linux-kernel - Re: [PATCH] iommu: arm-smmu-v3: Mark expected switch fall-through

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Date:   Fri, 26 Jul 2019 13:01:02 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Anders Roxell <anders.roxell@...aro.org>, will@...nel.org,
        joro@...tes.org
Cc:     stable@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] iommu: arm-smmu-v3: Mark expected switch fall-through

On 26/07/2019 12:28, Anders Roxell wrote:
> When fall-through warnings was enabled by default, commit d93512ef0f0e

That commit ID only exists in a handful of old linux-next tags.

> ("Makefile: Globally enable fall-through warning"), the following
> warning was starting to show up:
> 
> ../drivers/iommu/arm-smmu-v3.c: In function ‘arm_smmu_write_strtab_ent’:
> ../drivers/iommu/arm-smmu-v3.c:1189:7: warning: this statement may fall
>   through [-Wimplicit-fallthrough=]
>      if (disable_bypass)
>         ^
> ../drivers/iommu/arm-smmu-v3.c:1191:3: note: here
>     default:
>     ^~~~~~~
> 
> Rework so that the compiler doesn't warn about fall-through. Make it
> clearer by calling 'BUG()' when disable_bypass is set, and always
> 'break;'
> 
> Cc: stable@...r.kernel.org # v4.2+
> Fixes: 5bc0a11664e1 ("iommu/arm-smmu: Don't BUG() if we find aborting STEs with disable_bypass")

Why? There's no actual bug, and not even current kernels have that 
warning enabled.

> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>
> ---
>   drivers/iommu/arm-smmu-v3.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index a9a9fabd3968..8e5f0565996d 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -1186,8 +1186,9 @@ static void arm_smmu_write_strtab_ent(struct arm_smmu_master *master, u32 sid,
>   			ste_live = true;
>   			break;
>   		case STRTAB_STE_0_CFG_ABORT:
> -			if (disable_bypass)
> -				break;
> +			if (!disable_bypass)
> +				BUG();

You may as well just use BUG_ON().

Robin.

> +			break;
>   		default:
>   			BUG(); /* STE corruption */
>   		}
>