linux-kernel - Re: [PATCH 1/5] fbdev/efifb: Use stack memory for screeninfo structs

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Message-ID: <fef5bc70-3921-4562-b9d4-beccc76440a6@gmx.de>
Date: Wed, 28 Aug 2024 19:42:51 +0200
From: Helge Deller <deller@....de>
To: Thomas Weißschuh <linux@...ssschuh.net>,
 Peter Jones <pjones@...hat.com>, Daniel Vetter <daniel@...ll.ch>
Cc: linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] fbdev/efifb: Use stack memory for screeninfo structs

On 8/27/24 17:25, Thomas Weißschuh wrote:
> These variables are only used inside efifb_probe().
> Afterwards they are using memory unnecessarily.

Did you check if this change really saves some memory?
With your change, the compiler will either create a hidden
structure which it uses then, or it generates assembly
instructions to fill the struct at runtime.
Both options may not actually reduce the memory footprint...

Another option might be to mark the static struct __initdata
if you expect another card to take over before the memory is
freed at runtime. But I'm not sure if it's worth possible
implications.

Helge

> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
>   drivers/video/fbdev/efifb.c | 36 ++++++++++++++++++------------------
>   1 file changed, 18 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/video/fbdev/efifb.c b/drivers/video/fbdev/efifb.c
> index 8dd82afb3452..8bfe0ccbc67a 100644
> --- a/drivers/video/fbdev/efifb.c
> +++ b/drivers/video/fbdev/efifb.c
> @@ -52,24 +52,6 @@ struct efifb_par {
>   	resource_size_t size;
>   };
>
> -static struct fb_var_screeninfo efifb_defined = {
> -	.activate		= FB_ACTIVATE_NOW,
> -	.height			= -1,
> -	.width			= -1,
> -	.right_margin		= 32,
> -	.upper_margin		= 16,
> -	.lower_margin		= 4,
> -	.vsync_len		= 4,
> -	.vmode			= FB_VMODE_NONINTERLACED,
> -};
> -
> -static struct fb_fix_screeninfo efifb_fix = {
> -	.id			= "EFI VGA",
> -	.type			= FB_TYPE_PACKED_PIXELS,
> -	.accel			= FB_ACCEL_NONE,
> -	.visual			= FB_VISUAL_TRUECOLOR,
> -};
> -
>   static int efifb_setcolreg(unsigned regno, unsigned red, unsigned green,
>   			   unsigned blue, unsigned transp,
>   			   struct fb_info *info)
> @@ -357,6 +339,24 @@ static int efifb_probe(struct platform_device *dev)
>   	char *option = NULL;
>   	efi_memory_desc_t md;
>
> +	struct fb_var_screeninfo efifb_defined = {
> +		.activate		= FB_ACTIVATE_NOW,
> +		.height			= -1,
> +		.width			= -1,
> +		.right_margin		= 32,
> +		.upper_margin		= 16,
> +		.lower_margin		= 4,
> +		.vsync_len		= 4,
> +		.vmode			= FB_VMODE_NONINTERLACED,
> +	};
> +
> +	struct fb_fix_screeninfo efifb_fix = {
> +		.id			= "EFI VGA",
> +		.type			= FB_TYPE_PACKED_PIXELS,
> +		.accel			= FB_ACCEL_NONE,
> +		.visual			= FB_VISUAL_TRUECOLOR,
> +	};
> +
>   	/*
>   	 * If we fail probing the device, the kernel might try a different
>   	 * driver. We get a copy of the attached screen_info, so that we can
>