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[PATCH] pwm: cros_ec: add __packed to prevent padding

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  [PATCH] pwm: cros_ec: add __packed to prevent padding Brian Norris <briannorris@chromium.org> - 2016-07-26 20:30 +0200
    Re: [PATCH] pwm: cros_ec: add __packed to prevent padding Dmitry Torokhov <dmitry.torokhov@gmail.com> - 2016-07-26 21:10 +0200
    Re: [PATCH] pwm: cros_ec: add __packed to prevent padding Guenter Roeck <linux@roeck-us.net> - 2016-07-26 22:20 +0200

#1450773 — [PATCH] pwm: cros_ec: add __packed to prevent padding

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@chromium.org>
---
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

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#1450794

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@chromium.org>

Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

> ---
> 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
> 

-- 
Dmitry

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#1450848

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@chromium.org>

Reviewed-by: Guenter Roeck <linux@roeck-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
> 

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