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: <20080310141939.ce1169c0.akpm@linux-foundation.org>
Date:	Mon, 10 Mar 2008 14:19:39 -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, "Rafael J. Wysocki" <rjw@...k.pl>,
	Samuel Thibault <samuel.thibault@...-lyon.org>
Subject: Re: [PATCH 6/6] PM/FB/gxfb: add config option that allows disabling
 of VT switch during suspend

On Sat, 8 Mar 2008 20:49:27 -0500
Andres Salomon <dilinger@...ued.net> wrote:

> This is purely for suspend/resume time optimization.
> 
> 
> >From 874aa20022a2501a30704163ed31a7baba7de9e5 Mon Sep 17 00:00:00 2001
> From: Andres Salomon <dilinger@...ian.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@...ian.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.

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