// SPDX-License-Identifier: GPL-2.0+
/*
 * Copyright (C) 2009 by Bart Hartgers (bart.hartgers+ark3116@gmail.com)
 * Original version:
 * Copyright (C) 2006
 *   Simon Schulz (ark3116_driver <at> auctionant.de)
 *
 * ark3116
 * - implements a driver for the arkmicro ark3116 chipset (vendor=0x6547,
 *   productid=0x0232) (used in a datacable called KQ-U8A)
 *
 * Supports full modem status lines, break, hardware flow control. Does not
 * support software flow control, since I do not know how to enable it in hw.
 *
 * This driver is a essentially new implementation. I initially dug
 * into the old ark3116.c driver and suddenly realized the ark3116 is
 * a 16450 with a USB interface glued to it. See comments at the
 * bottom of this file.
 */

#include <linux/kernel.h>
#include <linux/ioctl.h>
#include <linux/tty.h>
#include <linux/slab.h>
#include <linux/tty_flip.h>
#include <linux/module.h>
#include <linux/usb.h>
#include <linux/usb/serial.h>
#include <linux/serial.h>
#include <linux/serial_reg.h>
#include <linux/uaccess.h>
#include <linux/mutex.h>
#include <linux/spinlock.h>

#define DRIVER_AUTHOR "Bart Hartgers <bart.hartgers+ark3116@gmail.com>"
#define DRIVER_DESC "USB ARK3116 serial/IrDA driver"
#define DRIVER_DEV_DESC "ARK3116 RS232/IrDA"
#define DRIVER_NAME "ark3116"

/* usb timeout of 1 second */
#define ARK_TIMEOUT 1000

static const struct usb_device_id id_table[] = {
        { USB_DEVICE(0x6547, 0x0232) },
        { USB_DEVICE(0x18ec, 0x3118) },         /* USB to IrDA adapter */
        { },
};
MODULE_DEVICE_TABLE(usb, id_table);

static int is_irda(struct usb_serial *serial)
{
        struct usb_device *dev = serial->dev;
        if (le16_to_cpu(dev->descriptor.idVendor) == 0x18ec &&
                        le16_to_cpu(dev->descriptor.idProduct) == 0x3118)
                return 1;
        return 0;
}

struct ark3116_private {
        int                     irda;   /* 1 for irda device */

        /* protects hw register updates */
        struct mutex            hw_lock;

        int                     quot;   /* baudrate divisor */
        __u32                   lcr;    /* line control register value */
        __u32                   hcr;    /* handshake control register (0x8)
                                         * value */
        __u32                   mcr;    /* modem control register value */

        /* protects the status values below */
        spinlock_t              status_lock;
        __u32                   msr;    /* modem status register value */
        __u32                   lsr;    /* line status register value */
};

static int ark3116_write_reg(struct usb_serial *serial,
                             unsigned reg, __u8 val)
{
        int result;
         /* 0xfe 0x40 are magic values taken from original driver */
        result = usb_control_msg(serial->dev,
                                 usb_sndctrlpipe(serial->dev, 0),
                                 0xfe, 0x40, val, reg,
                                 NULL, 0, ARK_TIMEOUT);
        if (result)
                return result;

        return 0;
}

static int ark3116_read_reg(struct usb_serial *serial,
                            unsigned reg, unsigned char *buf)
{
        int result;
        /* 0xfe 0xc0 are magic values taken from original driver */
        result = usb_control_msg(serial->dev,
                                 usb_rcvctrlpipe(serial->dev, 0),
                                 0xfe, 0xc0, 0, reg,
                                 buf, 1, ARK_TIMEOUT);
        if (result < 1) {
                dev_err(&serial->interface->dev,
                                "failed to read register %u: %d\n",
                                reg, result);
                if (result >= 0)
                        result = -EIO;

                return result;
        }

        return 0;
}

static inline int calc_divisor(int bps)
{
        /* Original ark3116 made some exceptions in rounding here
         * because windows did the same. Assume that is not really
         * necessary.
         * Crystal is 12MHz, probably because of USB, but we divide by 4?
         */
        return (12000000 + 2*bps) / (4*bps);
}

static int ark3116_port_probe(struct usb_serial_port *port)
{
        struct usb_serial *serial = port->serial;
        struct ark3116_private *priv;

        priv = kzalloc(sizeof(*priv), GFP_KERNEL);
        if (!priv)
                return -ENOMEM;

        mutex_init(&priv->hw_lock);
        spin_lock_init(&priv->status_lock);

        priv->irda = is_irda(serial);

        usb_set_serial_port_data(port, priv);

