An Uart bug for sifive model

[陈纪源]

Hi,
I recently use the opensbi as the bios on the sifive unmatched board.
I use the ecall instruction (set register a0 to the character and
register a7 to 2) to output a character, but failed. I read the source
code and find the get character function sifive_uart_getc, it only
receive one character from FIFO queue.

static int sifive_uart_getc(void)
{
    u32 ret = get_reg(UART_REG_RXFIFO);
    if (!(ret & UART_RXFIFO_EMPTY))
        return ret & UART_RXFIFO_DATA;
    return -1;
}

Actually the get character process should be the same with the putchar
process.When the kernel wait for one character, it should keep waiting
until we type on the keyboard.The process should change like this:

static int sifive_uart_getc(void)
{
    u32 ret = get_reg(UART_REG_RXFIFO);
    while (ret & UART_RXFIFO_EMPTY) {
        ret = get_reg(UART_REG_RXFIFO);
    }
    return ret & UART_RXFIFO_DATA;
}

If I have the wrong understanding of the uart, please let me know as
soon as possible.

Yours sincerely,
ZergingChen3