ESP-WHO（一）ESP32 摄像头驱动分析

ESP32 摄像头驱动分析

摄像头代码

摄像头初始化

Step:

1. 寻找摄像头
   1. 提供摄像头时钟、初始化 SCCB 总线、硬件复位摄像头
   2. 轮询地址寻找摄像头，通过 SCCB 总线读取摄像头 ID 等信息
   3. 更改摄像头的 ID 判断型号，并绑定对应的相关函数（摄像头传感器配置相关函数）
2. 初始化摄像头
   1. 根据选择的图像格式、和是否是高速模式，选择对应的 DMA BUF 处理函数和 DMA FIFO 模式
   2. 初始化 I2S 总线，使能 I2S_IN_DONE_INT 中断：当前 DMA 接收链表描述符被处理时即触发此中断 3. 初始化 DMA 相关变量（链表描述符、DMA 使用的数据缓冲区链表等），DMA 单次最多 4KB 、每行 DMA 采集几次
   3. 初始化存储图像的数据缓冲区（添加到一个链表中）并清空
   4. 初始化相关信号量：DMA 数据采集完成、一帧图像采集完成信号量、图像数据缓冲区进出信号量
   5. 创建 dma_filter_task 将 DMA 数据转换成像素数据并存储到图像缓冲区，这里也会检查是否有脏数据产生，判断是否接收完一帧完整的图像
   6. 初始化 vsync io 中断：每一帧图像开始结尾都会发生电平翻转
   7. 摄像头传感器相关配置（图像大小、格式等）

函数说明：

1. esp_camera_init 函数：
   1. 寻找摄像头
   2. 摄像头相关初始化

esp_err_t esp_camera_init(const camera_config_t* config)
{
    camera_model_t camera_model = CAMERA_NONE;
    esp_err_t err = camera_probe(config, &camera_model);
    if (err != ESP_OK) {
        ESP_LOGE(TAG, "Camera probe failed with error 0x%x", err);
        goto fail;
    }
    err = camera_init(config);
    if (err != ESP_OK) {
        ESP_LOGE(TAG, "Camera init failed with error 0x%x", err);
        return err;
    }
    return ESP_OK;

fail:
    free(s_state);
    s_state = NULL;
    camera_disable_out_clock();
    return err;
}

函数说明：

1. camera_probe 函数：
   1. 摄像头时钟配置、SCCB 总线初始化、硬件复位摄像头
   2. 轮询 127 以内的地址进行寻找摄像头、读取基本信息 PID、VER、MIDL、MIDH
   3. 根据摄像头 PID 判断设备类型，并绑定相应的函数、软件复位摄像头

esp_err_t camera_probe(const camera_config_t* config, camera_model_t* out_camera_model)
{
    if (s_state != NULL) {
        return ESP_ERR_INVALID_STATE;
    }

    s_state = (camera_state_t*) calloc(sizeof(*s_state), 1);
    if (!s_state) {
        return ESP_ERR_NO_MEM;
    }

    ESP_LOGD(TAG, "Enabling XCLK output");
    camera_enable_out_clock(config);

    ESP_LOGD(TAG, "Initializing SSCB");
    SCCB_Init(config->pin_sscb_sda, config->pin_sscb_scl);
	
    if(config->pin_pwdn >= 0) {
        ESP_LOGD(TAG, "Resetting camera by power down line");
        gpio_config_t conf = { 0 };
        conf.pin_bit_mask = 1LL << config->pin_pwdn;
        conf.mode = GPIO_MODE_OUTPUT;
        gpio_config(&conf);

