linux-kernel - Re: [PATCH 6/6] PM/FB/gxfb: add config option that allows disabling of VT switch during suspend

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Message-Id: <20080310152828.8f102f52.akpm@linux-foundation.org>
Date:	Mon, 10 Mar 2008 15:28:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andres Salomon <dilinger@...ued.net>
Cc:	adaplas@...il.com, linux-kernel@...r.kernel.org,
	linux-fbdev-devel@...ts.sourceforge.net, info-linux@...de.amd.com,
	jordan.crouse@....com, rjw@...k.pl, samuel.thibault@...-lyon.org
Subject: Re: [PATCH 6/6] PM/FB/gxfb: add config option that allows disabling
 of VT switch during suspend

On Mon, 10 Mar 2008 18:14:46 -0400 Andres Salomon <dilinger@...ued.net> wrote:

> On Mon, 10 Mar 2008 14:51:18 -0700
> Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Mon, 10 Mar 2008 17:44:24 -0400
> > Andres Salomon <dilinger@...ued.net> wrote:
> > 
> > > On Mon, 10 Mar 2008 14:19:39 -0700
> > > Andrew Morton <akpm@...ux-foundation.org> wrote:
> > > 
> > > > 
> > > > It would be far preferable to do this via a kernel boot parameter rather
> > > > than via a kernel rebuild.
> > > > 
> > > 
> > > Well, it shouldn't be supported at _all_ unless the framebuffer driver
> > > supports it; you're asking for it to be converted to something like the
> > > following?
> > > 
> > > 
> > > 
> > > #ifdef CONFIG_FBDEV_SUPPORTS_NOSWITCH
> > > static int no_vt_switch;
> > > #endif
> > > 
> > > int pm_prepare_console(void)
> > > {
> > > #ifdef CONFIG_FBDEV_SUPPORTS_NOSWITCH
> > > 	if (no_vt_switch)
> > > 		return 0;
> > > #endif
> > > 	...
> > > }
> > > 
> > > #ifdef CONFIG_FBDEV_SUPPORTS_NOSWITCH
> > > module_param(no_vt_switch, int, 0);
> > > MODULE_PARM_DESC(no_vt_switch, "...");
> > > #endif
> > > 
> > > 
> > > If so, that means extra #ifdef's sprinkled throughout the code.  Note
> > > that most framebuffer drivers won't be supporting this, and those that
> > > do will almost certainly always want vt switching turned off..
> > > 
> > 
> > I don't think you need any ifdefs or config options at all, do you?  Just
> > add a new module parameter to this driver and some little interface into
> > the PM core which allows the driver to set no_vt_switch?
> > 
> 
> That's a possibility, but doesn't adding hooks into PM core for two
> drivers (lxfb and gxfb) seems like overkill?

It's just a few bytes of code?

>  I also don't really see
> anyone needing to switch back and forth between VT_SWITCH and
> NO_VT_SWITCH other than to work around possible bugs in the register
> save/restore code..

Making driver behaviour depend upon compile-time settings is really to be
avoided.  A distributor has to choose one setting or the other, and a) that
means that some people will get the wrong setting and b) there's a decent
chance that the distributor will get it wrong.


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