UART initialization fails from loaded kernel

[jjfahner]

After porting raspbootcom to Windows I wanted to use your UART code as a basis instead of the MiniUART (which I was having reliability issues with) for a project on a RPi B+. I've scoured the internet for other info but find the same basic code for initialization of the UART.

For some reason though, after the UART has been initialized by raspbootin, I can't re-initialize it from the kernel_main in the kernel loaded by raspbootin. I can use the UART without issues from the loaded kernel, since it's already been initialized.

Debugging doesn't get me further than the call to disable pin 14/15, which never returns (basically the MMIO::write(GPPUDCLK0, (1 << 14) | (1 << 15))). Or maybe it causes an exception, but I'm trapping them so I should have noticed that.

I'm probably just not aware of an important detail (like needing to de-initialize something first), but I'm stuck. Hope there's something you can tell me!

[hsreina]

When I was using rapbootin, I used this code to initialize UART from my own kernel booted by raspbootin.
Now, I use a special version of my kernel to work same as raspbootin and I still don't have any problem to reinitialize

    // Disable UART0.
    memory::write32(UART0_CR, 0x00000000);
    // Setup the GPIO pin 14 && 15.

    // Disable pull up/down for all GPIO pins & delay for 150 cycles.
    memory::write32(GPIO::GPPUD, 0x00000000);
    delay(150);

    // Disable pull up/down for pin 14,15 & delay for 150 cycles.
    memory::write32(GPIO::GPPUDCLK0, (1 << 14) | (1 << 15));
    delay(150);

    // Write 0 to GPPUDCLK0 to make it take effect.
    memory::write32(GPIO::GPPUDCLK0, 0x00000000);

    // Clear pending interrupts.
    memory::write32(UART0_ICR, 0x7FF);

    memory::write32(UART0_IBRD, 1); // divider
    memory::write32(UART0_FBRD, 0); // fraction

    // Enable FIFO & 8 bit data transmissio (1 stop bit, no parity).
    memory::write32(
        UART0_LCRH, 
        (1 << 4) | // FEN - enable FIFO */ 
        (1 << 5) | (1 << 6) // WLEN
    );

    memory::write32(UART0_IMSC, 
        IMSC_RXI |  // enable interrupt on RX
        IMSC_RTI |  // enable receive timeout
        IMSC_PEI |  // enable parity error interrupt
        IMSC_BEI |  // enable Break error interrupt
        IMSC_OEI |  // enable Overrun error interrupt
        0
    );

    // Enable UART0, receive & transfer part of UART.
    memory::write32(UART0_CR, (1 << 0) | (1 << 8) | (1 << 9));

    interrupt::enableIrq(interrupt::uartInt);

I only got problem when trying to initialize mmu stuff before uart. Are you initializing uart before everything?
Or maybe you can share more of your code to try it out.

[mrvn]

Must be a problem with the kernel code, not raspbootin. Best guess the MMIO registers are not mapped or the address is wrong and the exception catching code doesn't work.