        // carefull, logic is inverted compared to reset pin
        gpio_set_level(config->pin_pwdn, 1);
        vTaskDelay(10 / portTICK_PERIOD_MS);
        gpio_set_level(config->pin_pwdn, 0);
        vTaskDelay(10 / portTICK_PERIOD_MS);
    }

    if(config->pin_reset >= 0) {
        ESP_LOGD(TAG, "Resetting camera");
        gpio_config_t conf = { 0 };
        conf.pin_bit_mask = 1LL << config->pin_reset;
        conf.mode = GPIO_MODE_OUTPUT;
        gpio_config(&conf);

        gpio_set_level(config->pin_reset, 0);
        vTaskDelay(10 / portTICK_PERIOD_MS);
        gpio_set_level(config->pin_reset, 1);
        vTaskDelay(10 / portTICK_PERIOD_MS);
#if CONFIG_OV2640_SUPPORT
    } else {
        //reset OV2640
        SCCB_Write(0x30, 0xFF, 0x01);//bank sensor
        SCCB_Write(0x30, 0x12, 0x80);//reset
        vTaskDelay(10 / portTICK_PERIOD_MS);
#endif
    }

    ESP_LOGD(TAG, "Searching for camera address");
    vTaskDelay(10 / portTICK_PERIOD_MS);
    uint8_t slv_addr = SCCB_Probe();
    if (slv_addr == 0) {
        *out_camera_model = CAMERA_NONE;
        camera_disable_out_clock();
        return ESP_ERR_CAMERA_NOT_DETECTED;
    }
    s_state->sensor.slv_addr = slv_addr;

    //s_state->sensor.slv_addr = 0x30;
    ESP_LOGD(TAG, "Detected camera at address=0x%02x", s_state->sensor.slv_addr);
    sensor_id_t* id = &s_state->sensor.id;

    id->PID = SCCB_Read(s_state->sensor.slv_addr, REG_PID);
    id->VER = SCCB_Read(s_state->sensor.slv_addr, REG_VER);
    id->MIDL = SCCB_Read(s_state->sensor.slv_addr, REG_MIDL);
    id->MIDH = SCCB_Read(s_state->sensor.slv_addr, REG_MIDH);
    vTaskDelay(10 / portTICK_PERIOD_MS);
    ESP_LOGD(TAG, "Camera PID=0x%02x VER=0x%02x MIDL=0x%02x MIDH=0x%02x",
             id->PID, id->VER, id->MIDH, id->MIDL);

    switch (id->PID) {
#if CONFIG_OV2640_SUPPORT
    case OV2640_PID:
        *out_camera_model = CAMERA_OV2640;
        ov2640_init(&s_state->sensor);
        break;
#endif
#if CONFIG_OV7725_SUPPORT
    case OV7725_PID:
        *out_camera_model = CAMERA_OV7725;
        ov7725_init(&s_state->sensor);
        break;
#endif
    default:
        id->PID = 0;
        *out_camera_model = CAMERA_UNKNOWN;
        camera_disable_out_clock();
        ESP_LOGE(TAG, "Detected camera not supported.");
        return ESP_ERR_CAMERA_NOT_SUPPORTED;
    }

    ESP_LOGD(TAG, "Doing SW reset of sensor");
    s_state->sensor.reset(&s_state->sensor);

    return ESP_OK;
}

函数说明：

1. camera_init 函数：
   1. 根据选择的图像格式、和是否是高速模式，选择对应的 DMA BUF 处理函数和 DMA FIFO 模式
   2. 初始化 I2S 总线，相关引脚配置并使能 I2S_IN_DONE_INT 中断：当前 DMA 接收链表描述符被处理时即触发此中断 3. 初始化 DMA 相关变量（链表描述符、DMA 使用的数据缓冲区链表等），DMA 单次最多 4KB 、每行 DMA 采集几次
   3. 初始化存储图像的数据缓冲区（添加到一个链表中）并清空
   4. 初始化相关信号量：DMA 数据采集完成、一帧图像采集完成信号量、图像数据缓冲区进出信号量
   5. 创建 dma_filter_task 将 DMA 数据转换成像素数据并存储到图像缓冲区，这里也会检查是否有脏数据产生，判断是否接收完一帧完整的图像
   6. 初始化 vsync io 中断：每一帧图像开始结尾都会发生电平翻转
   7. 摄像头传感器相关配置（图像大小、格式等）
   8. 跳过前面几帧数据

