[U-Boot] [PATCH 06/30] riscv: add Kconfig entries for the C and A ISA extensions

[Auer, Lukas]

Hi Bin,

On Wed, 2018-10-24 at 23:32 +0800, Bin Meng wrote:
> Hi Lukas,
> 
> On Wed, Oct 24, 2018 at 11:21 PM Auer, Lukas
> <lukas.auer at aisec.fraunhofer.de> wrote:
> > 
> > Hi Bin,
> > 
> > On Mon, 2018-10-22 at 15:21 +0800, Bin Meng wrote:
> > > Hi Lukas,
> > > 
> > > On Sat, Oct 20, 2018 at 6:09 AM Lukas Auer
> > > <lukas.auer at aisec.fraunhofer.de> wrote:
> > > > 
> > > > Add Kconfig entries for the C (compressed instructions) and A
> > > > (atomic
> > > > instructions) ISA extensions. Only the C ISA extension is
> > > > selectable.
> > > > This matches the configuration in Linux.
> > > > 
> > > > The Kconfig entries are not used yet. A follow-up patch will
> > > > select
> > > > the
> > > > appropriate compiler flags based on the Kconfig configuration.
> > > > 
> > > > Signed-off-by: Lukas Auer <lukas.auer at aisec.fraunhofer.de>
> > > > ---
> > > > 
> > > >  arch/riscv/Kconfig | 9 +++++++++
> > > >  1 file changed, 9 insertions(+)
> > > > 
> > > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > > > index b81e0d990a..e15329c35e 100644
> > > > --- a/arch/riscv/Kconfig
> > > > +++ b/arch/riscv/Kconfig
> > > > @@ -38,6 +38,15 @@ config ARCH_RV64I
> > > > 
> > > >  endchoice
> > > > 
> > > > +config RISCV_ISA_C
> > > > +       bool "Emit compressed instructions"
> > > > +       default y
> > > > +       help
> > > > +         This enables the compressed instructions ("C") ISA
> > > > extension.
> > > 
> > > This is a little bit confusing. Can we use Linux kernel's
> > > description
> > > instead like below?
> > > 
> > >           Adds "C" to the ISA subsets that the toolchain is
> > > allowed
> > > to emit
> > >           when building U-Boot, which results in compressed
> > > instructions in the
> > >           U-Boot binary.
> > > 
> > 
> > Sure, I will change it in v2.
> > 
> > > > +
> > > > +config RISCV_ISA_A
> > > > +       def_bool y
> > > 
> > > I do not believe U-Boot need to care about 'A' extension. So this
> > > can
> > > be dropped?
> > > 
> > 
> > That's right. The only reason it might be used in U-Boot is on
> > multi-
> > core systems. Linux chooses the hart to boot on with the hart
> > lottery,
> > which requires atomics. The first core to increment a flag wins and
> > starts the boot process. We could do something similar in U-Boot,
> > but I
> > don't think it's necessary. In my patches (for the next patch
> > series) I
> > have added CONFIG_MAIN_HART to select the hart U-Boot should run
> > on.
> > 
> 
> Thanks for the additional details. I think we will need
> CONFIG_MAIN_HART to select the hart U-Boot should run on instead of
> Linux's lottery game, since we need handle situation like SiFive
> Freedom U540 where hart0 is something that does not run Linux. But
> even with CONFIG_MAIN_HART, U-Boot still doesn't need support 'A',
> right?
> 

Yes that's right, at least I can't think of a reason for why we might
need it. Should I drop it then?

Thanks,
Lukas

> > I have included it here so that I can use it in the next patch to
> > build
> > the ISA string. I can remove it, but that would change the current
> > behavior, which has atomics enabled.
> > 
> 
> Regards,
> Bin