        /* setup the hardware */
        ark3116_write_reg(serial, UART_IER, 0);
        /* disable DMA */
        ark3116_write_reg(serial, UART_FCR, 0);
        /* handshake control */
        priv->hcr = 0;
        ark3116_write_reg(serial, 0x8     , 0);
        /* modem control */
        priv->mcr = 0;
        ark3116_write_reg(serial, UART_MCR, 0);

        if (!(priv->irda)) {
                ark3116_write_reg(serial, 0xb , 0);
        } else {
                ark3116_write_reg(serial, 0xb , 1);
                ark3116_write_reg(serial, 0xc , 0);
                ark3116_write_reg(serial, 0xd , 0x41);
                ark3116_write_reg(serial, 0xa , 1);
        }

        /* setup baudrate */
        ark3116_write_reg(serial, UART_LCR, UART_LCR_DLAB);

        /* setup for 9600 8N1 */
        priv->quot = calc_divisor(9600);
        ark3116_write_reg(serial, UART_DLL, priv->quot & 0xff);
        ark3116_write_reg(serial, UART_DLM, (priv->quot>>8) & 0xff);

        priv->lcr = UART_LCR_WLEN8;
        ark3116_write_reg(serial, UART_LCR, UART_LCR_WLEN8);

        ark3116_write_reg(serial, 0xe, 0);

        if (priv->irda)
                ark3116_write_reg(serial, 0x9, 0);

        dev_info(&port->dev, "using %s mode\n", priv->irda ? "IrDA" : "RS232");

        return 0;
}

static int ark3116_port_remove(struct usb_serial_port *port)
{
        struct ark3116_private *priv = usb_get_serial_port_data(port);

        /* device is closed, so URBs and DMA should be down */
        mutex_destroy(&priv->hw_lock);
        kfree(priv);

        return 0;
}

static void ark3116_set_termios(struct tty_struct *tty,
                                struct usb_serial_port *port,
                                struct ktermios *old_termios)
{
        struct usb_serial *serial = port->serial;
        struct ark3116_private *priv = usb_get_serial_port_data(port);
        struct ktermios *termios = &tty->termios;
        unsigned int cflag = termios->c_cflag;
        int bps = tty_get_baud_rate(tty);
        int quot;
        __u8 lcr, hcr, eval;

        /* set data bit count */
        switch (cflag & CSIZE) {
        case CS5:
                lcr = UART_LCR_WLEN5;
                break;
        case CS6:
                lcr = UART_LCR_WLEN6;
                break;
        case CS7:
                lcr = UART_LCR_WLEN7;
                break;
        default:
        case CS8:
                lcr = UART_LCR_WLEN8;
                break;
        }
        if (cflag & CSTOPB)
                lcr |= UART_LCR_STOP;
        if (cflag & PARENB)
                lcr |= UART_LCR_PARITY;
        if (!(cflag & PARODD))
                lcr |= UART_LCR_EPAR;
#ifdef CMSPAR
        if (cflag & CMSPAR)
                lcr |= UART_LCR_SPAR;
#endif
        /* handshake control */
        hcr = (cflag & CRTSCTS) ? 0x03 : 0x00;

        /* calc baudrate */
        dev_dbg(&port->dev, "%s - setting bps to %d\n", __func__, bps);
        eval = 0;
        switch (bps) {
        case 0:
                quot = calc_divisor(9600);
                break;
        default:
                if ((bps < 75) || (bps > 3000000))
                        bps = 9600;
                quot = calc_divisor(bps);
                break;
        case 460800:
                eval = 1;
                quot = calc_divisor(bps);
                break;
        case 921600:
                eval = 2;
                quot = calc_divisor(bps);
                break;
        }

        /* Update state: synchronize */
        mutex_lock(&priv->hw_lock);

        /* keep old LCR_SBC bit */
        lcr |= (priv->lcr & UART_LCR_SBC);

        dev_dbg(&port->dev, "%s - setting hcr:0x%02x,lcr:0x%02x,quot:%d\n",
                __func__, hcr, lcr, quot);

