linux-kernel - Re: [PATCH] MIPS: cavium_octeon: Fix syncw generation.

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Date:   Mon, 17 Feb 2020 01:22:50 +0100
From:   Philippe Mathieu-Daudé <f4bug@...at.org>
To:     Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
Cc:     Paul Burton <paulburton@...nel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        chris.packham@...iedtelesis.co.nz,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: cavium_octeon: Fix syncw generation.

Hi Mark,

On Tue, Feb 11, 2020 at 10:42 PM Mark Tomlinson
<mark.tomlinson@...iedtelesis.co.nz> wrote:
>
> The Cavium Octeon CPU uses a special sync instruction for implementing
> wmb, and due to a CPU bug, the instruction must appear twice. A macro
> had been defined to hide this:
>
>  #define __SYNC_rpt(type)     (1 + (type == __SYNC_wmb))
>
> which was intended to evaluate to 2 for __SYNC_wmb, and 1 for any other
> type of sync. However, this expression is evaluated by the assembler,
> and not the compiler, and the result of '==' in the assembler is 0 or
> -1, not 0 or 1 as it is in C. The net result was wmb() producing no code
> at all. The simple fix in this patch is to change the '+' to '-'.

Isn't this particular to the assembler implementation?
Can you explicit the assembler you are using in the commit description?
Assuming we have to look at your commit in 3 years from now, we'll
wonder what assembler you were using.

Thanks,

Phil.

> Fixes: bf92927251b3 ("MIPS: barrier: Add __SYNC() infrastructure")
> Signed-off-by: Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
> ---
>  arch/mips/include/asm/sync.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/mips/include/asm/sync.h b/arch/mips/include/asm/sync.h
> index 7c6a1095f5..aabd097933 100644
> --- a/arch/mips/include/asm/sync.h
> +++ b/arch/mips/include/asm/sync.h
> @@ -155,9 +155,11 @@
>   * effective barrier as noted by commit 6b07d38aaa52 ("MIPS: Octeon: Use
>   * optimized memory barrier primitives."). Here we specify that the affected
>   * sync instructions should be emitted twice.
> + * Note that this expression is evaluated by the assembler (not the compiler),
> + * and that the assembler evaluates '==' as 0 or -1, not 0 or 1.
>   */
>  #ifdef CONFIG_CPU_CAVIUM_OCTEON
> -# define __SYNC_rpt(type)      (1 + (type == __SYNC_wmb))
> +# define __SYNC_rpt(type)      (1 - (type == __SYNC_wmb))
>  #else
>  # define __SYNC_rpt(type)      1
>  #endif
> --
> 2.25.0
>