esp_err_t camera_init(const camera_config_t* config)
{
    if (!s_state) {
        return ESP_ERR_INVALID_STATE;
    }
    if (s_state->sensor.id.PID == 0) {
        return ESP_ERR_CAMERA_NOT_SUPPORTED;
    }
    memcpy(&s_state->config, config, sizeof(*config));
    esp_err_t err = ESP_OK;
    framesize_t frame_size = (framesize_t) config->frame_size;
    pixformat_t pix_format = (pixformat_t) config->pixel_format;
    s_state->width = resolution[frame_size][0];
    s_state->height = resolution[frame_size][1];

    if (pix_format == PIXFORMAT_GRAYSCALE) {
        s_state->fb_size = s_state->width * s_state->height;
        if (is_hs_mode()) {
            s_state->sampling_mode = SM_0A00_0B00;
            s_state->dma_filter = &dma_filter_grayscale_highspeed;
        } else {
            s_state->sampling_mode = SM_0A0B_0C0D;
            s_state->dma_filter = &dma_filter_grayscale;
        }
        s_state->in_bytes_per_pixel = 2;       // camera sends YUYV
        s_state->fb_bytes_per_pixel = 1;       // frame buffer stores Y8
    } else if (pix_format == PIXFORMAT_YUV422 || pix_format == PIXFORMAT_RGB565) {
        s_state->fb_size = s_state->width * s_state->height * 2;
        if (is_hs_mode()) {
            s_state->sampling_mode = SM_0A00_0B00;
            s_state->dma_filter = &dma_filter_yuyv_highspeed;
        } else {
            s_state->sampling_mode = SM_0A0B_0C0D;
            s_state->dma_filter = &dma_filter_yuyv;
        }
        s_state->in_bytes_per_pixel = 2;       // camera sends YUYV
        s_state->fb_bytes_per_pixel = 2;       // frame buffer stores Y8
    } else if (pix_format == PIXFORMAT_JPEG) {
        if (s_state->sensor.id.PID != OV2640_PID) {
            ESP_LOGE(TAG, "JPEG format is only supported for ov2640");
            err = ESP_ERR_NOT_SUPPORTED;
            goto fail;
        }
        int qp = config->jpeg_quality;
        int compression_ratio_bound = 1;
        if (qp > 10) {
            compression_ratio_bound = 16;
        } else if (qp > 5) {
            compression_ratio_bound = 10;
        } else {
            compression_ratio_bound = 4;
        }
        (*s_state->sensor.set_quality)(&s_state->sensor, qp);
        s_state->in_bytes_per_pixel = 2;
        s_state->fb_bytes_per_pixel = 2;
        s_state->fb_size = (s_state->width * s_state->height * s_state->fb_bytes_per_pixel) / compression_ratio_bound;
        s_state->dma_filter = &dma_filter_jpeg;
        s_state->sampling_mode = SM_0A00_0B00;
    } else {
        ESP_LOGE(TAG, "Requested format is not supported");
        err = ESP_ERR_NOT_SUPPORTED;
        goto fail;
    }

    ESP_LOGD(TAG, "in_bpp: %d, fb_bpp: %d, fb_size: %d, mode: %d, width: %d height: %d",
             s_state->in_bytes_per_pixel, s_state->fb_bytes_per_pixel,
             s_state->fb_size, s_state->sampling_mode,
             s_state->width, s_state->height);

    i2s_init();

    err = dma_desc_init();
    if (err != ESP_OK) {
        ESP_LOGE(TAG, "Failed to initialize I2S and DMA");
        goto fail;
    }

