int hw_device_init_from_string(const char *arg, HWDevice **dev_out)
{
    // "type=name:device,key=value,key2=value2"
    // "type:device,key=value,key2=value2"
    // -> av_hwdevice_ctx_create()
    // "type=name@name"
    // "type@name"
    // -> av_hwdevice_ctx_create_derived()

    AVDictionary *options = NULL;
    char *type_name = NULL, *name = NULL, *device = NULL;
    enum AVHWDeviceType type;
    HWDevice *dev, *src;
    AVBufferRef *device_ref = NULL;
    int err;
    const char *errmsg, *p, *q;
    size_t k;

    k = strcspn(arg, ":=@");
    p = arg + k;

    type_name = av_strndup(arg, k);
    if (!type_name) {
        err = AVERROR(ENOMEM);
        goto fail;
    }
    type = av_hwdevice_find_type_by_name(type_name);
    if (type == AV_HWDEVICE_TYPE_NONE) {
        errmsg = "unknown device type";
        goto invalid;
    }

    if (*p == '=') {
        k = strcspn(p + 1, ":@");

        name = av_strndup(p + 1, k);
        if (!name) {
            err = AVERROR(ENOMEM);
            goto fail;
        }
        if (hw_device_get_by_name(name)) {
            errmsg = "named device already exists";
            goto invalid;
        }

        p += 1 + k;
    } else {
        name = hw_device_default_name(type);
        if (!name) {
            err = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (!*p) {
        // New device with no parameters.
        err = av_hwdevice_ctx_create(&device_ref, type,
                                     NULL, NULL, 0);
        if (err < 0)
            goto fail;

    } else if (*p == ':') {
        // New device with some parameters.
        ++p;
        q = strchr(p, ',');
        if (q) {
            device = av_strndup(p, q - p);
            if (!device) {
                err = AVERROR(ENOMEM);
                goto fail;
            }
            err = av_dict_parse_string(&options, q + 1, "=", ",", 0);
            if (err < 0) {
                errmsg = "failed to parse options";
                goto invalid;
            }
        }

        err = av_hwdevice_ctx_create(&device_ref, type,
                                     device ? device : p, options, 0);
        if (err < 0)
            goto fail;

    } else if (*p == '@') {
        // Derive from existing device.

        src = hw_device_get_by_name(p + 1);
        if (!src) {
            errmsg = "invalid source device name";
            goto invalid;
        }

        err = av_hwdevice_ctx_create_derived(&device_ref, type,
                                             src->device_ref, 0);
        if (err < 0)
            goto fail;
    } else {
        errmsg = "parse error";
        goto invalid;
    }

    dev = hw_device_add();
    if (!dev) {
        err = AVERROR(ENOMEM);
        goto fail;
    }

    dev->name = name;
    dev->type = type;
    dev->device_ref = device_ref;

    if (dev_out)
        *dev_out = dev;

    name = NULL;
    err = 0;
done:
    av_freep(&type_name);
    av_freep(&name);
    av_freep(&device);
    av_dict_free(&options);
    return err;
invalid:
    av_log(NULL, AV_LOG_ERROR,
           "Invalid device specification \"%s\": %s\n", arg, errmsg);
    err = AVERROR(EINVAL);
    goto done;
fail:
    av_log(NULL, AV_LOG_ERROR,
           "Device creation failed: %d.\n", err);
    av_buffer_unref(&device_ref);
    goto done;
}