From 709b1b702a8d9c2b202a0de72c6e6041486366a4 Mon Sep 17 00:00:00 2001
From: Martin Whitaker <memtest@martin-whitaker.me.uk>
Date: Sat, 21 Jan 2023 16:37:44 +0000
Subject: [PATCH] Allow use on headless EFI systems.

A headless EFI system may have no GOP devices. In this case, disable
output to the physical display, but continue to write to the shadow
buffer. This allows operation via a serial console.
---
 boot/bootparams.h |  1 +
 boot/efisetup.c   | 15 +++++++--------
 system/screen.c   |  8 ++++++--
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/boot/bootparams.h b/boot/bootparams.h
index 45d95c4..273a562 100644
--- a/boot/bootparams.h
+++ b/boot/bootparams.h
@@ -58,6 +58,7 @@ typedef struct {
 
 #define VIDEO_TYPE_VLFB             0x23    // VESA VGA in graphic mode
 #define VIDEO_TYPE_EFI              0x70    // EFI graphic mode
+#define VIDEO_TYPE_NONE             0xff    // no video display (added for Memtest86+)
 
 #define LFB_CAPABILITY_64BIT_BASE   (1 << 1)
 
diff --git a/boot/efisetup.c b/boot/efisetup.c
index 5fc92df..4362215 100644
--- a/boot/efisetup.c
+++ b/boot/efisetup.c
@@ -334,9 +334,7 @@ static efi_status_t set_screen_info_from_gop(screen_info_t *si, efi_handle_t *ha
 
     efi_graphics_output_t *gop = find_gop(handles, handles_size);
     if (!gop) {
-#if DEBUG
-        print_string("GOP not found\n");
-#endif
+        print_string("No graphics display found\n");
         return EFI_NOT_FOUND;
     }
 
@@ -365,9 +363,7 @@ static efi_status_t set_screen_info_from_gop(screen_info_t *si, efi_handle_t *ha
         efi_call_bs(free_pool, info);
     }
     if (best_mode == UINT32_MAX) {
-#if DEBUG
-        print_string("No suitable GOP screen resolution\n");
-#endif
+        print_string("No suitable screen resolution found\n");
         return EFI_NOT_FOUND;
     }
 
@@ -469,9 +465,7 @@ static efi_status_t set_screen_info_from_gop(screen_info_t *si, efi_handle_t *ha
 
     status = efi_call_proto(gop, set_mode, best_mode);
     if (status != EFI_SUCCESS) {
-#if DEBUG
         print_string("Set GOP mode failed\n");
-#endif
         return status;
     }
 
@@ -502,6 +496,11 @@ static efi_status_t set_screen_info(boot_params_t *boot_params)
         if (status == EFI_SUCCESS) {
             status = set_screen_info_from_gop(&boot_params->screen_info, handles, handles_size);
         }
+        if (status == EFI_NOT_FOUND) {
+            // This may be a headless system. We can still output to a serial console.
+            boot_params->screen_info.orig_video_isVGA = VIDEO_TYPE_NONE;
+            status = EFI_SUCCESS;
+        }
         efi_call_bs(free_pool, handles);
     }
 
diff --git a/system/screen.c b/system/screen.c
index 4c63513..b745f16 100644
--- a/system/screen.c
+++ b/system/screen.c
@@ -42,7 +42,7 @@ static const rgb_value_t vga_pallete[16] = {
     { 255, 255, 255 }   // BOLD+WHITE
 };
 
-static vga_buffer_t *vga_buffer = (vga_buffer_t *)(0xb8000);
+static vga_buffer_t *vga_buffer = NULL;
 
 vga_buffer_t shadow_buffer;
 
@@ -64,7 +64,9 @@ static void vga_put_char(int row, int col, uint8_t ch, uint8_t attr)
     shadow_buffer[row][col].ch   = ch;
     shadow_buffer[row][col].attr = attr;
 
-    (*vga_buffer)[row][col].value = shadow_buffer[row][col].value;
+    if (vga_buffer) {
+        (*vga_buffer)[row][col].value = shadow_buffer[row][col].value;
+    }
 }
 
 static void lfb8_put_char(int row, int col, uint8_t ch, uint8_t attr)
@@ -239,6 +241,8 @@ void screen_init(void)
             uint32_t b = ((vga_pallete[i].b * b_max) / 255) << screen_info->blue_pos;
             lfb_pallete[i] = r | g | b;
         }
+    } else if (screen_info->orig_video_isVGA != VIDEO_TYPE_NONE) {
+        vga_buffer = (vga_buffer_t *)(0xb8000);
     }
 }