Re: [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 [off-list ref] wrote:

On Sat, 8 Mar 2008 20:49:27 -0500
Andres Salomon [off-list ref] wrote:

quoted

This is purely for suspend/resume time optimization.

quoted

From 874aa20022a2501a30704163ed31a7baba7de9e5 Mon Sep 17 00:00:00 2001

From: Andres Salomon <redacted>
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 <redacted>
---
 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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