riscv: enable EFI framebuffer

Pass framebuffer information from loader(8) to the kernel via the
MODINFOMD_EFI_FB metadata field.

Enable the vt_efifb driver. A small tweak is required to work around the
lack of VM_MEMATTR_WRITE_COMBINING on this platform; we use
VM_MEMATTR_UNCACHEABLE instead.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D48884

stand/efi/loader/bootinfo.c

@@ -182,7 +182,7 @@ bi_load_efi_data(struct preloaded_file *kfp, bool exit_bs)
 	struct efi_map_header *efihdr;
 	bool do_vmap;
 
-#if defined(__amd64__) || defined(__aarch64__) || defined(__i386__)
+#ifdef MODINFOMD_EFI_FB
 	struct efi_fb efifb;
 
 	efifb.fb_addr = gfx_state.tg_fb.fb_addr;

sys/dev/vt/hw/efifb/efifb.c

@@ -96,9 +96,11 @@ vt_efifb_init(struct vt_device *vd)
 {
 	struct fb_info	*info;
 	struct efi_fb	*efifb;
-	int		memattr;
+	vm_memattr_t	memattr;
 	int		roff, goff, boff;
-	char		attr[16];
+
+#ifdef VM_MEMATTR_WRITE_COMBINING
+	char attr[16];
 
 	/*
 	 * XXX TODO: I think there's more nuance here than we're acknowledging,
@@ -122,6 +124,9 @@ vt_efifb_init(struct vt_device *vd)
 			memattr = VM_MEMATTR_UNCACHEABLE;
 		}
 	}
+#else
+	memattr = VM_MEMATTR_UNCACHEABLE;
+#endif
 
 	info = vd->vd_softc;
 	if (info == NULL)

sys/riscv/conf/GENERIC

@@ -140,6 +140,7 @@ device		uart_ns8250	# ns8250-type UART driver
 
 # Console
 device		vt
+device		vt_efifb
 device		kbdmux
 
 # RTC

sys/riscv/include/metadata.h

@@ -30,6 +30,7 @@
 
 #define	MODINFOMD_DTBP		0x1001
 #define	MODINFOMD_EFI_MAP	0x1002
+#define	MODINFOMD_EFI_FB	0x1003
 
 struct efi_map_header {
 	size_t		memory_size;