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

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Date:   Wed, 01 Aug 2018 10:55:28 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     alankao@...estech.com
CC:     albert@...ive.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Andrew Waterman <andrew@...ive.com>,
        Darius Rad <darius@...espec.com>, greentime@...estech.com,
        zong@...estech.com, alankao@...estech.com
Subject:     Re: [PATCH v2] riscv: Add support to no-FPU systems

On Tue, 26 Jun 2018 21:22:26 PDT (-0700), alankao@...estech.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@...estech.com>
> Cc: Greentime Hu <greentime@...estech.com>
> Cc: Zong Li <zong@...estech.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.

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?