[Buildroot] [PATCH] arch/arc: add support of ARC HS38 core

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Dear Alexey Brodkin,

On Thu, 30 Oct 2014 11:05:24 +0000, Alexey Brodkin wrote:

> Ok, here's an explanation.
> ARC has currently 4 families of CPU cores.
> 2 of those families may have MMU, they are:
>  1) ARC700 series (ARC750D and ARC770D have MMU while others like
> ARC705, ARC725 have no MMU)
>  2) ARC HS series (HS38 has MMU while HS34 and HS36 are MMU-less models)
> 
> But since Buildroot is used for building tools and packages for
> Linux-driven systems I don't even mention other HS family members.
> 
> Also what's important that ARC700 and ARC HS families implement
> different ISAs and that's why I had to add another type of CPU -
> different settings of gcc and uclibc are required for ARC700 and HS.
> 
> Probably what I need to do is to list explicitly all relevant CPU
> modifications that could be used for Linux. then we'll have ARC750D,
> ARC770D and ARC HS38. In this case there will be no confusions.

Thanks for the explanation!

> > >  # Choise of atomic instructions presence
> > >  config BR2_ARC_ATOMIC_EXT
> > > +	default y if BR2_archs
> > 
> > Why? Are atomic instructions *always* available on HS38, or are they
> > also optional like on ARC700 ?
> 
> Well with ARC things a bit more complex compared to other architectures.
> Because we provide a LEGO-like IP - user may select each and every tiny
> detail he wants or doesn't want.
> 
> And all those "names" mentioned above like ARC770D, ARC HS are only
> names of "templates" - sets of components and features that are most
> likely will be used. But nobody can stop user to down-configure
> anything.
> 
> So in case of HS38 by default atomic options are enabled - that's why I
> enabled them in Buildroot for ARC HS by default. But since there's a
> probability one customer decides to down-configure atomic instructions
> (even though we don't recommend to do it) we need to have an ability to
> build software without atomic ops.

Ok, that explains why you decided to make it 'default y' for some CPU,
but still keep it an option.


> > So from gcc's point of view, the processor is called archs, but from a
> > marketing point of view it's HS38. What happens if tomorrow Synopsys
> > creates a different CPU core called HS100 ?
> 
> See above. We need to have arc700 and archs to distinguish 2 different
> ISAs and ABIs. Still as I commented above I'll add selection of a
> particular CPU so for example we may pass fine-tuning options to gcc
> like "-mtune=ARCxxx".

Hum: beware, we are removing the support for -mtune in Buildroot. Since
Buildroot targets only one system, -mtune is normally not useful and
-mcpu should be used instead. On some architecture, -march indicates
the ISA, while -mcpu indicates the specific CPU. Maybe ARC should do
the same?

> > Is this processor already supported by the current binutils/gcc/uClibc
> > versions used for the ARC architecture in Buildroot?
> 
> Right, arc-2014.08 tools (gcc, binutils, uClibc) already support ARC HS.
> Moreover uClibc has ARC HS support even in upstream master branch.

Great!

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com