clem_smartport.c

#include "clem_smartport.h"
#include "clem_debug.h"
#include "clem_mmio_defs.h"

#include <inttypes.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

/* PH0 + PH2 ONLY */
#define CLEM_SMARTPORT_BUS_RESET_PHASE (1 + 4)
/* PH1 + PH3 */
#define CLEM_SMARTPORT_BUS_ENABLE_PHASE (2 + 8)

#define CLEM_SMARTPORT_UNIT_STATE_NULL  0x00000000
#define CLEM_SMARTPORT_UNIT_STATE_READY 0x00010000

/* sync bytes up to the packet begin byte*/
#define CLEM_SMARTPORT_UNIT_STATE_PACKET          0x00020000
#define CLEM_SMARTPORT_UNIT_STATE_PACKET_HEADER   0x00020001
#define CLEM_SMARTPORT_UNIT_STATE_PACKET_CONTENTS 0x00020002
#define CLEM_SMARTPORT_UNIT_STATE_PACKET_CHECKSUM 0x00020003
#define CLEM_SMARTPORT_UNIT_STATE_PACKET_END      0x00020004
#define CLEM_SMARTPORT_UNIT_STATE_PACKET_BAD      0x0002ffff
#define CLEM_SMARTPORT_UNIT_STATE_EXECUTING       0x00030000
#define CLEM_SMARTPORT_UNIT_STATE_RESPONSE        0x00040000
#define CLEM_SMARTPORT_UNIT_STATE_PENDING_DATA    0x00050000
#define CLEM_SMARTPORT_UNIT_STATE_EXPECT_DATA     0x00060000
#define CLEM_SMARTPORT_UNIT_STATE_COMPLETE        0x000f0000
#define CLEM_SMARTPORT_UNIT_STATE_ABORT_COMMAND   0x80000000

#define CLEM_SMARTPORT_BUS_WRITE    1
#define CLEM_SMARTPORT_BUS_READ     2
#define CLEM_SMARTPORT_BUS_DATA     4
#define CLEM_SMARTPORT_BUS_REQ      8
#define CLEM_SMARTPORT_BUS_WRITE_HI 16

//  Only for debugging.
// #define CLEM_SMARTPORT_FILE_LOGGING 1

#if CLEM_SMARTPORT_FILE_LOGGING
#define CLEM_SMARTPORT_DEBUG_RECORD_LIMIT 128
static FILE *s_clem_log_f = NULL;
struct ClemensSmartPortDebugRecord {
    uint64_t t;
    char code[8];
    unsigned unit;
    unsigned state;
    enum ClemensSmartPortPacketType packet_type;
    uint16_t packet_contents_length;
    uint8_t source_id;
    uint8_t dest_id;
    uint8_t data_reg;
    bool packet_is_extended;
    bool ack;
};
static struct ClemensSmartPortDebugRecord s_clem_debug_records[CLEM_SMARTPORT_DEBUG_RECORD_LIMIT];
static unsigned s_clem_debug_count;

static const char *s_packet_types[] = {"unk", "cmd", "stat", "data"};

static void _clem_debug_print(FILE *fp, struct ClemensSmartPortDebugRecord *record) {
    fprintf(fp, "[%20" PRIu64 "] [%u] %6s %04X %04X %3s: %02X", record->t, record->unit,
            record->code, (uint16_t)(record->state >> 16), (uint16_t)(record->state & 0xffff),
            record->ack ? "ACK" : "   ", record->data_reg);
    if ((record->state & 0xffff0000) == CLEM_SMARTPORT_UNIT_STATE_PACKET) {
        fprintf(fp, " , %4s (%02X->%02X) %d bytes (%s)", s_packet_types[record->packet_type],
                record->source_id, record->dest_id, record->packet_contents_length,
                record->packet_is_extended ? "ext" : "std");
    }
    fprintf(fp, "\n");
}

static void _clem_debug_flush(struct ClemensSmartPortUnit *unit) {
    unsigned i;
    for (i = 0; i < s_clem_debug_count; ++i) {
        struct ClemensSmartPortDebugRecord *record = &s_clem_debug_records[i];
        _clem_debug_print(s_clem_log_f, record);
    }
    fflush(s_clem_log_f);
    s_clem_debug_count = 0;
}