    //s_state->fb_size = 75 * 1024;
    err = camera_fb_init(s_state->config.fb_count);
    if (err != ESP_OK) {
        ESP_LOGE(TAG, "Failed to allocate frame buffer");
        goto fail;
    }

    s_state->data_ready = xQueueCreate(16, sizeof(size_t));
    if (s_state->data_ready == NULL) {
        ESP_LOGE(TAG, "Failed to dma queue");
        err = ESP_ERR_NO_MEM;
        goto fail;
    }

    if(s_state->config.fb_count == 1) {
        s_state->frame_ready = xSemaphoreCreateBinary();
        if (s_state->frame_ready == NULL) {
            ESP_LOGE(TAG, "Failed to create semaphore");
            err = ESP_ERR_NO_MEM;
            goto fail;
        }
    } else {
        s_state->fb_in = xQueueCreate(s_state->config.fb_count, sizeof(camera_fb_t *));
        s_state->fb_out = xQueueCreate(1, sizeof(camera_fb_t *));
        if (s_state->fb_in == NULL || s_state->fb_out == NULL) {
            ESP_LOGE(TAG, "Failed to fb queues");
            err = ESP_ERR_NO_MEM;
            goto fail;
        }
    }

    //ToDo: core affinity?
    if (!xTaskCreatePinnedToCore(&dma_filter_task, "dma_filter", 4096, NULL, 10, &s_state->dma_filter_task, 1)) {
        ESP_LOGE(TAG, "Failed to create DMA filter task");
        err = ESP_ERR_NO_MEM;
        goto fail;
    }

    vsync_intr_disable();
    gpio_install_isr_service(ESP_INTR_FLAG_LEVEL1 | ESP_INTR_FLAG_IRAM);
    err = gpio_isr_handler_add(s_state->config.pin_vsync, &vsync_isr, NULL);
    if (err != ESP_OK) {
        ESP_LOGE(TAG, "vsync_isr_handler_add failed (%x)", err);
        goto fail;
    }

    s_state->sensor.status.framesize = frame_size;
    s_state->sensor.pixformat = pix_format;
    ESP_LOGD(TAG, "Setting frame size to %dx%d", s_state->width, s_state->height);
    if (s_state->sensor.set_framesize(&s_state->sensor, frame_size) != 0) {
        ESP_LOGE(TAG, "Failed to set frame size");
        err = ESP_ERR_CAMERA_FAILED_TO_SET_FRAME_SIZE;
        goto fail;
    }
    s_state->sensor.set_pixformat(&s_state->sensor, pix_format);

    if (s_state->sensor.id.PID == OV2640_PID) {
        s_state->sensor.set_gainceiling(&s_state->sensor, GAINCEILING_2X);
        s_state->sensor.set_bpc(&s_state->sensor, false);
        s_state->sensor.set_wpc(&s_state->sensor, true);
        s_state->sensor.set_lenc(&s_state->sensor, true);
    }

    skip_frame();
    //todo: for some reason the first set of the quality does not work.
    if (pix_format == PIXFORMAT_JPEG) {
        (*s_state->sensor.set_quality)(&s_state->sensor, config->jpeg_quality);
    }
    s_state->sensor.init_status(&s_state->sensor);
    return ESP_OK;

fail:
    esp_camera_deinit();
    return err;
}

数据采集

Step：

1. DMA 链表描述符初始化，初始化 4 行图像数据大小的 buf 空间
2. 开始接收数据时，先运行 i2s_run() ，等待 VSYNC 信号变为高时，即表示摄像头端已经开始一帧图像传输，需要跳过 VSYNC 为低时的数据，再采集过程中出现 VSYNC 由高到低变化，则表示一帧图像传输完成，VSYNC 中断服务函数 vsync_isr
3. 当前 DMA 接收链表描述符被处理完成时将触发 I2S_IN_DONE_INT 中断，进入 i2s_isr()，中断服务函数中，首先清楚中断标志，之后通过 signal_dma_buf_received 函数发出一次 DMA 数据采集完成信号，若一帧图像数据接收完成就停止 i2s
4. 在 dma_filter_task 任务中会等待 DMA 数据采集完成信号，并处理 DMA buffer 中的数据放入图像缓冲区中
5. 循环 3、4 步骤直到一帧图像数据接收完全，发出帧接收完信号

