linux-kernel - Re: [PATCH] drm/i915/backlight: Remove a useless kstrdup

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Message-ID: <3c793f42-6cd1-40e7-a3f2-556b6e5b4094@wanadoo.fr>
Date: Tue, 1 Oct 2024 20:34:22 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Jani Nikula <jani.nikula@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>,
 Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
 Tvrtko Ursulin <tursulin@...ulin.net>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>
Cc: linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
 intel-gfx@...ts.freedesktop.org, intel-xe@...ts.freedesktop.org,
 dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/i915/backlight: Remove a useless kstrdup_const()

Le 30/09/2024 à 09:48, Jani Nikula a écrit :
> On Sat, 28 Sep 2024, Christophe JAILLET <christophe.jaillet@...adoo.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.

Hi,

that works for me. Thanks and sorry for the noise.

CJ

> 
> The code simplicity is much more important than saving a few bytes.
> 
> BR,
> Jani.
> 
> 
> 
>>
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.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;
>>   }
>