static void _clem_debug_record(struct ClemensSmartPortDebugRecord *record,
                               struct ClemensSmartPortUnit *unit, const char *prefix,
                               clem_clocks_time_t t) {
    record->t = t / CLEM_CLOCKS_14MHZ_CYCLE;
    strncpy(record->code, prefix, sizeof(record->code) - 1);
    record->unit = unit->unit_id;
    record->code[sizeof(record->code) - 1] = '\0';
    record->state = unit->packet_state;
    record->ack = unit->ack_hi;
    record->data_reg = unit->data_reg;
    record->packet_type = unit->packet.type;
    record->packet_contents_length = unit->packet.contents_length;
    record->source_id = unit->packet.source_unit_id;
    record->dest_id = unit->packet.dest_unit_id;
    record->packet_is_extended = unit->packet.is_extended;
    //_clem_iwm_debug_print(stdout, record);
}

static void _clem_debug_event(struct ClemensSmartPortUnit *unit, const char *prefix,
                              clem_clocks_time_t t) {
    struct ClemensSmartPortDebugRecord *record = &s_clem_debug_records[s_clem_debug_count];
    _clem_debug_record(record, unit, prefix, t);

    if (++s_clem_debug_count >= CLEM_SMARTPORT_DEBUG_RECORD_LIMIT) {
        _clem_debug_flush(unit);
    }
}

#define CLEM_SMARTPORT_DEBUG_EVENT(_unit_, _prefix_, _lvl_)                                        \
    if ((_unit_)->enable_debug && (_lvl_) <= (_unit_)->debug_level) {                              \
        _clem_debug_event(_unit_, _prefix_, (_unit_)->debug_ts);                                   \
    }

#else
#define CLEM_SMARTPORT_DEBUG_EVENT(_unit_, _prefix_, _lvl_)
#endif

static void _clem_debug_gate(struct ClemensSmartPortUnit *unit, clem_clocks_time_t ts) {
    unit->enable_debug = unit->enable_debug;
    unit->debug_ts = ts;
#if CLEM_SMARTPORT_FILE_LOGGING
    if (s_clem_log_f == NULL && unit->enable_debug) {
        s_clem_log_f = fopen("smartport.log", "wt");
        s_clem_debug_count = 0;
    } else if (s_clem_log_f != NULL && !unit->enable_debug) {
        _clem_debug_flush(unit);
        fclose(s_clem_log_f);
        s_clem_log_f = NULL;
    }
#endif
}

static inline void _clem_smartport_packet_state(struct ClemensSmartPortUnit *unit,
                                                unsigned packet_state) {
    if (unit->packet_state == packet_state)
        return;

    switch (packet_state) {
    case CLEM_SMARTPORT_UNIT_STATE_READY:
        unit->data_bit_count = 0;
        unit->data_reg = 0x00;
        unit->data_size = 0;
        unit->write_signal = 0;
        unit->command_id = 0xff;
        unit->ack_hi = true;
        break;
    case CLEM_SMARTPORT_UNIT_STATE_EXECUTING:
        unit->ack_hi = false;
        break;
    case CLEM_SMARTPORT_UNIT_STATE_RESPONSE:
        unit->data_bit_count = 0;
        unit->ack_hi = true;
        break;
    case CLEM_SMARTPORT_UNIT_STATE_EXPECT_DATA:
        unit->data_bit_count = 0;
        unit->ack_hi = true;
        break;
    case CLEM_SMARTPORT_UNIT_STATE_COMPLETE:
        unit->ack_hi = false;
        break;
    case CLEM_SMARTPORT_UNIT_STATE_PACKET_BAD:
        unit->ack_hi = false;
        break;
    }

    unit->packet_state = packet_state;
    unit->packet_state_byte_cnt = 0;
    CLEM_SMARTPORT_DEBUG_EVENT(unit, "STATE", 1);
}

