DMA on esp32-c3

[piotrcurious]

Hello.
I do not understand exactly what are problems of implementing dma for esp32-c3.
Currently it just dma init crashes cpu.

Below is discussion with AI (gpt4o) which suggests it might be theoretically possible - but of course AI might just want it really bad 😂.

Anyone has any more information on limitations and possible problems?

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The issue stems from the fact that the ESP32‑C3’s hardware (and its associated SDK support) handles SPI and DMA differently compared to the original ESP32 chips. In particular:

1. Different Peripheral and DMA Implementations:
   The TFT_eSPI library was originally written for the ESP32 (and similar chips) and makes direct use of certain registers and assumptions about the SPI/DMA hardware. The ESP32‑C3, being based on a different architecture (RISC‑V instead of Tensilica Xtensa) and having a redesigned SPI peripheral and DMA controller, uses different register layouts and initialization procedures. As a result, the routines in TFT_eSPI that set up and trigger DMA transfers may not correctly configure the ESP32‑C3’s hardware.

2. SDK and Hardware Limitations:
   At the time many developers ran into these issues, the ESP-IDF (or Arduino core) support for the ESP32‑C3 was still evolving. Some of the DMA features—especially those tied to the SPI peripherals—might not be as mature or as well documented for the C3 as for earlier chips. This means that even if the library code were updated, there might be lingering limitations or bugs in how the ESP32‑C3’s DMA is handled by the SDK.

3. Workarounds and Current Recommendations:
   Until the TFT_eSPI library (or the underlying ESP32‑C3 support in the SDK) is updated to accommodate these differences, many developers have found that it’s best to disable DMA for displays on the ESP32‑C3. Although this might lead to slower screen updates (since data is then transferred via the CPU rather than DMA), it avoids the erratic behavior caused by the mismatch in hardware expectations.

In summary, the “problem” with DMA in TFT_eSPI on the ESP32‑C3 is due to hardware differences and evolving software support. The library’s DMA code isn’t yet fully compatible with the way the ESP32‑C3 implements SPI DMA, which is why you’re encountering issues. For now, the recommended solution is to disable DMA when using the ESP32‑C3 with TFT_eSPI or look out for updates to the library that address these differences.

Implementing DMA support for the TFT_eSPI library on the ESP32-C3 is feasible, given the updated ESP32-C3 documentation and the capabilities of the ESP-IDF framework. Here's a structured approach to achieve this:

1. Understand the ESP32-C3's SPI and DMA Architecture:

The ESP32-C3 features a general-purpose SPI controller (SPI2) that supports DMA operations. The ESP-IDF provides an SPI master driver that facilitates DMA transfers, allowing for efficient data movement between memory and peripherals without burdening the CPU.

2. Initialize the SPI Bus with DMA:

To set up the SPI bus with DMA enabled, utilize the spi_bus_initialize function provided by the ESP-IDF. This function requires a spi_bus_config_t structure to define the bus parameters and a spi_dma_chan_t parameter to specify the DMA channel.

Here's an example of how to initialize the SPI bus with DMA:

#include "driver/spi_master.h"

spi_bus_config_t buscfg = {
.mosi_io_num = GPIO_NUM_7, // Set according to your wiring
.miso_io_num = GPIO_NUM_2, // Set according to your wiring
.sclk_io_num = GPIO_NUM_6, // Set according to your wiring
.quadwp_io_num = -1,
.quadhd_io_num = -1,
.max_transfer_sz = 4096, // Adjust based on your requirements
};

esp_err_t ret = spi_bus_initialize(SPI2_HOST, &buscfg, SPI_DMA_CH_AUTO);
if (ret != ESP_OK) {
// Handle initialization error
}

In this configuration:

mosi_io_num, miso_io_num, and sclk_io_num should be set to the GPIO pins connected to your display.

max_transfer_sz defines the maximum transfer size; adjust this based on your application's needs.

SPI_DMA_CH_AUTO allows the driver to automatically select an available DMA channel.

3. Configure the TFT_eSPI Library:

The TFT_eSPI library is designed for flexibility and performance. To enable DMA support on the ESP32-C3, you'll need to modify the library's configuration files.