        /* handshake control */
        if (priv->hcr != hcr) {
                priv->hcr = hcr;
                ark3116_write_reg(serial, 0x8, hcr);
        }

        /* baudrate */
        if (priv->quot != quot) {
                priv->quot = quot;
                priv->lcr = lcr; /* need to write lcr anyway */

                /* disable DMA since transmit/receive is
                 * shadowed by UART_DLL
                 */
                ark3116_write_reg(serial, UART_FCR, 0);

                ark3116_write_reg(serial, UART_LCR,
                                  lcr|UART_LCR_DLAB);
                ark3116_write_reg(serial, UART_DLL, quot & 0xff);
                ark3116_write_reg(serial, UART_DLM, (quot>>8) & 0xff);

                /* restore lcr */
                ark3116_write_reg(serial, UART_LCR, lcr);
                /* magic baudrate thingy: not sure what it does,
                 * but windows does this as well.
                 */
                ark3116_write_reg(serial, 0xe, eval);

                /* enable DMA */
                ark3116_write_reg(serial, UART_FCR, UART_FCR_DMA_SELECT);
        } else if (priv->lcr != lcr) {
                priv->lcr = lcr;
                ark3116_write_reg(serial, UART_LCR, lcr);
        }

        mutex_unlock(&priv->hw_lock);

        /* check for software flow control */
        if (I_IXOFF(tty) || I_IXON(tty)) {
                dev_warn(&port->dev,
                                "software flow control not implemented\n");
        }

        /* Don't rewrite B0 */
        if (tty_termios_baud_rate(termios))
                tty_termios_encode_baud_rate(termios, bps, bps);
}

static void ark3116_close(struct usb_serial_port *port)
{
        struct usb_serial *serial = port->serial;

        /* disable DMA */
        ark3116_write_reg(serial, UART_FCR, 0);

        /* deactivate interrupts */
        ark3116_write_reg(serial, UART_IER, 0);

        usb_serial_generic_close(port);

        usb_kill_urb(port->interrupt_in_urb);
}

static int ark3116_open(struct tty_struct *tty, struct usb_serial_port *port)
{
        struct ark3116_private *priv = usb_get_serial_port_data(port);
        struct usb_serial *serial = port->serial;
        unsigned char *buf;
        int result;

        buf = kmalloc(1, GFP_KERNEL);
        if (buf == NULL)
                return -ENOMEM;

        result = usb_serial_generic_open(tty, port);
        if (result) {
                dev_dbg(&port->dev,
                        "%s - usb_serial_generic_open failed: %d\n",
                        __func__, result);
                goto err_free;
        }

        /* remove any data still left: also clears error state */
        ark3116_read_reg(serial, UART_RX, buf);

        /* read modem status */
        result = ark3116_read_reg(serial, UART_MSR, buf);
        if (result)
                goto err_close;
        priv->msr = *buf;

        /* read line status */
        result = ark3116_read_reg(serial, UART_LSR, buf);
        if (result)
                goto err_close;
        priv->lsr = *buf;

        result = usb_submit_urb(port->interrupt_in_urb, GFP_KERNEL);
        if (result) {
                dev_err(&port->dev, "submit irq_in urb failed %d\n",
                        result);
                goto err_close;
        }

        /* activate interrupts */
        ark3116_write_reg(port->serial, UART_IER, UART_IER_MSI|UART_IER_RLSI);

        /* enable DMA */
        ark3116_write_reg(port->serial, UART_FCR, UART_FCR_DMA_SELECT);

        /* setup termios */
        if (tty)
                ark3116_set_termios(tty, port, NULL);

        kfree(buf);

        return 0;

err_close:
        usb_serial_generic_close(port);
err_free:
        kfree(buf);

        return result;
}

static int ark3116_tiocmget(struct tty_struct *tty)
{
        struct usb_serial_port *port = tty->driver_data;
        struct ark3116_private *priv = usb_get_serial_port_data(port);
        __u32 status;
        __u32 ctrl;
        unsigned long flags;

        mutex_lock(&priv->hw_lock);
        ctrl = priv->mcr;
        mutex_unlock(&priv->hw_lock);

        spin_lock_irqsave(&priv->status_lock, flags);
        status = priv->msr;
        spin_unlock_irqrestore(&priv->status_lock, flags);

