/* 
 * Copyright 2010 Stefan Lankes, Chair for Operating Systems,
 *                               RWTH Aachen University
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * This file is part of MetalSVM.
 */

/**
 * @author Stefan Lankes
 * @file arch/x86/kernel/idt.c
 * @brief Definitions and functions related to IDT
 *
 *
 * This file defines the interface for interrupts as like 
 * structures to describe interrupt descriptor table entries.\n
 * See idt.h for flag definitions.
 */

#include <metalsvm/string.h>
#include <asm/idt.h>

/** @brief Defines an IDT entry
 *
 * This structure defines interrupt descriptor table entries.\n
 * They consist of the handling function's base address, some flags 
 * and a segment selector.
 */
typedef struct {
	/// Handler function's lower 16 address bits
	unsigned short base_lo;
	/// Handler function's segment selector.
	unsigned short sel;
	/// These bits are reserved by Intel
	unsigned char always0;
	/// These 8 bits contain flags. Exact use depends on the type of interrupt gate.
	unsigned char flags;
	/// Higher 16 bits of handler function's base address
	unsigned short base_hi;
} __attribute__ ((packed)) idt_entry_t;


/** @brief Defines the idt pointer structure.
 *
 * This structure keeps information about 
 * base address and size of the interrupt descriptor table.
 */
typedef struct {
	/// Size of the IDT in bytes (not the number of entries!)
	unsigned short limit;
	/// Base address of the IDT
	unsigned int base;
} __attribute__ ((packed)) idt_ptr_t;

/* 
 * Declare an IDT of 256 entries. Although we will only use the
 * first 32 entries in this tutorial, the rest exists as a bit
 * of a trap. If any undefined IDT entry is hit, it normally
 * will cause an "Unhandled Interrupt" exception. Any descriptor
 * for which the 'presence' bit is cleared (0) will generate an
 * "Unhandled Interrupt" exception 
 */
static idt_entry_t idt[256] = {[0 ... 255] = {0, 0, 0, 0, 0}};
idt_ptr_t idtp;

/** @brief Loads the IDT
 *
 * The true definition lives in 'start.asm'
 */
extern void idt_load(void);

/* 
 * Use this function to set an entry in the IDT. Alot simpler
 * than twiddling with the GDT ;) 
 */
void idt_set_gate(unsigned char num, unsigned long base, unsigned short sel,
		  unsigned char flags)
{

	/* The interrupt routine's base address */
	idt[num].base_lo = (base & 0xFFFF);
	idt[num].base_hi = (base >> 16) & 0xFFFF;

	/* The segment or 'selector' that this IDT entry will use
	 *  is set here, along with any access flags */
	idt[num].sel = sel;
	idt[num].always0 = 0;
	idt[num].flags = flags;
}

extern void isrsyscall(void);

/* Installs the IDT */
void idt_install(void)
{
	/* Sets the special IDT pointer up, just like in 'gdt.c' */
	idtp.limit = (sizeof(idt_entry_t) * 256) - 1;
	idtp.base = (unsigned int)&idt;

	/* Add any new ISRs to the IDT here using idt_set_gate */
	idt_set_gate(INT_SYSCALL, (unsigned int)isrsyscall, KERNEL_CODE_SELECTOR, 
		IDT_FLAG_PRESENT|IDT_FLAG_RING3|IDT_FLAG_32BIT|IDT_FLAG_TRAPGATE);

	/* Points the processor's internal register to the new IDT */
	idt_load();
}