linux-kernel - Re: [PATCH v2] staging: greybus: use inline function for macros

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Message-ID: <ZBrGwZK5YA+hMVM4@kroah.com>
Date:   Wed, 22 Mar 2023 10:13:37 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Menna Mahmoud <eng.mennamahmoud.mm@...il.com>
Cc:     outreachy@...ts.linux.dev, johan@...nel.org, elder@...nel.org,
        linux-kernel@...r.kernel.org, linux-staging@...ts.linux.dev,
        Julia Lawall <julia.lawall@...ia.fr>
Subject: Re: [PATCH v2] staging: greybus: use inline function for macros

On Tue, Mar 21, 2023 at 08:34:56PM +0200, Menna Mahmoud wrote:
> Convert `to_gbphy_dev` and `to_gbphy_driver` macros into a
> static inline function.
> 
> It is not great to have macros that use the `container_of` macro,
> because from looking at the definition one cannot tell what type
> it applies to.

Note, the compiler will tell you if you get this wrong, so this really
is not an issue.

The container_of() macro is "special" in that it is very type safe and
is very commonly used just as a #define to make it simpler and the
compiler can just do the pointer math automatically without ever needing
a function call to be involved.

> One can get the same benefit from an efficiency point of view
> by making an inline function.

It's actually more efficient to be a macro, so this isn't correct.

But all of this is really moot, and I would normally just take this
patch, but it's not correct:

> 
> Suggested-by: Julia Lawall <julia.lawall@...ia.fr>
> Signed-off-by: Menna Mahmoud <eng.mennamahmoud.mm@...il.com>
> ---
> changes in v2:
> 	-send patch as a single patch.
> 	-edit the name of struct object.
> 	-edit commit message.
> ---
>  drivers/staging/greybus/gbphy.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/staging/greybus/gbphy.h b/drivers/staging/greybus/gbphy.h
> index d4a225b76338..e7ba232bada1 100644
> --- a/drivers/staging/greybus/gbphy.h
> +++ b/drivers/staging/greybus/gbphy.h
> @@ -15,7 +15,10 @@ struct gbphy_device {
>  	struct list_head list;
>  	struct device dev;
>  };
> -#define to_gbphy_dev(d) container_of(d, struct gbphy_device, dev)
> +static inline struct gbphy_device *to_gbphy_dev(const struct device *_dev)
> +{
> +	return container_of(_dev, struct gbphy_device, dev);
> +}

You need a newline right before your new function to properly set it
off.


>  
>  static inline void *gb_gbphy_get_data(struct gbphy_device *gdev)
>  {
> @@ -43,7 +46,10 @@ struct gbphy_driver {
>  
>  	struct device_driver driver;
>  };
> -#define to_gbphy_driver(d) container_of(d, struct gbphy_driver, driver)
> +static inline struct gbphy_driver *to_gbphy_driver(struct device_driver *drv)
> +{
> +	return container_of(drv, struct gbphy_driver, driver);
> +}

I'm going to be a stickler here, and say this really should be using the
new container_of_const() macro instead, and with that, you can NOT turn
it into an inline function at all due to the fun use of Generic in that
macro.

So I wouldn't recommend changing this macro at this time at all, sorry.

thanks,

greg k-h