        return  (status & UART_MSR_DSR  ? TIOCM_DSR  : 0) |
                (status & UART_MSR_CTS  ? TIOCM_CTS  : 0) |
                (status & UART_MSR_RI   ? TIOCM_RI   : 0) |
                (status & UART_MSR_DCD  ? TIOCM_CD   : 0) |
                (ctrl   & UART_MCR_DTR  ? TIOCM_DTR  : 0) |
                (ctrl   & UART_MCR_RTS  ? TIOCM_RTS  : 0) |
                (ctrl   & UART_MCR_OUT1 ? TIOCM_OUT1 : 0) |
                (ctrl   & UART_MCR_OUT2 ? TIOCM_OUT2 : 0);
}

static int ark3116_tiocmset(struct tty_struct *tty,
                        unsigned set, unsigned clr)
{
        struct usb_serial_port *port = tty->driver_data;
        struct ark3116_private *priv = usb_get_serial_port_data(port);

        /* we need to take the mutex here, to make sure that the value
         * in priv->mcr is actually the one that is in the hardware
         */

        mutex_lock(&priv->hw_lock);

        if (set & TIOCM_RTS)
                priv->mcr |= UART_MCR_RTS;
        if (set & TIOCM_DTR)
                priv->mcr |= UART_MCR_DTR;
        if (set & TIOCM_OUT1)
                priv->mcr |= UART_MCR_OUT1;
        if (set & TIOCM_OUT2)
                priv->mcr |= UART_MCR_OUT2;
        if (clr & TIOCM_RTS)
                priv->mcr &= ~UART_MCR_RTS;
        if (clr & TIOCM_DTR)
                priv->mcr &= ~UART_MCR_DTR;
        if (clr & TIOCM_OUT1)
                priv->mcr &= ~UART_MCR_OUT1;
        if (clr & TIOCM_OUT2)
                priv->mcr &= ~UART_MCR_OUT2;

        ark3116_write_reg(port->serial, UART_MCR, priv->mcr);

        mutex_unlock(&priv->hw_lock);

        return 0;
}

static void ark3116_break_ctl(struct tty_struct *tty, int break_state)
{
        struct usb_serial_port *port = tty->driver_data;
        struct ark3116_private *priv = usb_get_serial_port_data(port);

        /* LCR is also used for other things: protect access */
        mutex_lock(&priv->hw_lock);

        if (break_state)
                priv->lcr |= UART_LCR_SBC;
        else
                priv->lcr &= ~UART_LCR_SBC;

        ark3116_write_reg(port->serial, UART_LCR, priv->lcr);

        mutex_unlock(&priv->hw_lock);
}

static void ark3116_update_msr(struct usb_serial_port *port, __u8 msr)
{
        struct ark3116_private *priv = usb_get_serial_port_data(port);
        unsigned long flags;

        spin_lock_irqsave(&priv->status_lock, flags);
        priv->msr = msr;
        spin_unlock_irqrestore(&priv->status_lock, flags);

        if (msr & UART_MSR_ANY_DELTA) {
                /* update input line counters */
                if (msr & UART_MSR_DCTS)
                        port->icount.cts++;
                if (msr & UART_MSR_DDSR)
                        port->icount.dsr++;
                if (msr & UART_MSR_DDCD)
                        port->icount.dcd++;
                if (msr & UART_MSR_TERI)
                        port->icount.rng++;
                wake_up_interruptible(&port->port.delta_msr_wait);
        }
}

static void ark3116_update_lsr(struct usb_serial_port *port, __u8 lsr)
{
        struct ark3116_private *priv = usb_get_serial_port_data(port);
        unsigned long flags;

        spin_lock_irqsave(&priv->status_lock, flags);
        /* combine bits */
        priv->lsr |= lsr;
        spin_unlock_irqrestore(&priv->status_lock, flags);

        if (lsr&UART_LSR_BRK_ERROR_BITS) {
                if (lsr & UART_LSR_BI)
                        port->icount.brk++;
                if (lsr & UART_LSR_FE)
                        port->icount.frame++;
                if (lsr & UART_LSR_PE)
                        port->icount.parity++;
                if (lsr & UART_LSR_OE)
                        port->icount.overrun++;
        }
}

static void ark3116_read_int_callback(struct urb *urb)
{
        struct usb_serial_port *port = urb->context;
        int status = urb->status;
        const __u8 *data = urb->transfer_buffer;
        int result;

