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

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Date:   Fri, 03 Aug 2018 21:03:00 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     Andrew Waterman <andrew@...ive.com>
CC:     alankao@...estech.com, albert@...ive.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Darius Rad <darius@...espec.com>, greentime@...estech.com,
        zong@...estech.com
Subject:     Re: [PATCH v2] riscv: Add support to no-FPU systems

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@...ive.com> wrote:
>> 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.
>
> 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?