static inline uint8_t _clem_smartport_encode_byte(uint8_t data, uint8_t *chksum) {
    data |= 0x80;
    *chksum ^= data;
    return data;
}

static void _clem_smartport_packet_decode_data(uint8_t *dest, unsigned dest_size,
                                               const uint8_t *src, unsigned src_groups,
                                               unsigned src_odd) {
    // the input buffer contains encoded chunks of data:
    //      every chunk contains the MSB of each 7-bit encoded byte in the group
    const uint8_t *src_groups_data = src + 1;
    uint8_t msbs = src[0] << 1;
    uint8_t *dest_data = dest;
    while (src_groups_data < (src + src_odd + 1) && dest_data <= (dest + dest_size)) {
        dest_data[0] = (*src_groups_data & 0x7f) | (msbs & 0x80);
        msbs <<= 1;
        ++src_groups_data;
        ++dest_data;
    }
    while (dest_data <= (dest + dest_size - 7)) {
        msbs = src_groups_data[0] << 1;
        dest_data[0] = (src_groups_data[1] & 0x7f) | (msbs & 0x80);
        msbs <<= 1;
        dest_data[1] = (src_groups_data[2] & 0x7f) | (msbs & 0x80);
        msbs <<= 1;
        dest_data[2] = (src_groups_data[3] & 0x7f) | (msbs & 0x80);
        msbs <<= 1;
        dest_data[3] = (src_groups_data[4] & 0x7f) | (msbs & 0x80);
        msbs <<= 1;
        dest_data[4] = (src_groups_data[5] & 0x7f) | (msbs & 0x80);
        msbs <<= 1;
        dest_data[5] = (src_groups_data[6] & 0x7f) | (msbs & 0x80);
        msbs <<= 1;
        dest_data[6] = (src_groups_data[7] & 0x7f) | (msbs & 0x80);
        src_groups_data += 8;
        dest_data += 7;
    }
}

static bool _clem_smartport_packet_encode_data(uint8_t *out, unsigned *out_limit,
                                               struct ClemensSmartPortPacket *packet,
                                               uint8_t unit_id) {
    unsigned out_size = 0;
    unsigned index;
    uint8_t chksum = 0x00;
    uint8_t odd_cnt = (uint8_t)(packet->contents_length % 7);
    uint8_t g7_cnt = (uint8_t)(packet->contents_length / 7);
    uint8_t tmp;
    uint16_t raw_contents_size;

    uint8_t *packet_src;

    //  sync bytes
    if (out_size + 6 > *out_limit) {
        *out_limit = out_size;
        return false;
    }
    out[out_size++] = 0xFF;
    out[out_size++] = 0x3F;
    out[out_size++] = 0xCF;
    out[out_size++] = 0xF3;
    out[out_size++] = 0xFC;
    out[out_size++] = 0xFF;

    if (out_size + 8 > *out_limit) {
        *out_limit = out_size;
        return false;
    }
    //  NOTE: checksum is performed on the encoded bytes (high bit on)
    //        which differs from how the checksum is calculated for the
    //        contents (see below.)
    out[out_size++] = 0xC3;
    out[out_size++] = _clem_smartport_encode_byte(0x00, &chksum);
    out[out_size++] = _clem_smartport_encode_byte(unit_id, &chksum);
    if (packet->type == kClemensSmartPortPacketType_Command) {
        tmp = 0x00;
    } else if (packet->type == kClemensSmartPortPacketType_Status) {
        tmp = 0x01;
    } else if (packet->type == kClemensSmartPortPacketType_Data) {
        tmp = 0x02;
    } else {
        tmp = 0x7F;
    }
    out[out_size++] = _clem_smartport_encode_byte(tmp, &chksum);
    if (packet->is_extended) {
        out[out_size++] = _clem_smartport_encode_byte(0x40, &chksum);
    } else {
        out[out_size++] = _clem_smartport_encode_byte(0x00, &chksum);
    }
    out[out_size++] = _clem_smartport_encode_byte(packet->status, &chksum);
    out[out_size++] = _clem_smartport_encode_byte(odd_cnt, &chksum);
    out[out_size++] = _clem_smartport_encode_byte(g7_cnt, &chksum);

