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 17:45:12 +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 Thu, Sep 20, 2018 at 08:03:06PM +0300, Yasha Cherikovsky wrote:
> MIPSR6 doesn't support unaligned access instructions (lwl, lwr, swl, swr).
> The MIPS tree has some special cases to avoid these instructions,
> and currently the code is testing for CONFIG_CPU_MIPSR6.
> 
> Declare a new Kconfig variable: CONFIG_CPU_HAS_NO_UNALIGNED_LOAD_STORE,
> and make CONFIG_CPU_MIPSR6 select it.
> And switch all the special cases to test for the new variable.
> 
> Also, the new variable selects CONFIG_GENERIC_CSUM, to use
> generic C checksum code (to avoid special assembly code that uses
> the unsupported instructions).

Thanks for your patch :)

I think it would be cleaner to invert this logic & instead have the
Kconfig entry indicate when kernel's build target *does* support the
[ls]w[lr] instructions.

It would be good for the name to be clear that these instructions are
what it's about too - "unaligned load store" is a little too vague for
my liking. For example one could easily misconstrue it to mean something
akin to the inverse of CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, whereas
in the MIPSr6 case many CPUs actually handle unaligned accesses in
hardware when using the regular load/store instructions. They don't have
the [ls]w[lr] instructions, but they don't need them because they handle
unaligned accesses more naturally without needing us to be explicit
about them.

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,
    Paul