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SubjectRe: [PATCH 6/6] PM/FB/gxfb: add config option that allows disabling of VT switch during suspend
On Mon, 10 Mar 2008 14:19:39 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Sat, 8 Mar 2008 20:49:27 -0500
> Andres Salomon <dilinger@queued.net> wrote:
>
> > This is purely for suspend/resume time optimization.
> >
> >
> > >From 874aa20022a2501a30704163ed31a7baba7de9e5 Mon Sep 17 00:00:00 2001
> > From: Andres Salomon <dilinger@debian.org>
> > Date: Wed, 5 Mar 2008 16:40:25 -0500
> > Subject: [PATCH] PM: add config option that allows disabling of VT switch during suspend
> >
> > Prior to suspend, we allocate and switch to a new VT; after suspend, we
> > switch back to the original VT. This can be slow, and is completely
> > unnecessary if the framebuffer we're using can restore video properly.
> >
> > This adds CONFIG_DISABLE_SUSPEND_VT_SWITCH which allows the user to
> > select whether or not to do that switch. This has been tested with the
> > gxfb driver (on a Geode GX dev board and OLPC prototype machines). In
> > order for the user to even see this option, a framebuffer has to declare
> > support for it (via CONFIG_FBDEV_SUPPORTS_NOSWITCH).
> >
> > Signed-off-by: Andres Salomon <dilinger@debian.org>
> > ---
> > drivers/char/Kconfig | 21 +++++++++++++++++++++
> > drivers/video/geode/Kconfig | 1 +
> > include/linux/suspend.h | 2 +-
> > kernel/power/console.c | 2 +-
> > 4 files changed, 24 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
> > index 47c6be8..7fe0e85 100644
> > --- a/drivers/char/Kconfig
> > +++ b/drivers/char/Kconfig
> > @@ -58,6 +58,27 @@ config VT_CONSOLE
> >
> > If unsure, say Y.
> >
> > +config FBDEV_SUPPORTS_NOSWITCH
> > + bool
> > + default n
> > +
> > +config DISABLE_SUSPEND_VT_SWITCH
> > + bool "Disable the console switch prior to suspend (EXPERIMENTAL)"
> > + depends on VT_CONSOLE && PM && EXPERIMENTAL && FBDEV_SUPPORTS_NOSWITCH
> > + default n
> > + ---help---
> > + This option disables the automatic switch to VT console that happens
> > + prior to Linux going into a suspend/sleep. Your
> > + video card/framebuffer must be able to properly restore the
> > + display (even if X is doing something crazy!) in this scenario.
> > + This is useful for saving precious milliseconds during suspend and
> > + resume; however, if your framebuffer lacks support, it could leave
> > + your display in a weird state. If you're even seeing this option,
> > + it means at least one of your selected framebuffer drivers supports
> > + restoring the display.
> > +
> > + If unsure, say N.
>
> 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..




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