m1n1/src/uartproxy.c

/* SPDX-License-Identifier: MIT */

#include "uartproxy.h"
#include "exception.h"
#include "iodev.h"
#include "proxy.h"
#include "string.h"
#include "types.h"
#include "utils.h"

#define REQ_SIZE 64

typedef struct {
    u32 _pad;
    u32 type;
    union {
        ProxyRequest prequest;
        struct {
            u64 addr;
            u64 size;
            u32 dchecksum;
        } mrequest;
    };
    u32 checksum;
} UartRequest;

#define REPLY_SIZE 36

typedef struct {
    u32 type;
    s32 status;
    union {
        ProxyReply preply;
        struct {
            u32 dchecksum;
        } mreply;
    };
    u32 checksum;
} UartReply;

#define REQ_NOP      0x00AA55FF
#define REQ_PROXY    0x01AA55FF
#define REQ_MEMREAD  0x02AA55FF
#define REQ_MEMWRITE 0x03AA55FF
#define REQ_BOOT     0x04AA55FF

#define ST_OK      0
#define ST_BADCMD  -1
#define ST_INVAL   -2
#define ST_XFRERR  -3
#define ST_CSUMERR -4

static u32 iodev_proxy_buffer[IODEV_MAX];

// I just totally pulled this out of my arse
// Noinline so that this can be bailed out by exc_guard = EXC_RETURN
// We assume this function does not use the stack
static u64 __attribute__((noinline)) checksum(void *start, u32 length)
{
    u32 sum = 0xDEADBEEF;
    u8 *d = (u8 *)start;

    while (length--) {
        sum *= 31337;
        sum += (*d++) ^ 0x5A;
    }
    return sum ^ 0xADDEDBAD;
}

iodev_id_t uartproxy_iodev;

void uartproxy_run(void)
{
    int running = 1;
    size_t bytes;
    u64 checksum_val;

    iodev_id_t iodev = IODEV_MAX;

    UartRequest request;
    UartReply reply = {REQ_BOOT};
    reply.checksum = checksum(&reply, REPLY_SIZE - 4);

    // Startup notification only goes out via UART
    iodev_write(IODEV_UART, &reply, REPLY_SIZE);

    while (running) {
        for (iodev = 0; iodev < IODEV_MAX;) {
            u8 b;
            iodev_handle_events(iodev);
            if (iodev_can_read(iodev) && iodev_read(iodev, &b, 1) == 1) {
                iodev_proxy_buffer[iodev] >>= 8;
                iodev_proxy_buffer[iodev] |= b << 24;
                if ((iodev_proxy_buffer[iodev] & 0xffffff) == 0xAA55FF)
                    break;
            }
            iodev++;
            if (iodev == IODEV_MAX)
                iodev = 0;
        }

        memset(&request, 0, sizeof(request));
        request.type = iodev_proxy_buffer[iodev];
        bytes = iodev_read(iodev, (&request.type) + 1, REQ_SIZE - 4);
        if (bytes != REQ_SIZE - 4)
            continue;

        if (checksum(&(request.type), REQ_SIZE - 4) != request.checksum) {
            memset(&reply, 0, sizeof(reply));
            reply.type = request.type;
            reply.status = ST_CSUMERR;
            reply.checksum = checksum(&reply, REPLY_SIZE - 4);
            iodev_write(iodev, &reply, REPLY_SIZE);
            continue;
        }

        memset(&reply, 0, sizeof(reply));
        reply.type = request.type;
        reply.status = ST_OK;

        uartproxy_iodev = iodev;

        switch (request.type) {
            case REQ_NOP:
                break;
            case REQ_PROXY:
                running = proxy_process(&request.prequest, &reply.preply);
                break;
            case REQ_MEMREAD:
                if (request.mrequest.size == 0)
                    break;
                exc_count = 0;
                exc_guard = GUARD_RETURN;
                checksum_val = checksum((void *)request.mrequest.addr, request.mrequest.size);
                exc_guard = GUARD_OFF;
                if (exc_count)
                    reply.status = ST_XFRERR;
                reply.mreply.dchecksum = checksum_val;
                break;
            case REQ_MEMWRITE:
                exc_count = 0;
                exc_guard = GUARD_SKIP;
                if (request.mrequest.size != 0) {
                    // Probe for exception guard
                    // We can't do the whole buffer easily, because we'd drop UART data
                    write8(request.mrequest.addr, 0);
                    write8(request.mrequest.addr + request.mrequest.size - 1, 0);
                }
                exc_guard = GUARD_OFF;
                if (exc_count) {
                    reply.status = ST_XFRERR;
                    break;
                }
                bytes = iodev_read(iodev, (void *)request.mrequest.addr, request.mrequest.size);
                if (bytes != request.mrequest.size) {
                    reply.status = ST_XFRERR;
                    break;
                }
                checksum_val = checksum((void *)request.mrequest.addr, request.mrequest.size);
                reply.mreply.dchecksum = checksum_val;
                if (reply.mreply.dchecksum != request.mrequest.dchecksum)
                    reply.status = ST_XFRERR;
                break;
            default:
                reply.status = ST_BADCMD;
                break;
        }
        reply.checksum = checksum(&reply, REPLY_SIZE - 4);
        iodev_write(iodev, &reply, REPLY_SIZE);

        if ((request.type == REQ_MEMREAD) && (reply.status == ST_OK)) {
            iodev_write(iodev, (void *)request.mrequest.addr, request.mrequest.size);
        }
    }
}