函数说明：

1. i2s_run 函数:
   1. 跳过当前这一帧（VSYNC 为高的这段数据丢掉），等待 VSYNC 为低，开始 i2s 接收数据
   2. 并在 i2s_start_bus 函数中使能 VSYNC 中断

static void i2s_run()
{
    for (int i = 0; i < s_state->dma_desc_count; ++i) {
        lldesc_t* d = &s_state->dma_desc[i];
        ESP_LOGV(TAG, "DMA desc %2d: %u %u %u %u %u %u %p %p",
                 i, d->length, d->size, d->offset, d->eof, d->sosf, d->owner, d->buf, d->qe.stqe_next);
        memset(s_state->dma_buf[i], 0, d->length);
    }

    // wait for frame
    camera_fb_int_t * fb = s_state->fb;
    while(s_state->config.fb_count > 1) {
        while(s_state->fb->ref && s_state->fb->next != fb) {
            s_state->fb = s_state->fb->next;
        }
        if(s_state->fb->ref == 0) {
            break;
        }
        vTaskDelay(2);
    }

    // wait for vsync
    ESP_LOGV(TAG, "Waiting for negative edge on VSYNC");
    while (_gpio_get_level(s_state->config.pin_vsync) != 0) {
        ;
    }
    ESP_LOGV(TAG, "Got VSYNC");

    i2s_start_bus();
}

static void IRAM_ATTR i2s_start_bus()
{
    s_state->dma_desc_cur = 0;
    s_state->dma_received_count = 0;
    //s_state->dma_filtered_count = 0;
    esp_intr_disable(s_state->i2s_intr_handle);
    i2s_conf_reset();

    I2S0.rx_eof_num = s_state->dma_sample_count;
    I2S0.in_link.addr = (uint32_t) &s_state->dma_desc[0];
    I2S0.in_link.start = 1;
    I2S0.int_clr.val = I2S0.int_raw.val;
    I2S0.int_ena.val = 0;
    I2S0.int_ena.in_done = 1;

    esp_intr_enable(s_state->i2s_intr_handle);
    I2S0.conf.rx_start = 1;
    if (s_state->config.pixel_format == PIXFORMAT_JPEG) {
        vsync_intr_enable();
    }
}

函数说明：

1. dma_desc_init 函数：
   1. DMA 链表描述符初始化，初始化 4 行图像数据大小的 buf 空间
   2. 分配 DMA buf 空间

static esp_err_t dma_desc_init()
{
    assert(s_state->width % 4 == 0);
    size_t line_size = s_state->width * s_state->in_bytes_per_pixel *
                       i2s_bytes_per_sample(s_state->sampling_mode);
    ESP_LOGD(TAG, "Line width (for DMA): %d bytes", line_size);
    size_t dma_per_line = 1;
    size_t buf_size = line_size;
    while (buf_size >= 4096) {
        buf_size /= 2;
        dma_per_line *= 2;
    }
    size_t dma_desc_count = dma_per_line * 4;
    s_state->dma_buf_width = line_size;
    s_state->dma_per_line = dma_per_line;
    s_state->dma_desc_count = dma_desc_count;
    ESP_LOGD(TAG, "DMA buffer size: %d, DMA buffers per line: %d", buf_size, dma_per_line);
    ESP_LOGD(TAG, "DMA buffer count: %d", dma_desc_count);
    ESP_LOGD(TAG, "DMA buffer total: %d bytes", buf_size * dma_desc_count);

