/*
References:
  https://wiki.osdev.org/Raspberry_Pi_Bare_Bones
*/
#include <uart.h>

#include <gpio.h>
#include <mmio.h>

enum
{
    // The base address for UART.
    // For raspi4 0xFE201000, raspi2 & 3 0x3F201000, and 0x20201000 for raspi1.
    UART0_BASE   = (GPIO_BASE + 0x1000),
    // The offsets for reach register for the UART.
    UART0_DR     = (UART0_BASE + 0x00),
    UART0_RSRECR = (UART0_BASE + 0x04),
    UART0_FR     = (UART0_BASE + 0x18),
    UART0_ILPR   = (UART0_BASE + 0x20),
    UART0_IBRD   = (UART0_BASE + 0x24),
    UART0_FBRD   = (UART0_BASE + 0x28),
    UART0_LCRH   = (UART0_BASE + 0x2C),
    UART0_CR     = (UART0_BASE + 0x30),
    UART0_IFLS   = (UART0_BASE + 0x34),
    UART0_IMSC   = (UART0_BASE + 0x38),
    UART0_RIS    = (UART0_BASE + 0x3C),
    UART0_MIS    = (UART0_BASE + 0x40),
    UART0_ICR    = (UART0_BASE + 0x44),
    UART0_DMACR  = (UART0_BASE + 0x48),
    UART0_ITCR   = (UART0_BASE + 0x80),
    UART0_ITIP   = (UART0_BASE + 0x84),
    UART0_ITOP   = (UART0_BASE + 0x88),
    UART0_TDR    = (UART0_BASE + 0x8C),
    // The offsets for Mailbox registers.
    MBOX_BASE    = 0xB880,
    MBOX_READ    = (MBOX_BASE + 0x00),
    MBOX_STATUS  = (MBOX_BASE + 0x18),
    MBOX_WRITE   = (MBOX_BASE + 0x20)
};

// A mailbox message with set clock rate of PL011 to 3MHz tag.
static volatile unsigned int  __attribute__((aligned(16))) mbox[9] = {
  9*4, 0, 0x38002, 12, 8, 2, 3000000, 0, 0
};

// Loop <delay> times in a way that the compiler won't optimize away.
static inline void delay(int32_t count) {
  asm volatile("__delay_%=: subs %[count], %[count], #1; bne __delay_%=\n"
    : "=r"(count): [count]"0"(count) : "cc");
}

void uart_init(int raspi) {
  // Disable UART0.
  mmio_write(UART0_CR, 0x00000000);
  
  // Set up GPIO pins 14 and 15.

  // Disable pull up/down for all GPIO pins.
  mmio_write(GPPUD, 0x00000000);
  delay(150);

  // Disable pull up/down for pins 14-15.
  mmio_write(GPPUDCLK0, (1 << 14) | (1 << 15));
  delay(150);

  // Write 0 to GPPUDCLK0 to make it take effect.
  mmio_write(GPPUDCLK0, 0x00000000);

  // Clear pending interrupts.
  mmio_write(UART0_ICR, 0x7FF);

  // Set integer & fractional part of baud rate.
  // Divider = UART_CLOCK/(16 * Baud)
  // Fraction part register = (Fractional part * 64) + 0.5
  // Baud = 115200.

  // For Raspi3 and 4 the UART_CLOCK is system-clock dependent by default.
  // Set it to 3Mhz so that we can consistently set the baud rate
  if (raspi >= 3) {
    // UART_CLOCK = 30000000;
    unsigned int r = (unsigned int) (((uint64_t)(&mbox) & ~0xF) | 8);
    // Wait until we can talk to the VC.
    while (mmio_read(MBOX_STATUS) & 0x80000000);
    // Send our message to property channel and wait for the response.
    mmio_write(MBOX_WRITE, r);
    while ((mmio_read(MBOX_STATUS) & 0x40000000) ||
           (mmio_read(MBOX_READ) != r));
  }

  // Divider = 3000000 / (16 * 115200) = 1.627 = ~1.
  mmio_write(UART0_IBRD, 1);
  // Fractional part register = (.627 * 64) + 0.5 = 40.6 = ~40.
  mmio_write(UART0_FBRD, 40);

  // Enable FIFO & 8 bit data transmission (1 stop bit, no parity).
  mmio_write(UART0_LCRH, (1 << 4) | (1 << 5) | (1 << 6));

  // Disable all interrupts.
  mmio_write(UART0_IMSC, (1 << 1) | (1 << 4) | (1 << 5) | (1 << 6) |
                         (1 << 7) | (1 << 8) | (1 << 9) | (1 << 10));

  // Enable UART0, receive & transfer part of UART.
  mmio_write(UART0_CR, (1 << 0) | (1 << 8) | (1 << 9));
}

void uart_putc(unsigned char c) {
  // Wait for UART to become ready to transmit.
  while (mmio_read(UART0_FR) & (1 << 5));
  mmio_write(UART0_DR, c);
}

unsigned char uart_getc() {
  // Wait for UART to have received something.
  while (mmio_read(UART0_FR) & (1 << 4));
  return mmio_read(UART0_DR);
}

void uart_write(const void* buffer, size_t length) {
  const uint8_t* bytes = buffer;
  for (size_t i = 0; i < length; ++i) {
    uart_putc(*bytes++);
  }
}

void uart_print(const char* string) {
  while (*string) {
    uart_putc(*string++);
  }
}