    raw_contents_size = odd_cnt > 0 ? odd_cnt + 1 : 0;
    raw_contents_size += (g7_cnt * 8);
    packet_src = packet->contents;

    if (out_size + raw_contents_size > *out_limit) {
        *out_limit = out_size;
        return false;
    }
    //  NOTE: checksums only on the decoded contents, so perform the checksum
    //        calculation on the source bytes
    tmp = 0;
    for (index = 0; index < (unsigned)odd_cnt; ++index) {
        if (packet_src[index] & 0x80) {
            tmp |= (1 << (6 - index));
        }
    }
    if (odd_cnt > 0) {
        out[out_size++] = 0x80 | tmp;
        while (odd_cnt > 0) {
            out[out_size++] = 0x80 | packet_src[0];
            chksum ^= packet_src[0];
            ++packet_src;
            --odd_cnt;
        }
    }
    while (g7_cnt > 0) {
        tmp = (packet_src[0] & 0x80) >> 1;
        tmp |= ((packet_src[1] & 0x80) >> 2);
        tmp |= ((packet_src[2] & 0x80) >> 3);
        tmp |= ((packet_src[3] & 0x80) >> 4);
        tmp |= ((packet_src[4] & 0x80) >> 5);
        tmp |= ((packet_src[5] & 0x80) >> 6);
        tmp |= ((packet_src[6] & 0x80) >> 7);
        out[out_size++] = 0x80 | tmp;
        out[out_size++] = 0x80 | packet_src[0];
        chksum ^= packet_src[0];
        out[out_size++] = 0x80 | packet_src[1];
        chksum ^= packet_src[1];
        out[out_size++] = 0x80 | packet_src[2];
        chksum ^= packet_src[2];
        out[out_size++] = 0x80 | packet_src[3];
        chksum ^= packet_src[3];
        out[out_size++] = 0x80 | packet_src[4];
        chksum ^= packet_src[4];
        out[out_size++] = 0x80 | packet_src[5];
        chksum ^= packet_src[5];
        out[out_size++] = 0x80 | packet_src[6];
        chksum ^= packet_src[6];
        packet_src += 7;
        --g7_cnt;
    }

    if (out_size + 3 > *out_limit) {
        *out_limit = out_size;
        return false;
    }
    tmp = 0x80 | (chksum & 0x40) | 0x20 | (chksum & 0x10) | 0x08 | (chksum & 0x04) | 0x02 |
          (chksum & 0x1);
    out[out_size++] = tmp;
    tmp =
        (chksum & 0x80) | 0x40 | (chksum & 0x20) | 0x10 | (chksum & 0x08) | 0x04 | (chksum & 0x02);
    out[out_size++] = 0x80 | (tmp >> 1);
    out[out_size++] = 0xC8;

    *out_limit = out_size;

    return true;
}

static unsigned _clem_smartport_packet_encode_response(struct ClemensSmartPortUnit *unit,
                                                       uint8_t dest_unit_id, uint8_t status) {
    unit->packet.status = status;
    unit->packet.dest_unit_id = dest_unit_id;
    unit->packet.source_unit_id = unit->unit_id;
    unit->data_size = CLEM_SMARTPORT_DATA_BUFFER_LIMIT;
    if (!_clem_smartport_packet_encode_data(unit->data, &unit->data_size, &unit->packet,
                                            unit->unit_id)) {
        return CLEM_SMARTPORT_UNIT_STATE_ABORT_COMMAND;
    }
    return CLEM_SMARTPORT_UNIT_STATE_RESPONSE;
};

static unsigned _clem_smartport_handle_packet(struct ClemensSmartPortUnit *unit,
                                              unsigned delta_ns) {
    //  Parse the decoded packet based on the packet type (For commands, obtain the command type
    //  and parameter count, for example)
    unsigned next_state = unit->packet_state;
    unsigned u32;
    uint8_t call_status;