    s_state->dma_buf = (dma_elem_t**) malloc(sizeof(dma_elem_t*) * dma_desc_count);
    if (s_state->dma_buf == NULL) {
        return ESP_ERR_NO_MEM;
    }
    s_state->dma_desc = (lldesc_t*) malloc(sizeof(lldesc_t) * dma_desc_count);
    if (s_state->dma_desc == NULL) {
        return ESP_ERR_NO_MEM;
    }
    size_t dma_sample_count = 0;
    for (int i = 0; i < dma_desc_count; ++i) {
        ESP_LOGD(TAG, "Allocating DMA buffer #%d, size=%d", i, buf_size);
        dma_elem_t* buf = (dma_elem_t*) malloc(buf_size);
        if (buf == NULL) {
            return ESP_ERR_NO_MEM;
        }
        s_state->dma_buf[i] = buf;
        ESP_LOGV(TAG, "dma_buf[%d]=%p", i, buf);

        lldesc_t* pd = &s_state->dma_desc[i];
        pd->length = buf_size;
        if (s_state->sampling_mode == SM_0A0B_0B0C &&
                (i + 1) % dma_per_line == 0) {
            pd->length -= 4;
        }
        dma_sample_count += pd->length / 4;
        pd->size = pd->length;
        pd->owner = 1;
        pd->sosf = 1;
        pd->buf = (uint8_t*) buf;
        pd->offset = 0;
        pd->empty = 0;
        pd->eof = 1;
        pd->qe.stqe_next = &s_state->dma_desc[(i + 1) % dma_desc_count];
    }
    s_state->dma_sample_count = dma_sample_count;
    return ESP_OK;
}

函数说明：

1. vsync_isr 函数：
   1. 当摄像头一帧数据发送完， VSYNC 由高变低触发中断
   2. 如果是多帧缓存的方式，将继续下一次采集

static void IRAM_ATTR vsync_isr(void* arg)
{
    GPIO.status1_w1tc.val = GPIO.status1.val;
    GPIO.status_w1tc = GPIO.status;
    bool need_yield = false;
    //if vsync is low and we have received some data, frame is done
    if (_gpio_get_level(s_state->config.pin_vsync) == 0) {
        if(s_state->dma_received_count > 0) {
            signal_dma_buf_received(&need_yield);
            //ets_printf("end_vsync\n");
            if(s_state->dma_filtered_count > 1 || s_state->config.fb_count > 1) {
                i2s_stop(&need_yield);
            }
        }
        if(s_state->config.fb_count > 1 || s_state->dma_filtered_count < 2) {
            I2S0.conf.rx_start = 0;
            I2S0.in_link.start = 0;
            I2S0.int_clr.val = I2S0.int_raw.val;
            i2s_conf_reset();
            s_state->dma_desc_cur = (s_state->dma_desc_cur + 1) % s_state->dma_desc_count;
            //I2S0.rx_eof_num = s_state->dma_sample_count;
            I2S0.in_link.addr = (uint32_t) &s_state->dma_desc[s_state->dma_desc_cur];
            I2S0.in_link.start = 1;
            I2S0.conf.rx_start = 1;
            s_state->dma_received_count = 0;
        }
    }
    if (need_yield) {
        portYIELD_FROM_ISR();
    }
}

函数说明：

1. i2s_isr 函数：
   1. 当前 DMA 接收链表描述符被处理完成时将触发 I2S_IN_DONE_INT 中断，在中断服务函数中判断是否接收完成（非 JPEG 格式），并调用 signal_dma_buf_received 函数

static void IRAM_ATTR i2s_isr(void* arg)
{
    I2S0.int_clr.val = I2S0.int_raw.val;
    bool need_yield = false;
    signal_dma_buf_received(&need_yield);
    if (s_state->config.pixel_format != PIXFORMAT_JPEG
     && s_state->dma_received_count == s_state->height * s_state->dma_per_line) {
        i2s_stop(&need_yield);
    }
    if (need_yield) {
        portYIELD_FROM_ISR();
    }
}

