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

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Date:	Mon, 9 Feb 2015 10:03:38 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Thomas Niederprüm <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 Sat, Feb 07, 2015 at 05:12:11PM +0100, Thomas Niederprüm wrote:
> Am Sat, 7 Feb 2015 12:26:27 +0100
> schrieb Maxime Ripard <maxime.ripard@...e-electrons.com>:
> 
> > 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.
> 
> Good idea! I'll try to do it that way.
> 
> > 
> > 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.
> 
> True! I'll try to keep the existing behaviour when rewriting this to
> use the refresh rate.
> 
> > 
> > >  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.
> 
> Could you please elaborate why? I'm not seeing it...

On a general basis, because the structure is shared by all the
instances of the driver, so it's usually not such a good idea to mix
the static declaration of the structure and the dynamic one.

From a more fundamental point of view, because this parameter will
most likely be different from one instance to another?

Maxime

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

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