    if (unit->packet.type == kClemensSmartPortPacketType_Command) {
        switch (unit->command_id) {
        case CLEM_SMARTPORT_COMMAND_INIT:
            CLEM_DEBUG("SmartPort: [%02X] Init", unit->unit_id);
            unit->unit_id = unit->packet.dest_unit_id;
            unit->ph3_latch_lo = false;
            if (unit->device.do_reset) {
                /* This is not a necessary override */
                call_status = (*unit->device.do_reset)(&unit->device, delta_ns);
            } else {
                call_status = CLEM_SMARTPORT_STATUS_CODE_OK;
            }
            //  generate response
            next_state = _clem_smartport_packet_encode_response(unit, unit->packet.source_unit_id,
                                                                call_status);
            break;
        case CLEM_SMARTPORT_COMMAND_STATUS:
            CLEM_DEBUG("SmartPort: [%02X] Status", unit->unit_id);
            if (unit->device.do_status) {
                call_status = (*unit->device.do_status)(&unit->device, &unit->packet, delta_ns);
            } else {
                call_status = CLEM_SMARTPORT_STATUS_CODE_BAD_CTL;
            }
            unit->packet.type = kClemensSmartPortPacketType_Status;
            next_state = _clem_smartport_packet_encode_response(unit, unit->packet.source_unit_id,
                                                                call_status);
            break;
        case CLEM_SMARTPORT_COMMAND_READBLOCK:
            CLEM_DEBUG("SmartPort: [%02X] ReadBlock", unit->unit_id);
            u32 = ((unsigned)(unit->packet.contents[6]) << 16) |
                  ((unsigned)(unit->packet.contents[5]) << 8) | unit->packet.contents[4];
            if (unit->device.do_read_block) {
                call_status =
                    (*unit->device.do_read_block)(&unit->device, &unit->packet, u32, delta_ns);
            } else {
                call_status = CLEM_SMARTPORT_STATUS_CODE_OFFLINE;
            }
            if (call_status == CLEM_SMARTPORT_STATUS_CODE_OK) {
                unit->packet.type = kClemensSmartPortPacketType_Data;
            } else {
                unit->packet.type = kClemensSmartPortPacketType_Status;
            }
            next_state = _clem_smartport_packet_encode_response(unit, unit->packet.source_unit_id,
                                                                call_status);
            break;
        case CLEM_SMARTPORT_COMMAND_WRITEBLOCK:
            /* WriteBlock will expect two Host initiated transactions, and as such
               we allow the implementation to handle both transactions.  The implementation
               must assume this and return a response after the data transmission. */
            CLEM_DEBUG("SmartPort: [%02X] WriteBlock", unit->unit_id);
            u32 = ((unsigned)(unit->packet.contents[6]) << 16) |
                  ((unsigned)(unit->packet.contents[5]) << 8) | unit->packet.contents[4];
            if (unit->device.do_write_block) {
                (*unit->device.do_write_block)(&unit->device, &unit->packet, u32, delta_ns);
            }
            next_state = CLEM_SMARTPORT_UNIT_STATE_PENDING_DATA;
            break;
        case CLEM_SMARTPORT_COMMAND_FORMAT:
            if (unit->device.do_format) {
                call_status = (*unit->device.do_format)(&unit->device, &unit->packet, delta_ns);
            } else {
                call_status = CLEM_SMARTPORT_STATUS_CODE_OK;
            }
            next_state = _clem_smartport_packet_encode_response(unit, unit->packet.source_unit_id,
                                                                call_status);
            break;
        case CLEM_SMARTPORT_COMMAND_CONTROL:
            break;
        default:
            CLEM_WARN("SmartPort: [%02X] Unhandled command %02X", unit->unit_id, unit->command_id);
            next_state = CLEM_SMARTPORT_UNIT_STATE_ABORT_COMMAND;
            break;
        }
    } else {
        switch (unit->command_id) {
        case CLEM_SMARTPORT_COMMAND_WRITEBLOCK:
            if (unit->device.do_write_block) {
                call_status = (*unit->device.do_write_block)(&unit->device, &unit->packet,
                                                             0xffffffff, delta_ns);
            } else {
                call_status = CLEM_SMARTPORT_STATUS_CODE_OFFLINE;
            }
            unit->packet.type = kClemensSmartPortPacketType_Status;
            next_state = _clem_smartport_packet_encode_response(unit, unit->packet.source_unit_id,
                                                                call_status);
            break;
        }
    }
    return next_state;
}

