Messages in this thread | | | Date | Fri, 03 Aug 2018 21:03:00 -0700 (PDT) | Subject | Re: [PATCH v2] riscv: Add support to no-FPU systems | From | Palmer Dabbelt <> |
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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@sifive.com> wrote: >> On Tue, 26 Jun 2018 21:22:26 PDT (-0700), alankao@andestech.com 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@andestech.com> >>> Cc: Greentime Hu <greentime@andestech.com> >>> Cc: Zong Li <zong@andestech.com> >>> --- >>> 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?
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