uart.c

Go to the documentation of this file.

/*
 * Copyright (C) 2014 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );

/** @file
 *
 * 16550-compatible UART
 *
 */

#include <unistd.h>
#include <errno.h>
#include <ipxe/uart.h>

/** Timeout for transmit holding register to become empty */
#define UART_THRE_TIMEOUT_MS 100

/** Timeout for transmitter to become empty */
#define UART_TEMT_TIMEOUT_MS 1000

/**
 * Transmit data
 *
 * @v uart              UART
 * @v data              Data
 */
void uart_transmit ( struct uart *uart, uint8_t data ) {
        unsigned int i;
        uint8_t lsr;

        /* Wait for transmitter holding register to become empty */
        for ( i = 0 ; i < UART_THRE_TIMEOUT_MS ; i++ ) {
                lsr = uart_read ( uart, UART_LSR );
                if ( lsr & UART_LSR_THRE )
                        break;
                mdelay ( 1 );
        }

        /* Transmit data (even if we timed out) */
        uart_write ( uart, UART_THR, data );
}

/**
 * Flush data
 *
 * @v uart              UART
 */
void uart_flush ( struct uart *uart ) {
        unsigned int i;
        uint8_t lsr;

        /* Wait for transmitter and receiver to become empty */
        for ( i = 0 ; i < UART_TEMT_TIMEOUT_MS ; i++ ) {
                uart_read ( uart, UART_RBR );
                lsr = uart_read ( uart, UART_LSR );
                if ( ( lsr & UART_LSR_TEMT ) && ! ( lsr & UART_LSR_DR ) )
                        break;
        }
}

/**
 * Check for existence of UART
 *
 * @v uart              UART
 * @ret rc              Return status code
 */
int uart_exists ( struct uart *uart ) {

        /* Fail if no UART port is defined */
        if ( ! uart->base )
                return -ENODEV;

        /* Fail if UART scratch register seems not to be present */
        uart_write ( uart, UART_SCR, 0x18 );
        if ( uart_read ( uart, UART_SCR ) != 0x18 )
                return -ENODEV;
        uart_write ( uart, UART_SCR, 0xae );
        if ( uart_read ( uart, UART_SCR ) != 0xae )
                return -ENODEV;

        return 0;
}

/**
 * Initialise UART
 *
 * @v uart              UART
 * @v baud              Baud rate, or zero to leave unchanged
 * @v lcr               Line control register value, or zero to leave unchanged
 * @ret rc              Return status code
 */
int uart_init ( struct uart *uart, unsigned int baud, uint8_t lcr ) {
        uint8_t dlm;
        uint8_t dll;
        int rc;

        /* Check for existence of UART */
        if ( ( rc = uart_exists ( uart ) ) != 0 )
                return rc;

        /* Configure divisor and line control register, if applicable */
        if ( ! lcr )
                lcr = uart_read ( uart, UART_LCR );
        uart->lcr = lcr;
        uart_write ( uart, UART_LCR, ( lcr | UART_LCR_DLAB ) );
        if ( baud ) {
                uart->divisor = ( UART_MAX_BAUD / baud );
                dlm = ( ( uart->divisor >> 8 ) & 0xff );
                dll = ( ( uart->divisor >> 0 ) & 0xff );
                uart_write ( uart, UART_DLM, dlm );
                uart_write ( uart, UART_DLL, dll );
        } else {
                dlm = uart_read ( uart, UART_DLM );
                dll = uart_read ( uart, UART_DLL );
                uart->divisor = ( ( dlm << 8 ) | dll );
        }
        uart_write ( uart, UART_LCR, ( lcr & ~UART_LCR_DLAB ) );

        /* Disable interrupts */
        uart_write ( uart, UART_IER, 0 );

        /* Enable FIFOs */
        uart_write ( uart, UART_FCR, UART_FCR_FE );

        /* Assert DTR and RTS */
        uart_write ( uart, UART_MCR, ( UART_MCR_DTR | UART_MCR_RTS ) );

        /* Flush any stale data */
        uart_flush ( uart );

        return 0;
}