acpi/x86: add a kernel parameter to disable ACPI BGRT

BGRT is for displaying seamless OEM logo from booting to login screen;
however, this mechanism does not always work well on all configurations
and the OEM logo can be displayed multiple times. This looks worse than
without BGRT enabled.

This patch adds a kernel parameter to disable BGRT in boot time. This is
easier than re-compiling a kernel with CONFIG_ACPI_BGRT disabled.
Signed-off-by: Alex Hung <alex.hung@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -450,6 +450,9 @@
 	bert_disable	[ACPI]
 			Disable BERT OS support on buggy BIOSes.
 
+	bgrt_disable	[ACPI][X86]
+			Disable BGRT to avoid flickering OEM logo.
+
 	bttv.card=	[HW,V4L] bttv (bt848 + bt878 based grabber cards)
 	bttv.radio=	Most important insmod options are available as
 			kernel args too.

arch/x86/kernel/acpi/boot.c

@@ -45,6 +45,7 @@ EXPORT_SYMBOL(acpi_disabled);
 #define PREFIX			"ACPI: "
 
 int acpi_noirq;				/* skip ACPI IRQ initialization */
+int acpi_nobgrt;			/* skip ACPI BGRT */
 int acpi_pci_disabled;		/* skip ACPI PCI scan and IRQ initialization */
 EXPORT_SYMBOL(acpi_pci_disabled);
 
@@ -1619,7 +1620,7 @@ int __init acpi_boot_init(void)
 	acpi_process_madt();
 
 	acpi_table_parse(ACPI_SIG_HPET, acpi_parse_hpet);
-	if (IS_ENABLED(CONFIG_ACPI_BGRT))
+	if (IS_ENABLED(CONFIG_ACPI_BGRT) && !acpi_nobgrt)
 		acpi_table_parse(ACPI_SIG_BGRT, acpi_parse_bgrt);
 
 	if (!acpi_noirq)
@@ -1671,6 +1672,13 @@ static int __init parse_acpi(char *arg)
 }
 early_param("acpi", parse_acpi);
 
+static int __init parse_acpi_bgrt(char *arg)
+{
+	acpi_nobgrt = true;
+	return 0;
+}
+early_param("bgrt_disable", parse_acpi_bgrt);
+
 /* FIXME: Using pci= for an ACPI parameter is a travesty. */
 static int __init parse_pci(char *arg)
 {