stand: Add print_delay environment variable in loader

This adds support for a new `print_delay` environment variable,
which inserts a delay in microseconds when `putchar` encounters a
newline character. This can be useful when debugging.

Reviewed by:	markj, imp, ziaee, mckusick (mentor)
Approved by:	markj, imp, ziaee, mckusick (mentor)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D50832

stand/common/bootstrap.h

@@ -372,6 +372,8 @@ extern struct arch_switch archsw;
 /* This must be provided by the MD code, but should it be in the archsw? */
 void	delay(int delay);
 
+int setprint_delay(struct env_var *ev, int flags, const void *value);
+
 /* common code to set currdev variable. */
 int gen_setcurrdev(struct env_var *ev, int flags, const void *value);
 int mount_currdev(struct env_var *, int, const void *);

stand/common/console.c

@@ -44,6 +44,8 @@ static int	twiddle_set(struct env_var *ev, int flags, const void *value);
 #endif
 int module_verbose = MODULE_VERBOSE;
 
+static uint32_t print_delay_usec = 0;
+
 static int
 module_verbose_set(struct env_var *ev, int flags, const void *value)
 {
@@ -65,6 +67,23 @@ module_verbose_set(struct env_var *ev, int flags, const void *value)
 	return (CMD_OK);
 }
 
+/*
+ * Hook to set the print delay
+ */
+int
+setprint_delay(struct env_var *ev, int flags, const void *value)
+{
+	char *end;
+	int usec = strtol(value, &end, 10);
+
+	if (*(char *)value == '\0' || *end != '\0')
+		return (EINVAL);
+	if (usec < 0)
+		return (EINVAL);
+	print_delay_usec = usec;
+	return (0);
+}
+
 /*
  * Detect possible console(s) to use.  If preferred console(s) have been
  * specified, mark them as active. Else, mark the first probed console
@@ -178,6 +197,10 @@ putchar(int c)
 		    (C_PRESENTOUT | C_ACTIVEOUT))
 			consoles[cons]->c_out(c);
 	}
+
+	/* Pause after printing newline character if a print delay is set */
+	if (print_delay_usec != 0 && c == '\n')
+		delay(print_delay_usec);
 }
 
 /*

stand/defaults/loader.conf

@@ -95,6 +95,8 @@ audit_event_type="etc_security_audit_event"
 				# Default is unset and disabled (no delay).
 #autoboot_delay="10"		# Delay in seconds before autobooting,
 				# -1 for no user interrupts, NO to disable
+#print_delay="1000000"		# Slow printing of loader messages, useful for
+				# debugging.  Given in microseconds.
 #password=""			# Prevent changes to boot options
 #bootlock_password=""		# Prevent booting (see check-password.4th(8))
 #geom_eli_passphrase_prompt="NO" # Prompt for geli(8) passphrase to mount root

stand/defaults/loader.conf.5

@@ -21,7 +21,7 @@
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
-.Dd June 5, 2025
+.Dd June 12, 2025
 .Dt LOADER.CONF 5
 .Os
 .Sh NAME
@@ -116,6 +116,10 @@ option in this manner,
 .Va beastie_disable
 must be set to
 .Dq Li YES .
+.It Ar print_delay
+Add a delay in microseconds after printing each line.
+Default
+.Dq Li 0 .
 .It Ar boot_*
 See list in
 .Xr loader.efi 8

stand/efi/loader/main.c

@@ -1241,6 +1241,9 @@ main(int argc, CHAR16 *argv[])
 #endif
 	cons_probe();
 
+	/* Set print_delay variable to have hooks in place. */
+	env_setenv("print_delay", EV_VOLATILE, "", setprint_delay, env_nounset);
+
 	/* Set up currdev variable to have hooks in place. */
 	env_setenv("currdev", EV_VOLATILE, "", gen_setcurrdev, env_nounset);