static unsigned _clem_smartport_bus_handshake(struct ClemensSmartPortUnit *unit, unsigned bus_state,
                                              unsigned delta_ns) {
    uint8_t *data_tail;
    uint8_t *data_start;

    if (!(bus_state & CLEM_SMARTPORT_BUS_REQ)) {
        if (unit->packet_state == CLEM_SMARTPORT_UNIT_STATE_EXECUTING) {
            _clem_smartport_packet_state(unit, _clem_smartport_handle_packet(unit, delta_ns));
        } else if (unit->packet_state == CLEM_SMARTPORT_UNIT_STATE_COMPLETE) {
            _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_READY);
        } else if (unit->packet_state == CLEM_SMARTPORT_UNIT_STATE_PENDING_DATA) {
            _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_EXPECT_DATA);
        } else if (unit->packet_state == CLEM_SMARTPORT_UNIT_STATE_PACKET_BAD) {
            //  TODO: we should probably send a response or some type?
            _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_READY);
            printf("SmartPort: [%02X] received a bad packet\n", unit->unit_id);
        }
        return bus_state;
    }

    if (bus_state & CLEM_SMARTPORT_BUS_WRITE) {
        //  write into data buffer from the disk port
        bool data_signal = (bus_state & CLEM_SMARTPORT_BUS_DATA) != 0;
        if (data_signal != unit->write_signal) {
            unit->data_reg |= 1;
            bus_state |= CLEM_SMARTPORT_BUS_WRITE_HI;
        } else {
            unit->data_reg &= ~1;
        }
        unit->write_signal = data_signal;
        CLEM_SMARTPORT_DEBUG_EVENT(unit, "INBIT", 3)

        if (unit->data_bit_count > 0 || (unit->data_reg & 0x01)) {
            unit->data_bit_count++;
            if (unit->data_bit_count >= 8) {
                if (unit->data_size < CLEM_SMARTPORT_DATA_BUFFER_LIMIT) {
                    // printf("SmartPort => %02X\n", unit->data_reg);
                    unit->data[unit->data_size++] = unit->data_reg;
                    unit->packet_state_byte_cnt++;
                    CLEM_SMARTPORT_DEBUG_EVENT(unit, "INBYTE", 2)
                } else {
                    CLEM_LOG("SmartPort: Data overrun at unit %u, device %u", unit->unit_id,
                             unit->device.device_id);
                }
                unit->data_reg = 0;
                unit->data_bit_count = 0;
            } else {
                unit->data_reg <<= 1;
            }
        }
    } else if (unit->packet_state == CLEM_SMARTPORT_UNIT_STATE_RESPONSE) {
        bus_state &= ~CLEM_SMARTPORT_BUS_DATA;
        bus_state |= CLEM_SMARTPORT_BUS_READ;
        if (unit->data_bit_count == 0) {
            if (unit->packet_state_byte_cnt >= unit->data_size) {
                _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_COMPLETE);
                return bus_state;
            } else {
                unit->data_reg = unit->data[unit->packet_state_byte_cnt++];
                // printf("SmartPort <= %02X\n", unit->data_reg);
                unit->data_bit_count = 8;
            }
        }
        --unit->data_bit_count;
        if (unit->data_reg & 0x80) {
            bus_state |= CLEM_SMARTPORT_BUS_DATA;
        }
        unit->data_reg <<= 1;
        return bus_state;
    }

    if (unit->packet_state_byte_cnt < 1)
        return bus_state;

    data_start = unit->data + unit->data_size - unit->packet_state_byte_cnt;
    data_tail = unit->data + unit->data_size;

