linux-kernel - Re: [PATCH 1/1] MIPS: Add new Kconfig variable to avoid unaligned access instructions

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Date:   Tue, 25 Sep 2018 19:57:59 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Yasha Cherikovsky <yasha.che3@...il.com>
CC:     Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] MIPS: Add new Kconfig variable to avoid unaligned
 access instructions

Hi Yasha,

On Tue, Sep 25, 2018 at 10:30:52PM +0300, Yasha Cherikovsky wrote:
> On Tue, 2018-09-25 at 17:45 +0000, Paul Burton wrote:
> > How about we:
> > 
> >   - Add a Kconfig option CONFIG_CPU_SUPPORTS_LOAD_STORE_LR, and
> > select
> >     it for all existing pre-r6 targets (probably from CONFIG_CPU_*).
> > 
> >   - Change CONFIG_GENERIC_CSUM to default y if
> >     !CONFIG_CPU_SUPPORTS_LOAD_STORE_LR, and drop the selects of it.
> > 
> > That would avoid the double-negative ("if we don't not support this")
> > that the #ifndef's currently represent. It would also mean any future
> > architecture/ISA targets beyond MIPSr6 automatically avoid the
> > instructions.
> 
> Thanks for your feedback, I'll start preparing v2.
> 
> Looking in arch/mips/Kconfig, some CPU options start
> with CPU_SUPPORTS_ and some with CPU_HAS_.
> Which perfix should we use here?

That's a good question :)

To be honest I don't think either of them is perfect, since what we're
really describing is what's supported by the ISA that the kernel build
is targeting - and in theory the CPU we actually run on could support
extra things.

But considering it I think CPU_HAS_ is probably the best choice for
this, since it's already used similarly to indicate support for pref &
sync instructions.

Thanks,
    Paul