Re: cortexa9t2hf instead of cortexa9hf

[Martin Jansa <martin.jansa@gmail.com>]

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On Mon, Jun 17, 2019 at 07:47:36PM +0200, Arno Steffens wrote:
> Thanks for explaining this.
> I take some time to read about thumb/thumb2. The feedback is mixed. It seems to generate more compact code, but some say it speeds up, others it slows down because of reduced function set - and it can cause strange effects.
> And mixing this causes time to switch processor mode. So, as I am not an expert in this and can't decide what ist best on per function base and speed is of highest priority, I think I better should use not thumb(2).
> 
> So, do I get it right that with this cortexa9t2hf I just have the option to compile it for thumb2? But without using a dedicated compiler option it generates same "standard" arm code and the difference is just to adapt all the Makefiles for this suffix.
> 
> According to Martin I can get the previous setting by just set ARM_INSTRUCTION_SET to "arm" instead of "armv7a". Mh - I just afraid that I lose other kinds of optimisation. (I am just a user not an expert in arm architecture).

I didn't say anything about changing the DEFAULTTUNE.

Just ARM_INSTRUCTION_SET to "arm" which will give you "cortexa9hf" again
(not "armv7a").

> On the other hand for those like me it is better go the standard way. Once I am sure compiler results will not become worse (see above) I go for the pain and renaming my toolchain/makefiles/stuff.

The results should be almost the same, maybe slightly better, depends on
the actual code as Khem mentioned.

But if this causes you a lot of pain "renaming my toolchain/makefiles/stuff"
then you should probably spend the time on your tooling instead of
replacing one hardcoded value with another.

Cheers,

> 
> Thanks for you taking the time.
> Arno
> 
> > Hello Arno,
> >
> > Let me try to explain my point of view. Since here (my best guess) we
> > have some asynchronous bitbake code which went South upon introducing
> > T2 HW extension.
> >
> > Point [1]: as far as I understand arm, cortexa9t2hf is is just a
> > superset of previous cortexa9hf (HW wise). NEON HW extension (NEON
> > media coprocessor) exists in both of them. In other words:
> > cortexa9t2hf = cortexa9hf HW + T2 HW extension.
> >
> > Point [2]:
> > > bitbake gives me in 2.5:
> > > TUNE_FEATURES        = "arm armv7a vfp thumb neon callconvention-hard cortexa9"
> > > TARGET_FPU           = "hard"
> > >
> > > and in 2.7:
> > > TUNE_FEATURES        = "arm vfp cortexa9 neon thumb callconvention-hard"
> > > TARGET_FPU           = "hard"
> >
> > These two lines are the same: you are able to use 32b arm mode, 16bit
> > thumb mode, using armv7 HW with neon HW extension, and using HW FP
> > extension as well. The Cortex in both cases is A9.
> >
> > I expect that somebody somewhere in bitbake version 1.42 - 100% sure
> > (since 2.5/Sumo uses bitbake 1.38) dropped "armv7a" as TUNE FEATURE,
> > and I have no idea if this is done intentionally or not.
> >
> > Because of that I copied Alex and Ross to CC: into email, so they
> > should unveil this mystery (I would prefer "armv7" to stay in bitbake
> > 1.42, since A8 and A9 belongs to armv7, A15 belongs to armv8 (IIRC).
> >
> > Bottom line: nothing to be done by you, Arno, seems that bitbake 1.42
> > should return "armv7" as TUNE FEATURE.
> >
> > Best Regards,
> > Zoran
> > _______
> >
> >
> > On Mon, Jun 17, 2019 at 3:00 PM Arno Steffens <star@gmx.li> wrote:
> > >
> > > Hello Zoran,
> > > thanks. As far as I understand is thumb2 another mode of coding, that might create more compact code.
> > > But I want to keep all compatible to my previous tool-chain and settings.
> > > The only file where I can found this "cortexa9t2hf" is
> > > ./meta/conf/machine/include/tune-cortexa9.inc
> > > but I can't see how I can control Yocto to generate "cortexa9hf-neon" as before.
> > > Or have I been wrong the time before?
> > >
> > > bitbake gives me in 2.5:
> > >
> > > TUNE_FEATURES        = "arm armv7a vfp thumb neon callconvention-hard cortexa9"
> > > TARGET_FPU           = "hard"
> > >
> > > and in 2.7:
> > > TUNE_FEATURES        = "arm vfp cortexa9 neon thumb callconvention-hard"
> > > TARGET_FPU           = "hard"
> > >
> > > so armv7a seem to be missing. In terms of thumb both is same. But is that the reason? Where to set it?
> > > Arno
> > >
> > > >
> > > > Hello Arno,
> > > >
> > > > Your question, per say, has little to do with YOCTO forum. But I'll
> > > > try (as my best) to answer your question.
> > > >
> > > > Cortexa9hf should be armv7 A9 Hard Floating (it contains HW FP unit).
> > > >
> > > > Cortexa9t2hf is by analogy armv7 A9 T2 Hard Floating. Now, the
> > > > question is what is T2? T2 is addition to the previous architecture
> > > > Cortexa9hf, and addition is Thumb-2 mode.
> > > >
> > > > Hope this helps,
> > > >
> > > > Zoran
> > > > _______
> > > >
> > > > On Mon, Jun 17, 2019 at 2:03 PM Arno Steffens <star@gmx.li> wrote:
> > > > >
> > > > > I switched from Yocto 2.5 to 2.7 and recognised a new architetcure name.
> > > > > Instead of cortexa9hf it is now build for cortexa9t2hf? Did I do something wrong or what exactly does this t2 mean?
> > > > > Target system is a Zynq7020 system.
> > > > > --
> > > > > _______________________________________________
> > > > > yocto mailing list
> > > > > yocto@yoctoproject.org
> > > > > https://lists.yoctoproject.org/listinfo/yocto
> > > >
> >
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-- 
Martin 'JaMa' Jansa     jabber: Martin.Jansa@gmail.com

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