函数说明：

1. signal_dma_buf_received 函数：
   1. 发出 DMA 接收完成信号，给出当前处理的 DMA 链表描述符索引

static void IRAM_ATTR signal_dma_buf_received(bool* need_yield)
{
    size_t dma_desc_filled = s_state->dma_desc_cur;
    s_state->dma_desc_cur = (dma_desc_filled + 1) % s_state->dma_desc_count;
    s_state->dma_received_count++;
    if(!s_state->fb->ref && s_state->fb->bad){
        *need_yield = false;
        return;
    }
    BaseType_t higher_priority_task_woken;
    BaseType_t ret = xQueueSendFromISR(s_state->data_ready, &dma_desc_filled, &higher_priority_task_woken);
    if (ret != pdTRUE) {
        if(!s_state->fb->ref) {
            s_state->fb->bad = 1;
        }
        //ESP_EARLY_LOGW(TAG, "qsf:%d", s_state->dma_received_count);
        //ets_printf("qsf:%d\n", s_state->dma_received_count);
    }
    *need_yield = (ret == pdTRUE && higher_priority_task_woken == pdTRUE);
}

函数说明：

1. dma_filter_task 函数：
   1. 接收 DMA 接收完成信号，判断是否为最后一次处理，是则调用 dma_finish_frame 函数，否则，调用 dma_filter_buffer 函数将 DMA BUF 中的数据放入图像数据缓冲区中

static void IRAM_ATTR dma_filter_task(void *pvParameters)
{
    s_state->dma_filtered_count = 0;
    while (true) {
        size_t buf_idx;
        if(xQueueReceive(s_state->data_ready, &buf_idx, portMAX_DELAY) == pdTRUE) {
            if (buf_idx == SIZE_MAX) {
                //this is the end of the frame
                dma_finish_frame();
            } else {
                dma_filter_buffer(buf_idx);
            }
        }
    }
}

函数说明：

1. dma_finish_frame 函数：
   1. 如果是脏数据重新采集一次数据

static void IRAM_ATTR dma_finish_frame()
{
    size_t buf_len = s_state->width * s_state->fb_bytes_per_pixel / s_state->dma_per_line;

    if(!s_state->fb->ref) {
        // is the frame bad?
        if(s_state->fb->bad){
            s_state->fb->bad = 0;
            s_state->fb->len = 0;
            *((uint32_t *)s_state->fb->buf) = 0;
            if(s_state->config.fb_count == 1) {
                i2s_start_bus();
            }
        } else {
            s_state->fb->len = s_state->dma_filtered_count * buf_len;
            if(s_state->fb->len) {
                //find the end marker for JPEG. Data after that can be discarded
                if(s_state->fb->format == PIXFORMAT_JPEG){
                    uint8_t * dptr = &s_state->fb->buf[s_state->fb->len - 1];
                    while(dptr > s_state->fb->buf){
                        if(dptr[0] == 0xFF && dptr[1] == 0xD9 && dptr[2] == 0x00 && dptr[3] == 0x00){
                            dptr += 2;
                            s_state->fb->len = dptr - s_state->fb->buf;
                            if((s_state->fb->len & 0x1FF) == 0){
                                s_state->fb->len += 1;
                            }
                            if((s_state->fb->len % 100) == 0){
                                s_state->fb->len += 1;
                            }
                            break;
                        }
                        dptr--;
                    }
                }
                //send out the frame
                camera_fb_done();
            } else if(s_state->config.fb_count == 1){
                //frame was empty?
                i2s_start_bus();
            }
        }
    } else if(s_state->fb->len) {
        camera_fb_done();
    }
    s_state->dma_filtered_count = 0;
}

函数说明：

1. dma_filter_buffer 函数：
   1. 调用 DMA BUF 处理函数，将数据转换（根据 DMA FIFO 模式）并放入图像数据缓冲区
   2. 通过检查第一次 DMA 采集的数据是否错误判断本次数据是否为脏数据

static void IRAM_ATTR dma_filter_buffer(size_t buf_idx)
{
    //no need to process the data if frame is in use or is bad
    if(s_state->fb->ref || s_state->fb->bad) {
        return;
    }

