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

>
> 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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