        switch (status) {
        case -ECONNRESET:
        case -ENOENT:
        case -ESHUTDOWN:
                /* this urb is terminated, clean up */
                dev_dbg(&port->dev, "%s - urb shutting down with status: %d\n",
                        __func__, status);
                return;
        default:
                dev_dbg(&port->dev, "%s - nonzero urb status received: %d\n",
                        __func__, status);
                break;
        case 0: /* success */
                /* discovered this by trail and error... */
                if ((urb->actual_length == 4) && (data[0] == 0xe8)) {
                        const __u8 id = data[1]&UART_IIR_ID;
                        dev_dbg(&port->dev, "%s: iir=%02x\n", __func__, data[1]);
                        if (id == UART_IIR_MSI) {
                                dev_dbg(&port->dev, "%s: msr=%02x\n",
                                        __func__, data[3]);
                                ark3116_update_msr(port, data[3]);
                                break;
                        } else if (id == UART_IIR_RLSI) {
                                dev_dbg(&port->dev, "%s: lsr=%02x\n",
                                        __func__, data[2]);
                                ark3116_update_lsr(port, data[2]);
                                break;
                        }
                }
                /*
                 * Not sure what this data meant...
                 */
                usb_serial_debug_data(&port->dev, __func__,
                                      urb->actual_length,
                                      urb->transfer_buffer);
                break;
        }

        result = usb_submit_urb(urb, GFP_ATOMIC);
        if (result)
                dev_err(&port->dev, "failed to resubmit interrupt urb: %d\n",
                        result);
}


/* Data comes in via the bulk (data) URB, errors/interrupts via the int URB.
 * This means that we cannot be sure which data byte has an associated error
 * condition, so we report an error for all data in the next bulk read.
 *
 * Actually, there might even be a window between the bulk data leaving the
 * ark and reading/resetting the lsr in the read_bulk_callback where an
 * interrupt for the next data block could come in.
 * Without somekind of ordering on the ark, we would have to report the
 * error for the next block of data as well...
 * For now, let's pretend this can't happen.
 */
static void ark3116_process_read_urb(struct urb *urb)
{
        struct usb_serial_port *port = urb->context;
        struct ark3116_private *priv = usb_get_serial_port_data(port);
        unsigned char *data = urb->transfer_buffer;
        char tty_flag = TTY_NORMAL;
        unsigned long flags;
        __u32 lsr;

        /* update line status */
        spin_lock_irqsave(&priv->status_lock, flags);
        lsr = priv->lsr;
        priv->lsr &= ~UART_LSR_BRK_ERROR_BITS;
        spin_unlock_irqrestore(&priv->status_lock, flags);

        if (!urb->actual_length)
                return;

        if (lsr & UART_LSR_BRK_ERROR_BITS) {
                if (lsr & UART_LSR_BI)
                        tty_flag = TTY_BREAK;
                else if (lsr & UART_LSR_PE)
                        tty_flag = TTY_PARITY;
                else if (lsr & UART_LSR_FE)
                        tty_flag = TTY_FRAME;

                /* overrun is special, not associated with a char */
                if (lsr & UART_LSR_OE)
                        tty_insert_flip_char(&port->port, 0, TTY_OVERRUN);
        }
        tty_insert_flip_string_fixed_flag(&port->port, data, tty_flag,
                                                        urb->actual_length);
        tty_flip_buffer_push(&port->port);
}

static struct usb_serial_driver ark3116_device = {
        .driver = {
                .owner =        THIS_MODULE,
                .name =         "ark3116",
        },
        .id_table =             id_table,
        .num_ports =            1,
        .num_bulk_in =          1,
        .num_bulk_out =         1,
        .num_interrupt_in =     1,
        .port_probe =           ark3116_port_probe,
        .port_remove =          ark3116_port_remove,
        .set_termios =          ark3116_set_termios,
        .tiocmget =             ark3116_tiocmget,
        .tiocmset =             ark3116_tiocmset,
        .tiocmiwait =           usb_serial_generic_tiocmiwait,
        .get_icount =           usb_serial_generic_get_icount,
        .open =                 ark3116_open,
        .close =                ark3116_close,
        .break_ctl =            ark3116_break_ctl,
        .read_int_callback =    ark3116_read_int_callback,
        .process_read_urb =     ark3116_process_read_urb,
};

static struct usb_serial_driver * const serial_drivers[] = {
        &ark3116_device, NULL
};

module_usb_serial_driver(serial_drivers, id_table);

