drivers/ukconsole/vga: Add initial sanity check

On initialization, ensure that the VGA controller is present and in
the expected state, otherwise cancel the initialization and
registration of the VGA driver, but continue booting.

For platforms without an VGA controller (such as firecracker), this
avoids aborts due to an illicit registration of a non-existing VGA
memory region.

Signed-off-by: Florian Rommel <mail@florommel.de>
Approved-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Simon Kuenzer <simon@unikraft.io>
GitHub-Closes: #1530

diff --git a/drivers/ukconsole/vgacons/vga.c b/drivers/ukconsole/vgacons/vga.c
index 1ba8c3ded8bf5bdf643c7656cfebdca92dcbb3ae..9785825e3c5a7a8a8f74af0526d452b339f948ba 100644 (file)
--- a/drivers/ukconsole/vgacons/vga.c
+++ b/drivers/ukconsole/vgacons/vga.c
@@ -213,6 +213,35 @@ static struct uk_console_ops vga_ops = { .out = vga_console_out };
 
 static struct uk_console vga_dev;
 
+#define VGA_MISC_OUTPUT_REG 0x3cc
+#define VGA_IO_ADDRESS_SELECT_MASK 0b01
+#define VGA_ERAM_MASK 0b10
+#define VGA_GRAPHICS_ADDRESS_REG 0x3ce
+#define VGA_GRAPHICS_DATA_REG 0x3cf
+#define VGA_MISC_GRAPHICS_REG 0x06
+#define VGA_MEMORY_MAP_SELECT_MASK 0b1100
+
+static int vga_check_settings(void)
+{
+       __u8 eram, memory_map_select;
+
+       /* First, read the 'ERAM' bit in the 'Miscellaneous Output Register'.
+        * It should be set, signifying that the VGA controller detects writes
+        * to the VGA memory region.
+        */
+       eram = inb(VGA_MISC_OUTPUT_REG) & VGA_ERAM_MASK;
+
+       /* Second, read 'Memory Map Select' filed in the 'Miscellaneous
+        * Graphics Register'. It should be 0b11 in order to select the
+        * 0xb8000-0xbffff address range.
+        */
+       outb(VGA_GRAPHICS_ADDRESS_REG, VGA_MISC_GRAPHICS_REG);
+       memory_map_select = inb(VGA_GRAPHICS_DATA_REG)
+                           & VGA_MEMORY_MAP_SELECT_MASK;
+
+       return eram && memory_map_select == VGA_MEMORY_MAP_SELECT_MASK;
+}
+
 static int vga_init(struct ukplat_bootinfo *bi)
 {
        unsigned long irq_flags;
@@ -224,12 +253,33 @@ static int vga_init(struct ukplat_bootinfo *bi)
        terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK);
 
        local_irq_save(irq_flags);
+       if (!vga_check_settings()) {
+               __u8 val;
+
+               /* Set 'ERAM' in 'Miscellaneous Output Register'. */
+               val = inb(VGA_MISC_OUTPUT_REG);
+               outb(VGA_MISC_OUTPUT_REG, val | VGA_ERAM_MASK);
+
+               /* Set 'Memory Map Select' in 'Miscellaneous Graphics Reg'. */
+               outb(VGA_GRAPHICS_ADDRESS_REG, VGA_MISC_GRAPHICS_REG);
+               val = inb(VGA_GRAPHICS_DATA_REG);
+               outb(VGA_GRAPHICS_DATA_REG, val | VGA_MEMORY_MAP_SELECT_MASK);
+
+               /* When the settings are still not correct, we assume that there
+                * is no VGA controller.
+                */
+               if (!vga_check_settings()) {
+                       uk_pr_err("Could not initialize the VGA driver\n");
+                       return 0;
+               }
+       }
+
        /* Location of the address and data registers is variable and denoted
         * by the least significant bit in the Input/Output register located
         * at 0x3cc. For our emulated color display, they should always be
         * 0x3d{4,5}, but better safe than sorry, so let's check at init time.
         */
-       if (inb(0x3cc) & 0x1) {
+       if (inb(VGA_MISC_OUTPUT_REG) & VGA_IO_ADDRESS_SELECT_MASK) {
                areg = 0x3d4;
                dreg = 0x3d5;
        } else {