efifb: Skip DMI checks if the bootloader knows what it's doing

The majority of the DMI checks in efifb are for cases where the bootloader
has provided invalid information. However, on some machines the overrides
may do more harm than good due to configuration differences between machines
with the same machine identifier. It turns out that it's possible for the
bootloader to get the correct information on GOP-based systems, but we
can't guarantee that the kernel's being booted with one that's been updated
to do so. Add support for a capabilities flag that can be set by the
bootloader, and skip the DMI checks in that case. Additionally, set this
flag in the UEFI stub code.
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

arch/x86/boot/compressed/eboot.c

@@ -379,6 +379,8 @@ static efi_status_t setup_gop(struct screen_info *si, efi_guid_t *proto,
 		si->rsvd_pos = 0;
 	}
 
+	si->capabilities |= VIDEO_CAPABILITY_SKIP_QUIRKS;
+
 free_handle:
 	efi_call_phys1(sys_table->boottime->free_pool, gop_handle);
 	return status;

drivers/video/efifb.c

@@ -553,7 +553,9 @@ static int __init efifb_init(void)
 	int ret;
 	char *option = NULL;
 
-	dmi_check_system(dmi_system_table);
+	if (screen_info.orig_video_isVGA != VIDEO_TYPE_EFI ||
+	    !(screen_info.capabilities & VIDEO_CAPABILITY_SKIP_QUIRKS))
+		dmi_check_system(dmi_system_table);
 
 	if (screen_info.orig_video_isVGA != VIDEO_TYPE_EFI)
 		return -ENODEV;

include/linux/screen_info.h

@@ -68,6 +68,8 @@ struct screen_info {
 
 #define VIDEO_FLAGS_NOCURSOR	(1 << 0) /* The video mode has no cursor set */
 
+#define VIDEO_CAPABILITY_SKIP_QUIRKS	(1 << 0)
+
 #ifdef __KERNEL__
 extern struct screen_info screen_info;