UART write-only driver for A6 SecureROM

uart.c

/*
 * The algorithm is mostly stolen from iBoot source,
 * cleaned up for the purpose
 * 
 * TODO: analyze S5L8700X datasheet in order
 * to understand if UART peripherial is the same,
 * and if so - clean up this mess
 */

#include <stdint.h>
#include <securerom.h>
#include <target/offsets.h>

#define rULCON				(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x00))
#define rUCON				(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x04))
#define rUFCON				(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x08))
#define rUMCON				(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x0C))
#define rUTRSTAT			(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x10))
#define rUERSTAT			(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x14))
#define rUFSTAT 			(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x18))
#define rUMSTAT 			(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x1C))
#define rUBRDIV 			(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x28))
#define rUABRCNT 			(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x2C))
#define rUTXH 				(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x20))
#define rURXH				(*(volatile uint32_t *)(TARGET_DEBUG_UART_BASEADDR + 0x24))

static void uart_gpio_init() {
    *(volatile uint32_t *)(TARGET_GPIO_BASEADDR + sizeof(uint32_t) * (TARGET_GPIO_DEBUG_UART_PORT * 8 + TARGET_GPIO_DEBUG_UART_PIN)) = \
        TARGET_GPIO_DEBUG_UART_CONFIG;
}

#define SAMPLE_RATE 16
#define BAUD_RATE   115200

int uart_init() {
    /* configuring UART TX line GPIO */
    uart_gpio_init();

    /* enabling UART clocking */
    clock_gate(TARGET_DEBUG_UART_CLK_GATE, true);

    /* setting data sizes */
    rULCON = 0x03; // 81N, not IR
    rUCON  = 0x05;

    /* setting NCLK */
    rUCON |= (1 << 10);

    /* setting sample rate */
    rUBRDIV = (rUBRDIV & ~(0xF << 16)) | ((16 - SAMPLE_RATE) << 16);

    /* setting baud rate */
    uint32_t freq = clock_get_frequency(TARGET_NCLK_CLK_GATE);

    uint16_t div = freq / (BAUD_RATE * SAMPLE_RATE);

    uint32_t baud_low, actual_baud;
    actual_baud = freq / ((div + 0) * SAMPLE_RATE);
    baud_low    = freq / ((div + 1) * SAMPLE_RATE);

    if ((BAUD_RATE - baud_low) < (actual_baud - BAUD_RATE)) {
        div++;
    }

    rUBRDIV = (rUBRDIV & ~0xFFFF) | (div - 1);

    /* setting flow control */
    rUMCON = 0x01;  // no flow control - assert RTS

    /* setting FIFO */
    rUFCON = 0; // no FIFO

    /* 
     * setting mode:
     * TX and RX (GPIO is disabled for RX, so we should be OK),
     * no interrupts
     */

    rUCON &= ~((0xf << 11) | (1 << 7));
    rUCON = (rUCON & ~0xf) | 0x5; 

    return 0;
}

int uart_putc(char c) {
    /* wait */
    while (!((rUTRSTAT) & 0x04));

    /* write! */
    rUTXH = c;

    return 1;
}

int uart_puts(const char *s) {
    char c;
    size_t size = 0;

    while ((c = *s++)) {
        uart_putc(c);
        size++;
    }

    return size;
}