User Setup Configuration:

In the User_Setup.h file, ensure that the correct driver and pin configurations are defined for your display. Additionally, define the TFT_DMA_BUFFER_SIZE to specify the size of the DMA buffer.

#define ILI9341_DRIVER
#define TFT_MISO 2
#define TFT_MOSI 7
#define TFT_SCLK 6
#define TFT_CS 10 // Set according to your wiring
#define TFT_DC 9 // Set according to your wiring
#define TFT_RST 8 // Set according to your wiring
#define TFT_DMA_BUFFER_SIZE 1024

Processor-Specific Configuration:

The TFT_eSPI library contains processor-specific configurations. For the ESP32-C3, ensure that the TFT_eSPI_ESP32_C3.h file is correctly set up to handle DMA operations. This may involve defining specific macros or including necessary headers.

4. Allocate DMA-Capable Memory:

When performing DMA transfers, the data buffers must reside in DMA-capable memory. The ESP-IDF provides a helper function to allocate such memory:

void *spi_bus_dma_memory_alloc(spi_host_device_t host_id, size_t size, uint32_t extra_heap_caps);

Use this function to allocate memory for your display data buffers, ensuring that the MALLOC_CAP_DMA capability is specified.

5. Implement DMA Transactions:

With the SPI bus initialized and DMA-capable memory allocated, you can perform DMA transactions using the spi_device_queue_trans and spi_device_get_trans_result functions. These functions allow you to queue transactions and handle them asynchronously, leveraging DMA for efficient data transfer.

spi_transaction_t trans = {
.length = length_in_bits,
.tx_buffer = dma_buffer,
// Additional transaction parameters
};

esp_err_t ret = spi_device_queue_trans(spi_handle, &trans, portMAX_DELAY);
if (ret != ESP_OK) {
// Handle queuing error
}

// Later, retrieve the result
spi_transaction_t *rtrans;
ret = spi_device_get_trans_result(spi_handle, &rtrans, portMAX_DELAY);
if (ret != ESP_OK) {
// Handle retrieval error
}

6. Testing and Optimization:

After implementing the above steps, thoroughly test the DMA functionality with your display. Monitor for any anomalies or performance issues. Optimize buffer sizes and transaction parameters as needed to achieve the desired performance.

Conclusion:

By following this approach, you can implement DMA support for the TFT_eSPI library on the ESP32-C3. This will enhance the performance of display operations by offloading data transfers to the DMA controller, allowing the CPU to perform other tasks concurrently.
Implementing DMA support for the TFT_eSPI library on the ESP32-C3 requires modifications to the processor-specific configuration file, typically named TFT_eSPI_ESP32_C3.h. As of the latest updates, the TFT_eSPI library does not natively support DMA on the ESP32-C3 due to hardware differences and the evolving nature of software support.

Steps to Modify the Processor-Specific Configuration File:

1. Include Necessary Headers: Ensure that the required headers for SPI and DMA operations are included:

#include "driver/spi_master.h"
#include "esp_heap_caps.h"

2. Define DMA Settings: Set up macros or definitions that will be used to configure DMA operations:

#define DMA_CHANNEL SPI_DMA_CH_AUTO // Automatically select an available DMA channel
#define DMA_MAX_SIZE 4096 // Maximum transfer size; adjust as needed

3. Initialize SPI Bus with DMA: Modify the SPI bus initialization function to enable DMA:

void init_spi_bus(void) {
spi_bus_config_t buscfg = {
.mosi_io_num = TFT_MOSI,
.miso_io_num = TFT_MISO,
.sclk_io_num = TFT_SCLK,
.quadwp_io_num = -1,
.quadhd_io_num = -1,
.max_transfer_sz = DMA_MAX_SIZE,
};
esp_err_t ret = spi_bus_initialize(SPI2_HOST, &buscfg, DMA_CHANNEL);
if (ret != ESP_OK) {
// Handle initialization error
}
}

