simple uart interrupt handler

uart_interrupt.c

/* simple uart interrupt : echo when something received on serial */
/* does not work -- u-boot remaps the interrupt table : 
 * http://stackoverflow.com/questions/23964400/am335x-freertos-port-unable-to-handle-irqs-and-swi */

#define INTC_BASE 0x48200000
#define INTC_SIR_IRQ 0x40

#define IRQ_HANDLER 0x4030CE38  /* RAM exception vector : TRM 26.1.3.2 */

#define CM_PER_TIMER7_CLKCTRL 0x44E0007C
#define DMTIMER7_TCLR 0x4804A038
#define UART0_BASE 0x44E09000
#define UART_LCR 0x0C
#define IIR_UART 0x08
#define IER_UART 0x04
#define UART0INT 72
#define IT_TYPE_RHR 0x02

#define REG(x) (*((volatile unsigned int *) x))

void timer_init() {
    REG(CM_PER_TIMER7_CLKCTRL) = 0x2;
    REG(DMTIMER7_TCLR) = 0x3;
}

void uart_putc(int c){
    REG(UART0_BASE + 0) = c;
}

void __attribute__((interrupt("IRQ"))) uart_isr(void){
    uart_putc((int) 'A');
    
    /* check INTC_SIR_IRQ[0:6] for UART0INT : TRM 6.5.1 */
    
    if((REG(INTC_BASE + INTC_SIR_IRQ) & 0x7F) == UART0INT){
        
        /* identify the UART interrupt type, if RHR: begin echo */
        
        unsigned int lcr = REG(UART0_BASE + UART_LCR); /* preserve LCR */
        REG(UART0_BASE + UART_LCR) &= 0x7F;            /* switch to register op 
                                                        * mode */
        unsigned int it_type = (REG(UART0_BASE + IIR_UART) & 0x3E) >> 1; 
                                                       /* TRM 19.5.1.9 */
        REG(UART0_BASE + UART_LCR) = lcr;              /* restore LCR */
        
        
        if(it_type == IT_TYPE_RHR){                    /* something received */
            while(1){                                  /* start echo */
                uart_putc((int) 'Z');
            }
        }
    }
}


void sleep(){
    int T = 1000000;
    while(T--){
        int TT = 100000;
        while(TT--){
            ;
        }
    }
}

#define INTC_PROTECTION 0x4C
void hello_world(){
 
    REG(IRQ_HANDLER) = uart_isr;
    REG(UART0_BASE + IER_UART) |= 0x01;                /* enable RHR interrupt */
    //REG(INTC_BASE + INTC_PROTECTION) = 0x00;    // disable protection
    uart_putc((int) 'X');

    while(1){
        sleep();
    }
}