    switch (unit->packet_state) {
    case CLEM_SMARTPORT_UNIT_STATE_READY:
        _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_PACKET);
        break;
    case CLEM_SMARTPORT_UNIT_STATE_EXPECT_DATA:
        _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_PACKET);
        break;
    case CLEM_SMARTPORT_UNIT_STATE_PACKET:
        if (data_tail[-1] != 0xff) {
            if (data_tail[-1] == 0xC3) {
                //  save off the start of the actual packet data
                _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_PACKET_HEADER);
                memset(&unit->packet, 0, sizeof(unit->packet));
            } else {
                _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_PACKET_BAD);
                CLEM_WARN("SmartPort: sync byte expected but got %02X", data_tail[-1]);
            }
        }
        break;
    case CLEM_SMARTPORT_UNIT_STATE_PACKET_HEADER:
        if (unit->packet_state_byte_cnt == 7) {
            //  the last two bytes of the header contain the packet length in unencoded bytes
            //  we must calculate the encoded length of 7-bit bytes to pull in from the packet.
            _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_PACKET_CONTENTS);
            unit->packet.source_unit_id = data_start[1] & 0x7f;
            unit->packet.dest_unit_id = data_start[0] & 0x7f;
            if (data_start[2] == 0x80) {
                unit->packet.type = kClemensSmartPortPacketType_Command;
            } else if (data_start[2] == 0x81) {
                unit->packet.type = kClemensSmartPortPacketType_Status;
            } else if (data_start[2] == 0x82) {
                unit->packet.type = kClemensSmartPortPacketType_Data;
            }
            if (data_start[3] == 0xC0) {
                unit->packet.is_extended = true;
            }
            unit->packet.status = data_start[4] & 0x7f;
            //  get the decoded contents counts necessary to decode the odd and
            //  7 byte groups.
            unit->packet.contents_length = (data_tail[-1] & 0x7f) << 8;
            unit->packet.contents_length |= (data_tail[-2] & 0x7f);
            //  calculate encoded content length
            unit->packet_cntr = (((unsigned)(data_tail[-1]) & 0x7f) * 7) + (data_tail[-2] & 0x7f);
            unit->packet_cntr = (unit->packet_cntr * 8 + 6) / 7;
        }
        break;
    case CLEM_SMARTPORT_UNIT_STATE_PACKET_CONTENTS:
        if (unit->packet_state_byte_cnt == unit->packet_cntr) {
            _clem_smartport_packet_decode_data(unit->packet.contents, unit->packet_cntr, data_start,
                                               unit->packet.contents_length >> 8,
                                               unit->packet.contents_length & 0xff);
            // store off the actual length of the packet in bytes vs its group/odd count
            unit->packet.contents_length =
                (unit->packet.contents_length >> 8) * 7 + (unit->packet.contents_length & 0xff);
            _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_PACKET_CHECKSUM);
        }
        break;
    case CLEM_SMARTPORT_UNIT_STATE_PACKET_CHECKSUM:
        if (unit->packet_state_byte_cnt == 2) {
            //  16-bit encoded checksum (8-bit actual)
            //  calc checksum up to (data_size - 2)
            _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_PACKET_END);
        }
        break;
    case CLEM_SMARTPORT_UNIT_STATE_PACKET_END:
        if (data_tail[-1] == 0xC8) {
            //  Ignore
            //  unit->packet_ours = true;
            if (unit->packet.type == kClemensSmartPortPacketType_Command) {
                unit->command_id = unit->packet.contents[0];
            }
            if (!unit->unit_id && unit->command_id == CLEM_SMARTPORT_COMMAND_INIT) {
                _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_EXECUTING);
            } else if (unit->unit_id && unit->packet.dest_unit_id == unit->unit_id) {
                _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_EXECUTING);
            } else {
                //  TODO: implement logic to ignore the remaining command as this state
                //        change will still result in reading packet data.  Fortunately
                //        we only support one device on this bus, so it won't matter...
                //        YET
                //  unit->packet_ours = false;
                _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_READY);
            }
        } else {
            _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_PACKET_BAD);
        }
        break;
    }

    return bus_state;
}

/*
   Bus Reset starts the unit assignment process from the host.

   Each device will force PH30 on its *output* phase, which is passed to the
   next device on the daisy-chained disk port.

