Re: [PATCH v2] riscv: Add support to no-FPU systems

[Palmer Dabbelt]

On Wed, 01 Aug 2018 11:23:43 PDT (-0700), Andrew Waterman wrote:
> On Wed, Aug 1, 2018 at 10:55 AM, Palmer Dabbelt <palmer@xxxxxxxxxx> wrote:
> > On Tue, 26 Jun 2018 21:22:26 PDT (-0700), alankao@xxxxxxxxxxxxx wrote:
> > > This patch adds an option, CONFIG_FPU, to enable/disable floating
> > > procedures.  Also, some style issues are fixed.
> > >
> > > Signed-off-by: Alan Kao <alankao@xxxxxxxxxxxxx>
> > > Cc: Greentime Hu <greentime@xxxxxxxxxxxxx>
> > > Cc: Zong Li <zong@xxxxxxxxxxxxx>
> > > ---
> > >  arch/riscv/Kconfig                 |  9 ++++
> > >  arch/riscv/Makefile                | 19 +++----
> > >  arch/riscv/include/asm/switch_to.h |  6 +++
> > >  arch/riscv/kernel/entry.S          |  3 +-
> > >  arch/riscv/kernel/process.c        |  7 ++-
> > >  arch/riscv/kernel/signal.c         | 82 +++++++++++++++++++++---------
> > >  6 files changed, 90 insertions(+), 36 deletions(-)
> > >
> > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > > index 6debcc4afc72..6069597ba73f 100644
> > > --- a/arch/riscv/Kconfig
> > > +++ b/arch/riscv/Kconfig
> > > @@ -232,6 +232,15 @@ config RISCV_BASE_PMU
> > >
> > >  endmenu
> > >
> > > +config FPU
> > > +       bool "FPU support"
> > > +       default y
> > > +       help
> > > +         Say N here if you want to disable all floating-point related
> > > procedure
> > > +         in the kernel.
> > > +
> > > +         If you don't know what to do here, say Y.
> > > +
> > >  endmenu
> >
> >
> > Sorry for letting this slide for a bit.  While I'm not opposed to a solution
> > that requires a FPU Kconfig option, it'd be a bit better if we could detect
> > this at boot time.  I think this should be possible because at one point
> > this actually worked and we could boot the same kernel on FPU and no-FPU
> > systems.
>
> I believe it would suffice to have start_thread set sstatus.FS to OFF
> for no-FPU systems (vs. INITIAL for systems with FPU).   The ISA
> string in the devicetree should indicate whether F/D extensions are
> present.
>
> That said, it makes sense to me to additionally provide the Kconfig
> option.  This would elide the sstatus.SD check for no-FPU systems,
> shaving a couple instructions off the context-switch path.  It would
> also enable mimicking the behavior of a no-FPU system even when the
> FPU is present.

That sounds like a good argument.  Do you mind submitting a two-part patch set, 
to:

* Allow FPU kernels to detect and run correctly on non-FPU systems.  You should 
 be able to detect these very early by writing to sstatus, or later by looking 
 at the device tree.
* Add a Kconfig option to disable the FPU entirely (which is pretty much this 
 patch).

Thanks!

> > If that's not possible then we'll have to take something like this.  There
> > were some comments on this v2 but I don't see a v3, did I miss one?