linux-kernel - Re: [PATCH 6/8] fbdev: ssd1307fb: Add module parameter to set update delay of the deffered io.

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Date:	Sat, 7 Feb 2015 12:26:27 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	niederp@...sik.uni-kl.de
Cc:	linux-fbdev@...r.kernel.org, plagnioj@...osoft.com,
	tomi.valkeinen@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/8]  fbdev: ssd1307fb: Add module parameter to set
 update delay of the  deffered io.

On Fri, Feb 06, 2015 at 11:28:12PM +0100, niederp@...sik.uni-kl.de wrote:
> From: Thomas Niederprüm <niederp@...sik.uni-kl.de>
> 
> This patch adds the module parameter "delaydivider" to set delay for the
> deferred io. Effectively this is setting the refresh rate for mmap access
> to the framebuffer. The delay for the deferred io is HZ/delaydivider.

So this is actually a refresh rate?

Maybe you could expose it as such, and pass a frequency in Hz as an
argument.

Exposing the divider directly has some issues, since the bootloader
that set the parameter won't know the HZ value, you'll end up with
different rates for different configurations, without any way to do
something about it.

> 
> Signed-off-by: Thomas Niederprüm <niederp@...sik.uni-kl.de>
> ---
>  drivers/video/fbdev/ssd1307fb.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/ssd1307fb.c b/drivers/video/fbdev/ssd1307fb.c
> index 1d81877..b38315d 100644
> --- a/drivers/video/fbdev/ssd1307fb.c
> +++ b/drivers/video/fbdev/ssd1307fb.c
> @@ -44,10 +44,14 @@
>  #define	SSD1307FB_SET_VCOMH		0xdb
>  
>  #define BITSPERPIXEL 1
> +#define DELAYDIVIDER 20
>  
>  static u_int bitsperpixel = BITSPERPIXEL;
>  module_param(bitsperpixel, uint, 0);
>  
> +static u_int delaydivider = DELAYDIVIDER;
> +module_param(delaydivider, uint, 0);
> +

You're breaking the existing behaviour.

>  struct ssd1307fb_par;
>  
>  struct ssd1307fb_deviceinfo {
> @@ -312,7 +316,7 @@ static void ssd1307fb_deferred_io(struct fb_info *info,
>  }
>  
>  static struct fb_deferred_io ssd1307fb_defio = {
> -	.delay		= HZ,
> +	.delay		= HZ/DELAYDIVIDER,
>  	.deferred_io	= ssd1307fb_deferred_io,
>  };
>  
> @@ -601,6 +605,7 @@ static int ssd1307fb_probe(struct i2c_client *client,
>  	info->fix = ssd1307fb_fix;
>  	info->fix.line_length = par->width * bitsperpixel / 8;
>  	info->fbdefio = &ssd1307fb_defio;
> +	info->fbdefio->delay = HZ/delaydivider;

That won't work with multiple instances of the same driver
unfortunately.

>  
>  	info->var = ssd1307fb_var;
>  	info->var.bits_per_pixel = bitsperpixel;
> -- 
> 2.1.1
> 

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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