linux-kernel - Re: [PATCH] drivers/macintosh/smu.c: Mark expected switch fall-through

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Message-ID: <878ssfzjdk.fsf@concordia.ellerman.id.au>
Date:   Wed, 31 Jul 2019 00:28:55 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     PowerPC <linuxppc-dev@...ts.ozlabs.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Kees Cook <keescook@...omium.org>,
        Linux kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drivers/macintosh/smu.c: Mark expected switch fall-through

Stephen Rothwell <sfr@...b.auug.org.au> writes:
> Mark switch cases where we are expecting to fall through.
>
> This patch fixes the following warning (Building: powerpc):
>
> drivers/macintosh/smu.c: In function 'smu_queue_i2c':
> drivers/macintosh/smu.c:854:21: warning: this statement may fall through [-Wimplicit-fallthrough=]
>    cmd->info.devaddr &= 0xfe;
>    ~~~~~~~~~~~~~~~~~~^~~~~~~
> drivers/macintosh/smu.c:855:2: note: here
>   case SMU_I2C_TRANSFER_STDSUB:
>   ^~~~
>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Gustavo A. R. Silva <gustavo@...eddedor.com>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Stephen Rothwell <sfr@...b.auug.org.au>
> ---
>  drivers/macintosh/smu.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/macintosh/smu.c b/drivers/macintosh/smu.c
> index 276065c888bc..23f1f41c8602 100644
> --- a/drivers/macintosh/smu.c
> +++ b/drivers/macintosh/smu.c
> @@ -852,6 +852,7 @@ int smu_queue_i2c(struct smu_i2c_cmd *cmd)
>  		break;
>  	case SMU_I2C_TRANSFER_COMBINED:
>  		cmd->info.devaddr &= 0xfe;
> +		/* fall through */
>  	case SMU_I2C_TRANSFER_STDSUB:
>  		if (cmd->info.sublen > 3)
>  			return -EINVAL;

Why do we think it's an expected fall through? I can't really convince
myself from the surrounding code that it's definitely intentional.

cheers