RE: git: 7749de244014 - main - Add new kern.vt.slow_down tunable.

Poul-Henning Kamp <phk_at_FreeBSD.org> wrote on
Date: Sat, 23 Nov 2024 15:03:55 UTC :

> The branch main has been updated by phk:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=7749de244014a057b55552ea9d68fd8aeb262ea0
> 
> commit 7749de244014a057b55552ea9d68fd8aeb262ea0
> Author:     Poul-Henning Kamp <phk@FreeBSD.org>
> AuthorDate: 2024-11-23 15:01:09 +0000
> Commit:     Poul-Henning Kamp <phk@FreeBSD.org>
> CommitDate: 2024-11-23 15:01:09 +0000
> 
>     Add new kern.vt.slow_down tunable.
>     
>     On a laptop with no other console devices than the screen, things
>     scroll of the screen faster than eye or camera can capture it.
>     
>     This tunable slows the console down and makes it update synchronously,
>     so console output continues when timers or interrupts do not.
>     
>     Differential Revision:  https://reviews.freebsd.org/D47710
> ---
>  share/man/man4/vt.4  | 11 +++++++++++
>  sys/dev/vt/vt_core.c |  9 +++++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/share/man/man4/vt.4 b/share/man/man4/vt.4
> index 4c115b68a80d..d3d3c4b38013 100644
> --- a/share/man/man4/vt.4
> +++ b/share/man/man4/vt.4
> @@ -50,6 +50,7 @@ In
>  .Cd kern.vt.color.<colornum>.rgb="<colorspec>"
>  .Cd kern.vt.fb.default_mode="<X>x<Y>"
>  .Cd kern.vt.fb.modes.<connector>="<X>x<Y>"
> +.Cd kern.vt.slow_down=<delay>"
>  .Cd screen.font="<X>x<Y>"
>  .Pp
>  In
> @@ -266,6 +267,16 @@ It will contain a list of connectors and their associated tunables.
>  This is currently only supported by the
>  .Cm vt_fb
>  backend when it is paired with a KMS video driver.
> +.It Va kern.vt.slow_down
> +When debugging the kernel on modern laptops, the screen is often
> +the only available console, and relevant information will scroll
> +out of view before it can be captured by eye or camera.
> +.Pp
> +Setting
> +.Va kern.vt.slow_down
> +to a non-zero number

Seems to not be true in the code for: less than 0.

I've no clue which is intended to be the intent: the wording above
or the code below.

> will make console output synchronous (ie:
> +not dependent on timers and interrupts) and slow it down in proportion
> +to the number.
>  .It Va screen.font
>  Set this value to the base name of the desired font file located in
>  .Pa /boot/fonts .
> diff --git a/sys/dev/vt/vt_core.c b/sys/dev/vt/vt_core.c
> index 87020b6e6f19..1be98466112e 100644
> --- a/sys/dev/vt/vt_core.c
> +++ b/sys/dev/vt/vt_core.c
> @@ -132,6 +132,9 @@ static VT_SYSCTL_INT(debug, 0, "vt(9) debug level");
>  static VT_SYSCTL_INT(deadtimer, 15, "Time to wait busy process in VT_PROCESS mode");
>  static VT_SYSCTL_INT(suspendswitch, 1, "Switch to VT0 before suspend");
>  
> +/* Slow down and dont rely on timers and interrupts */
> +static VT_SYSCTL_INT(slow_down, 0, "Non-zero make console slower and synchronous.");

I see in sys/dev/vt/vt.h :

#define VT_SYSCTL_INT(_name, _default, _descr)                          \
int vt_##_name = (_default);                                            \
SYSCTL_INT(_kern_vt, OID_AUTO, _name, CTLFLAG_RWTUN, &vt_##_name, 0, _descr)

so vt_slow_down has a signed value (int type, not unsigned int type).

> +
>  /* Allow to disable some keyboard combinations. */
>  static VT_SYSCTL_INT(kbd_halt, 1, "Enable halt keyboard combination.  "
>      "See kbdmap(5) to configure.");
> @@ -1657,6 +1660,12 @@ vtterm_done(struct terminal *tm)
>  		}
>  		vd->vd_flags &= ~VDF_SPLASH;
>  		vt_flush(vd);
> +	} else if (vt_slow_down > 0) {

That test treats < 0 the same as == 0, only > 0 is distinct.

> +		int i, j;
> +		for (i = 0; i < vt_slow_down; i++) {
> +			for (j = 0; j < 1000; j++)
> +				vt_flush(vd);
> +		}
>  	} else if (!(vd->vd_flags & VDF_ASYNC)) {
>  		vt_flush(vd);
>  	}


===
Mark Millard
marklmi at yahoo.com