, and for COMPILE_IN_THUMB, by passing -mthumb.
See
https://github.com/tpetazzoni/uclibc-ng/commits/arm-simplifications.
Build testing in progress. I'm testing:
* Cortex-M4, so a Thumb2-only core
* Cortex-A in Thumb2 mode
* ARM920T in Thumb mode
* FA526, which is an ARMv4 (and therefore doesn't support Thumb)
Any other config you think is relevant?
Thanks,
Thomas
--
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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