    //check if there is enough space in the frame buffer for the new data
    size_t buf_len = s_state->width * s_state->fb_bytes_per_pixel / s_state->dma_per_line;
    size_t fb_pos = s_state->dma_filtered_count * buf_len;
    if(fb_pos > s_state->fb_size - buf_len) {
        //size_t processed = s_state->dma_received_count * buf_len;
        //ets_printf("[%s:%u] ovf pos: %u, processed: %u\n", __FUNCTION__, __LINE__, fb_pos, processed);
        return;
    }

    //convert I2S DMA buffer to pixel data
    (*s_state->dma_filter)(s_state->dma_buf[buf_idx], &s_state->dma_desc[buf_idx], s_state->fb->buf + fb_pos);

    //first frame buffer
    if(!s_state->dma_filtered_count) {
        //check for correct JPEG header
        if(s_state->sensor.pixformat == PIXFORMAT_JPEG) {
            uint32_t sig = *((uint32_t *)s_state->fb->buf) & 0xFFFFFF;
            if(sig != 0xffd8ff) {
                //ets_printf("bad header\n");
                s_state->fb->bad = 1;
                return;
            }
        }
        //set the frame properties
        s_state->fb->width = resolution[s_state->sensor.status.framesize][0];
        s_state->fb->height = resolution[s_state->sensor.status.framesize][1];
        s_state->fb->format = s_state->sensor.pixformat;
    }
    s_state->dma_filtered_count++;
}

函数说明：

1. camera_fb_done 函数：
   1. 帧缓存完成，若是单帧缓存模式，将给出帧已经接收完的信号
   2. 若是多帧缓存模式，将当前的这一帧放入到出队的队列中，在其放入入队的队列前将不可用以缓存图像数据

static void IRAM_ATTR camera_fb_done()
{
    camera_fb_int_t * fb = NULL, * fb2 = NULL;
    BaseType_t taskAwoken = 0;

    if(s_state->config.fb_count == 1) {
        xSemaphoreGive(s_state->frame_ready);
        return;
    }

    fb = s_state->fb;
    if(!fb->ref && fb->len) {
        //add reference
        fb->ref = 1;

        //check if the queue is full
        if(xQueueIsQueueFullFromISR(s_state->fb_out) == pdTRUE) {
            //pop frame buffer from the queue
            if(xQueueReceiveFromISR(s_state->fb_out, &fb2, &taskAwoken) == pdTRUE) {
                //free the popped buffer
                fb2->ref = 0;
                fb2->len = 0;
                //push the new frame to the end of the queue
                xQueueSendFromISR(s_state->fb_out, &fb, &taskAwoken);
            } else {
                //queue is full and we could not pop a frame from it
            }
        } else {
            //push the new frame to the end of the queue
            xQueueSendFromISR(s_state->fb_out, &fb, &taskAwoken);
        }
    } else {
        //frame was referenced or empty
    }

    //return buffers to be filled
    while(xQueueReceiveFromISR(s_state->fb_in, &fb2, &taskAwoken) == pdTRUE) {
        fb2->ref = 0;
        fb2->len = 0;
    }

    //advance frame buffer only if the current one has data
    if(s_state->fb->len) {
        s_state->fb = s_state->fb->next;
    }
    //try to find the next free frame buffer
    while(s_state->fb->ref && s_state->fb->next != fb) {
        s_state->fb = s_state->fb->next;
    }
    //is the found frame buffer free?
    if(!s_state->fb->ref) {
        //buffer found. make sure it's empty
        s_state->fb->len = 0;
        *((uint32_t *)s_state->fb->buf) = 0;
    } else {
        //stay at the previous buffer
        s_state->fb = fb;
    }
}