4. Allocate DMA-Capable Memory: Ensure that memory allocated for data transfers is DMA-capable:

uint8_t* dma_buffer = (uint8_t*) heap_caps_malloc(DMA_MAX_SIZE, MALLOC_CAP_DMA);
if (dma_buffer == NULL) {
// Handle memory allocation error
}

5. Implement DMA Transfer Functions: Create functions to handle data transfers using DMA:

void dma_transfer(spi_device_handle_t spi, const uint8_t* data, size_t length) {
spi_transaction_t trans = {
.length = length * 8, // Length in bits
.tx_buffer = data,
};
esp_err_t ret = spi_device_polling_transmit(spi, &trans);
if (ret != ESP_OK) {
// Handle transmission error
}
}

6. Update TFT_eSPI Functions: Modify the existing functions in TFT_eSPI_ESP32_C3.h to utilize the new DMA transfer functions where appropriate. This may involve replacing standard SPI send/receive calls with dma_transfer calls.

Considerations:

Testing and Validation: After implementing these changes, thoroughly test the library with your specific display hardware to ensure stability and performance improvements.

Community Contributions: Given the complexities involved, consider collaborating with the TFT_eSPI community. Sharing your modifications can help others facing similar challenges and may lead to further optimizations.

By carefully implementing these changes, you can enable DMA support for the TFT_eSPI library on the ESP32-C3, leading to more efficient and faster display operations.

Implementing DMA support for the pushImageDMA function in the TFT_eSPI library on the ESP32-C3 involves several key steps. This function is designed to transfer image data to the display efficiently using Direct Memory Access (DMA), which offloads the data transfer task from the CPU, allowing for faster and more efficient operations.

1. Ensure DMA-Capable Memory Allocation:

DMA operations require that the data buffers reside in DMA-capable memory. In the ESP-IDF framework, you can allocate such memory using the heap_caps_malloc function with the MALLOC_CAP_DMA capability.

#include "esp_heap_caps.h"

// Allocate DMA-capable memory for the image buffer
uint16_t* image_buffer = (uint16_t*) heap_caps_malloc(buffer_size * sizeof(uint16_t), MALLOC_CAP_DMA);
if (image_buffer == NULL) {
// Handle memory allocation failure
}

2. Configure the SPI Device for DMA:

When initializing the SPI device, ensure that DMA is enabled. This is done by specifying a DMA channel during the SPI bus initialization.

#include "driver/spi_master.h"

spi_bus_config_t buscfg = {
.mosi_io_num = GPIO_NUM_7, // Set according to your wiring
.miso_io_num = GPIO_NUM_2, // Set according to your wiring
.sclk_io_num = GPIO_NUM_6, // Set according to your wiring
.quadwp_io_num = -1,
.quadhd_io_num = -1,
.max_transfer_sz = MAX_TRANSFER_SIZE, // Define based on your requirements
};

esp_err_t ret = spi_bus_initialize(SPI2_HOST, &buscfg, SPI_DMA_CH_AUTO);
if (ret != ESP_OK) {
// Handle initialization error
}

3. Implement the pushImageDMA Function:

The pushImageDMA function is responsible for initiating the DMA transfer of image data to the display. Below is an example implementation:

#include "driver/spi_master.h"

// Assume 'spi' is a valid spi_device_handle_t obtained during SPI device initialization

void pushImageDMA(int16_t x, int16_t y, int16_t w, int16_t h, uint16_t* data) {
// Calculate the total number of pixels
size_t len = w * h;

// Create a transaction descriptor
spi_transaction_t trans = {
    .length = len * 16,  // Total data length in bits (16 bits per pixel)
    .tx_buffer = data,
};

// Queue the transaction
esp_err_t ret = spi_device_queue_trans(spi, &trans, portMAX_DELAY);
if (ret != ESP_OK) {
    // Handle queuing error
}

// Wait for the transaction to complete
spi_transaction_t* rtrans;
ret = spi_device_get_trans_result(spi, &rtrans, portMAX_DELAY);
if (ret != ESP_OK) {
    // Handle retrieval error
}

}

Considerations:

Buffer Alignment: Ensure that the data buffer is 32-bit aligned and its size is a multiple of 4 bytes to meet DMA requirements.

