#include <kernel/x86/gdt.h>
#include <stdint.h>
#include <stdio.h>

#ifdef __TESTING__
#include <kernel/_kernel.h>
#endif

/**
 * What is this file?
 *
 * Well, to properly set up a lot of the system, we need something called a GDT
 * or, a Global Descriptor Table.
 *
 * This table, establishes a few things.
 *
 * Mainly it sets 4 segments,
 * 	A kernel code segment, with RING 0 permissions
 * 	A kernel data segment, with RING 0 permissions
 * 	A user code segment, with RING 3 permissions
 * 	A user data segment, with RING 3 permissions
 *
 * This allows for future userspace to properly segment code and data,
 * anything in userspace shouldn't have access to hardware like the kernel does
 * So by passing through this GDT, we can dish out authority to access certain data, functions,
 * etc, by going through the CPU permission system (RING 0 - 3)
 */

#define GDT_SIZE 5

// Each define here is for a specific flag in the descriptor.
// Refer to the intel documentation for a description of what each one does.
#define SEG_DESCTYPE(x)  ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
#define SEG_PRES(x)      ((x) << 0x07) // Present
#define SEG_SAVL(x)      ((x) << 0x0C) // Available for system use
#define SEG_LONG(x)      ((x) << 0x0D) // Long mode
#define SEG_SIZE(x)      ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
#define SEG_GRAN(x)      ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
#define SEG_PRIV(x)     (((x) &  0x03) << 0x05)   // Set privilege level (0 - 3)
 
#define SEG_DATA_RD        0x00 // Read-Only
#define SEG_DATA_RDA       0x01 // Read-Only, accessed
#define SEG_DATA_RDWR      0x02 // Read/Write
#define SEG_DATA_RDWRA     0x03 // Read/Write, accessed
#define SEG_DATA_RDEXPD    0x04 // Read-Only, expand-down
#define SEG_DATA_RDEXPDA   0x05 // Read-Only, expand-down, accessed
#define SEG_DATA_RDWREXPD  0x06 // Read/Write, expand-down
#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
#define SEG_CODE_EX        0x08 // Execute-Only
#define SEG_CODE_EXA       0x09 // Execute-Only, accessed
#define SEG_CODE_EXRD      0x0A // Execute/Read
#define SEG_CODE_EXRDA     0x0B // Execute/Read, accessed
#define SEG_CODE_EXC       0x0C // Execute-Only, conforming
#define SEG_CODE_EXCA      0x0D // Execute-Only, conforming, accessed
#define SEG_CODE_EXRDC     0x0E // Execute/Read, conforming
#define SEG_CODE_EXRDCA    0x0F // Execute/Read, conforming, accessed
 
#define GDT_CODE_PL0 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                     SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
                     SEG_PRIV(0)     | SEG_CODE_EXRD
 
#define GDT_DATA_PL0 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                     SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
                     SEG_PRIV(0)     | SEG_DATA_RDWR
 
#define GDT_CODE_PL3 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                     SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
                     SEG_PRIV(3)     | SEG_CODE_EXRD
 
#define GDT_DATA_PL3 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                     SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
                     SEG_PRIV(3)     | SEG_DATA_RDWR



uint64_t gdt[GDT_SIZE];

void set_gdt(unsigned short limit, uint64_t* base);
void reload_segments(void);

uint64_t create_descriptor(uint32_t base, uint32_t limit, uint16_t flag)
{
	uint64_t descriptor;

	descriptor = limit & 0x000F0000;
	descriptor |= (flag << 8) & 0x00F0FF00;
	descriptor |= (base >> 16) & 0x000000FF;
	descriptor |= base & 0xFF000000;

	descriptor <<= 32;

	descriptor |= base << 16;
	descriptor |= limit & 0x0000FFFF;

	return descriptor;
}

#ifdef __TESTING__
void dump_gdt(void)
{
	for (int i = 0; i < GDT_SIZE; i++) {
		printf("GDT_ENTRY %d: %4 | %2\n", i, gdt[i], gdt[i]);
	}
}
#endif


void gdt_init(void)
{ 
#ifdef __TESTING__
	kinfo("Initializing the GDT");
#endif
	gdt[0] = create_descriptor(0, 0, 0); // null
	gdt[1] = create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL0)); // kernel code
	gdt[2] = create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL0)); // kernel data
	gdt[3] = create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL3)); // user code
	gdt[4] = create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL3)); // user data

	set_gdt((sizeof(uint64_t) * GDT_SIZE) - 1, &(gdt[0])); // limit, base
	reload_segments();
#ifdef __TESTING__
	kinfo("Initialized the GDT");
#endif
#ifdef __TESTING__
	dump_gdt();
#endif
}

#undef SEG_DESCTYPE
#undef SEG_PRES
#undef SEG_SAVL
#undef SEG_LONG
#undef SEG_SIZE
#undef SEG_GRAN
#undef SEG_PRIV
#undef SEG_DATA_RD        
#undef SEG_DATA_RDA       
#undef SEG_DATA_RDWR      
#undef SEG_DATA_RDWRA     
#undef SEG_DATA_RDEXPD    
#undef SEG_DATA_RDEXPDA   
#undef SEG_DATA_RDWREXPD  
#undef SEG_DATA_RDWREXPDA 
#undef SEG_CODE_EX        
#undef SEG_CODE_EXA       
#undef SEG_CODE_EXRD      
#undef SEG_CODE_EXRDA     
#undef SEG_CODE_EXC       
#undef SEG_CODE_EXCA      
#undef SEG_CODE_EXRDC     
#undef SEG_CODE_EXRDCA    
#undef GDT_CODE_PL0 
#undef GDT_DATA_PL0 
#undef GDT_CODE_PL3 
#undef GDT_DATA_PL3 
#undef GDT_SIZE