MODULE_LICENSE("GPL");

MODULE_AUTHOR(DRIVER_AUTHOR);
MODULE_DESCRIPTION(DRIVER_DESC);

/*
 * The following describes what I learned from studying the old
 * ark3116.c driver, disassembling the windows driver, and some lucky
 * guesses. Since I do not have any datasheet or other
 * documentation, inaccuracies are almost guaranteed.
 *
 * Some specs for the ARK3116 can be found here:
 * http://web.archive.org/web/20060318000438/
 *   www.arkmicro.com/en/products/view.php?id=10
 * On that page, 2 GPIO pins are mentioned: I assume these are the
 * OUT1 and OUT2 pins of the UART, so I added support for those
 * through the MCR. Since the pins are not available on my hardware,
 * I could not verify this.
 * Also, it states there is "on-chip hardware flow control". I have
 * discovered how to enable that. Unfortunately, I do not know how to
 * enable XON/XOFF (software) flow control, which would need support
 * from the chip as well to work. Because of the wording on the web
 * page there is a real possibility the chip simply does not support
 * software flow control.
 *
 * I got my ark3116 as part of a mobile phone adapter cable. On the
 * PCB, the following numbered contacts are present:
 *
 *  1:- +5V
 *  2:o DTR
 *  3:i RX
 *  4:i DCD
 *  5:o RTS
 *  6:o TX
 *  7:i RI
 *  8:i DSR
 * 10:- 0V
 * 11:i CTS
 *
 * On my chip, all signals seem to be 3.3V, but 5V tolerant. But that
 * may be different for the one you have ;-).
 *
 * The windows driver limits the registers to 0-F, so I assume there
 * are actually 16 present on the device.
 *
 * On an UART interrupt, 4 bytes of data come in on the interrupt
 * endpoint. The bytes are 0xe8 IIR LSR MSR.
 *
 * The baudrate seems to be generated from the 12MHz crystal, using
 * 4-times subsampling. So quot=12e6/(4*baud). Also see description
 * of register E.
 *
 * Registers 0-7:
 * These seem to be the same as for a regular 16450. The FCR is set
 * to UART_FCR_DMA_SELECT (0x8), I guess to enable transfers between
 * the UART and the USB bridge/DMA engine.
 *
 * Register 8:
 * By trial and error, I found out that bit 0 enables hardware CTS,
 * stopping TX when CTS is +5V. Bit 1 does the same for RTS, making
 * RTS +5V when the 3116 cannot transfer the data to the USB bus
 * (verified by disabling the reading URB). Note that as far as I can
 * tell, the windows driver does NOT use this, so there might be some
 * hardware bug or something.
 *
 * According to a patch provided here
 * https://lore.kernel.org/lkml/200907261419.50702.linux@rainbow-software.org
 * the ARK3116 can also be used as an IrDA dongle. Since I do not have
 * such a thing, I could not investigate that aspect. However, I can
 * speculate ;-).
 *
 * - IrDA encodes data differently than RS232. Most likely, one of
 *   the bits in registers 9..E enables the IR ENDEC (encoder/decoder).
 * - Depending on the IR transceiver, the input and output need to be
 *   inverted, so there are probably bits for that as well.
 * - IrDA is half-duplex, so there should be a bit for selecting that.
 *
 * This still leaves at least two registers unaccounted for. Perhaps
 * The chip can do XON/XOFF or CRC in HW?
 *
 * Register 9:
 * Set to 0x00 for IrDA, when the baudrate is initialised.
 *
 * Register A:
 * Set to 0x01 for IrDA, at init.
 *
 * Register B:
 * Set to 0x01 for IrDA, 0x00 for RS232, at init.
 *
 * Register C:
 * Set to 00 for IrDA, at init.
 *
 * Register D:
 * Set to 0x41 for IrDA, at init.
 *
 * Register E:
 * Somekind of baudrate override. The windows driver seems to set
 * this to 0x00 for normal baudrates, 0x01 for 460800, 0x02 for 921600.
 * Since 460800 and 921600 cannot be obtained by dividing 3MHz by an integer,
 * it could be somekind of subdivisor thingy.
 * However,it does not seem to do anything: selecting 921600 (divisor 3,
 * reg E=2), still gets 1 MHz. I also checked if registers 9, C or F would
 * work, but they don't.
 *
 * Register F: unknown
 */