Re: [PATCH] drm/i915/backlight: Remove a useless kstrdup_const()

[Jani Nikula <jani.nikula@linux.intel.com>]

On Sat, 28 Sep 2024, Christophe JAILLET <christophe.jaillet@wanadoo.fr> wrote:
> "name" is allocated and freed in intel_backlight_device_register().
> The initial allocation just duplicates "intel_backlight".
>
> Later, if a device with this name has already been registered, another
> dynamically generated one is allocated using kasprintf().
>
> So at the end of the function, when "name" is freed, it can point either to
> the initial static literal "intel_backlight" or to the kasprintf()'ed one.
>
> So kfree_const() is used.
>
> However, when built as a module, kstrdup_const() and kfree_const() don't
> work as one would expect and are just plain kstrdup() and kfree().
>
>
> Slightly change the logic and introduce a new variable to hold the
> address returned by kasprintf() should it be used.
>
> This saves a memory allocation/free and avoids these _const functions,
> which names can be confusing when used with code built as module.

Okay, I'd rather revert your earlier commit 379b63e7e682
("drm/i915/display: Save a few bytes of memory in
intel_backlight_device_register()") than add this.

The code simplicity is much more important than saving a few bytes.

BR,
Jani.



>
> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
> Compile tested only.
>
> For the records, this patch is a clean-up effort related to discussions at:
>   - https://lore.kernel.org/all/ZvHurCYlCoi1ZTCX@skv.local/
>   - https://lore.kernel.org/all/20240924050937.697118-1-senozhatsky@chromium.org/
> ---
>  drivers/gpu/drm/i915/display/intel_backlight.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/drm/i915/display/intel_backlight.c
> index 9e05745d797d..bf7686aa044f 100644
> --- a/drivers/gpu/drm/i915/display/intel_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_backlight.c
> @@ -914,9 +914,9 @@ int intel_backlight_device_register(struct intel_connector *connector)
>  {
>  	struct drm_i915_private *i915 = to_i915(connector->base.dev);
>  	struct intel_panel *panel = &connector->panel;
> +	const char *name, *new_name = NULL;
>  	struct backlight_properties props;
>  	struct backlight_device *bd;
> -	const char *name;
>  	int ret = 0;
>  
>  	if (WARN_ON(panel->backlight.device))
> @@ -949,10 +949,7 @@ int intel_backlight_device_register(struct intel_connector *connector)
>  	else
>  		props.power = BACKLIGHT_POWER_OFF;
>  
> -	name = kstrdup_const("intel_backlight", GFP_KERNEL);
> -	if (!name)
> -		return -ENOMEM;
> -
> +	name = "intel_backlight";
>  	bd = backlight_device_get_by_name(name);
>  	if (bd) {
>  		put_device(&bd->dev);
> @@ -963,11 +960,11 @@ int intel_backlight_device_register(struct intel_connector *connector)
>  		 * compatibility. Use unique names for subsequent backlight devices as a
>  		 * fallback when the default name already exists.
>  		 */
> -		kfree_const(name);
> -		name = kasprintf(GFP_KERNEL, "card%d-%s-backlight",
> -				 i915->drm.primary->index, connector->base.name);
> -		if (!name)
> +		new_name = kasprintf(GFP_KERNEL, "card%d-%s-backlight",
> +				     i915->drm.primary->index, connector->base.name);
> +		if (!new_name)
>  			return -ENOMEM;
> +		name = new_name;
>  	}
>  	bd = backlight_device_register(name, connector->base.kdev, connector,
>  				       &intel_backlight_device_ops, &props);
> @@ -987,7 +984,7 @@ int intel_backlight_device_register(struct intel_connector *connector)
>  		    connector->base.base.id, connector->base.name, name);
>  
>  out:
> -	kfree_const(name);
> +	kfree(new_name);
>  
>  	return ret;
>  }

-- 
Jani Nikula, Intel