   The host will reenable the bus and assign a unit ID to the next available
   device in the chain. See do_enable() for details on subsequent events.

   To achieve this, on do_reset(), each device maintains an 'available' flag
   which is TRUE once it receives its ID assignment.  When 'available', the
   device no longer forces PH3 to 0 on its output and the next device will
   pick up on the PH3 as set by the host.
*/

/*
  Bus Enable marks that all 'available' bus residents can handle SmartPort
  commands.  See do_reset() for details on how a device becomes available.

  Much of the implementation follows the SmartPort protocol diagrams in
  Chapter 7 of the Apple IIgs firmware reference.

  Each device may have its own command handler.  They share common bus
  negotation logic with the host, which is provided below.
*/
bool clem_smartport_bus(struct ClemensSmartPortUnit *unit, unsigned unit_count, unsigned *io_flags,
                        unsigned *out_phase, clem_clocks_time_t ts, unsigned delta_ns) {
    struct ClemensSmartPortUnit *unit_end = unit + unit_count;
    unsigned select_bits = *out_phase;
    unsigned bus_state = 0;
    bool is_bus_enabled = false;
    bool is_ack_hi = false;

    _clem_debug_gate(unit, ts);

    for (; unit < unit_end; unit++) {
        if (unit->device.device_id == 0)
            continue;
        if (select_bits == CLEM_SMARTPORT_BUS_RESET_PHASE) {
            unit->unit_id = 0x00;
            unit->ph3_latch_lo = 1;
            unit->bus_enabled = false;
        } else if ((select_bits & CLEM_SMARTPORT_BUS_ENABLE_PHASE) ==
                   CLEM_SMARTPORT_BUS_ENABLE_PHASE) {
            if (!unit->bus_enabled) {
                unit->bus_enabled = true;
                _clem_smartport_packet_state(unit, CLEM_SMARTPORT_UNIT_STATE_READY);
            }
            if ((select_bits & 1) != 0) {
                bus_state |= CLEM_SMARTPORT_BUS_REQ;
            }
            if (*io_flags & CLEM_IWM_FLAG_WRITE_REQUEST) {
                bus_state |= CLEM_SMARTPORT_BUS_WRITE;
                if (*io_flags & CLEM_IWM_FLAG_WRITE_DATA) {
                    bus_state |= CLEM_SMARTPORT_BUS_DATA;
                }
            }
            if (delta_ns > 0) {
                bus_state = _clem_smartport_bus_handshake(unit, bus_state, delta_ns);
            }
        } else {
            unit->bus_enabled = false;
        }
        if (unit->ph3_latch_lo) {
            // PH3 OFF for downstream devices
            // This should only occur after a bus reset for a device that hasn't
            // received its ID from the hsot
            select_bits &= ~8;
        }
        is_bus_enabled = unit->bus_enabled;
        is_ack_hi = unit->ack_hi;
        unit++;
    }

    if (!is_bus_enabled) {
        is_bus_enabled =
            (select_bits & CLEM_SMARTPORT_BUS_ENABLE_PHASE) == CLEM_SMARTPORT_BUS_ENABLE_PHASE;
        is_ack_hi = is_bus_enabled;
    }

    if (is_bus_enabled) {
        if (is_ack_hi) {
            *io_flags |= CLEM_IWM_FLAG_WRPROTECT_SENSE;
        } else {
            *io_flags &= ~CLEM_IWM_FLAG_WRPROTECT_SENSE;
        }

        *io_flags &= ~CLEM_IWM_FLAG_READ_DATA;
        if (bus_state & CLEM_SMARTPORT_BUS_READ) {
            if (bus_state & CLEM_SMARTPORT_BUS_DATA) {
                *io_flags |= CLEM_IWM_FLAG_READ_DATA;
            }
        }
        if (bus_state & CLEM_SMARTPORT_BUS_WRITE_HI) {
            *io_flags |= CLEM_IWM_FLAG_WRITE_HI;
        }

        //    printf("SmartPort: REQ: %u,  ACK: %u\n", (select_bits & 1) != 0, is_ack_hi);
    }

    return is_bus_enabled;
}