Error Handling: Implement appropriate error handling to manage scenarios where SPI transactions fail.

Performance Optimization: Adjust the max_transfer_sz parameter in the spi_bus_config_t structure based on your application's requirements to optimize performance.

By following these steps, you can implement DMA support for the pushImageDMA function in the TFT_eSPI library on the ESP32-C3, resulting in more efficient image data transfers to your display.

Implementing a non-blocking pushImageDMA function in the TFT_eSPI library for the ESP32-C3 involves managing Direct Memory Access (DMA) transfers asynchronously. This approach allows the CPU to perform other tasks while the DMA controller handles data transfer to the display. A common method to achieve this is by using a dma_busy flag to monitor the status of the DMA transfer.

1. Define the dma_busy Flag:

Within your TFT_eSPI class, declare a volatile boolean flag to indicate the DMA transfer status:

volatile bool dma_busy = false;

2. Modify the pushImageDMA Function:

Update the pushImageDMA function to initiate the DMA transfer and set the dma_busy flag accordingly:

void TFT_eSPI::pushImageDMA(int32_t x, int32_t y, int32_t w, int32_t h, const uint16_t* data) {
// Ensure any previous DMA transfer is complete
while (dma_busy) {
// Optionally, implement a timeout or yield to other tasks
}

// Set the DMA busy flag
dma_busy = true;

// Configure the SPI transaction
spi_transaction_t trans = {};
trans.tx_buffer = data;
trans.length = w * h * 16; // Total bits to transfer

// Set up the user-defined callback to clear the dma_busy flag upon completion
trans.user = (void*)this;

// Queue the transaction
esp_err_t ret = spi_device_queue_trans(spi_handle, &trans, portMAX_DELAY);
if (ret != ESP_OK) {
    // Handle error
    dma_busy = false;
}

}

3. Implement the DMA Completion Callback:

Define a callback function that the SPI driver will call upon completion of the DMA transfer. This function will clear the dma_busy flag:

extern "C" void IRAM_ATTR spi_dma_complete_callback(spi_transaction_t* trans) {
TFT_eSPI* tft = (TFT_eSPI*)trans->user;
tft->dma_busy = false;
}

4. Initialize the SPI Bus with DMA and Register the Callback:

During the SPI bus initialization, ensure that DMA is enabled and the callback is registered:

void TFT_eSPI::initDMA() {
// SPI bus configuration
spi_bus_config_t buscfg = {
.mosi_io_num = TFT_MOSI,
.miso_io_num = -1,
.sclk_io_num = TFT_SCLK,
.quadwp_io_num = -1,
.quadhd_io_num = -1,
.max_transfer_sz = MAX_TRANSFER_SIZE,
};

// Initialize the SPI bus
esp_err_t ret = spi_bus_initialize(SPI2_HOST, &buscfg, SPI_DMA_CH_AUTO);
if (ret != ESP_OK) {
    // Handle initialization error
}

// SPI device configuration
spi_device_interface_config_t devcfg = {
    .clock_speed_hz = SPI_FREQUENCY,
    .mode = 0,
    .spics_io_num = TFT_CS,
    .queue_size = 1,
    .pre_cb = nullptr,
    .post_cb = spi_dma_complete_callback,
};

// Attach the device to the SPI bus
ret = spi_bus_add_device(SPI2_HOST, &devcfg, &spi_handle);
if (ret != ESP_OK) {
    // Handle device addition error
}

}

5. Usage Considerations:

Non-Blocking Behavior: The pushImageDMA function initiates the DMA transfer and returns immediately, allowing the CPU to continue executing other tasks.

Checking DMA Status: Before initiating a new DMA transfer, ensure that the previous transfer has completed by checking the dma_busy flag.

Thread Safety: If multiple tasks or interrupts can access the pushImageDMA function, implement appropriate synchronization mechanisms to prevent race conditions.

Conclusion:

By implementing the dma_busy flag and the associated callback mechanism, you can achieve non-blocking DMA transfers in the TFT_eSPI library on the ESP32-C3. This approach enhances performance by allowing the CPU to perform other operations while the DMA controller manages data transfer to the display.