linux-kernel - Re: [PATCH] pwm: cros_ec: add _

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Message-ID: <20160726201506.GB11345@roeck-us.net>
Date:	Tue, 26 Jul 2016 13:15:06 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Brian Norris <briannorris@...omium.org>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	linux-kernel@...r.kernel.org,
	Brian Norris <computersforpeace@...il.com>,
	linux-pwm@...r.kernel.org,
	Javier Martinez Canillas <javier@....samsung.com>,
	Enric Balletbo <enric.balletbo@...labora.co.uk>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Gwendal Grignou <gwendal@...omium.org>
Subject: Re: [PATCH] pwm: cros_ec: add __packed to prevent padding

On Tue, Jul 26, 2016 at 11:22:13AM -0700, Brian Norris wrote:
> While the particular usage in question is likely safe (struct
> cros_ec_command is 32-bit aligned, followed by <=32-bit fields), it's
> been suggested this is not a great pattern to follow for the general
> case -- for example, if we follow a 'struct cros_ec_command' (which is
> 32-bit- but not 64-bit-aligned) with a struct that starts with a 64-bit
> type (e.g., u64), the compiler may add padding.
> 
> Let's add __packed, to inform the compiler of our true intention -- to
> have no padding between these struct elements -- and to future proof for
> any refactorings that might occur.
> 
> Signed-off-by: Brian Norris <briannorris@...omium.org>

Reviewed-by: Guenter Roeck <linux@...ck-us.net>

> ---
> Hi Thierry,
> 
> I don't know of any bugs directly resolved by this patch, so take it for
> v4.8/v4.9 at your discretion.
> 
>  drivers/pwm/pwm-cros-ec.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-cros-ec.c b/drivers/pwm/pwm-cros-ec.c
> index 99b9acc1a420..f6ca4e8c6253 100644
> --- a/drivers/pwm/pwm-cros-ec.c
> +++ b/drivers/pwm/pwm-cros-ec.c
> @@ -38,7 +38,7 @@ static int cros_ec_pwm_set_duty(struct cros_ec_device *ec, u8 index, u16 duty)
>  	struct {
>  		struct cros_ec_command msg;
>  		struct ec_params_pwm_set_duty params;
> -	} buf;
> +	} __packed buf;
>  	struct ec_params_pwm_set_duty *params = &buf.params;
>  	struct cros_ec_command *msg = &buf.msg;
>  
> @@ -65,7 +65,7 @@ static int __cros_ec_pwm_get_duty(struct cros_ec_device *ec, u8 index,
>  			struct ec_params_pwm_get_duty params;
>  			struct ec_response_pwm_get_duty resp;
>  		};
> -	} buf;
> +	} __packed buf;
>  	struct ec_params_pwm_get_duty *params = &buf.params;
>  	struct ec_response_pwm_get_duty *resp = &buf.resp;
>  	struct cros_ec_command *msg = &buf.msg;
> -- 
> 2.8.0.rc3.226.g39d4020
>