Best practice for buffering data to be sent on UART [closed] - embedded

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I'm working on an embedded project using an STM32F7 device, writing bare metal C.
I want to be able to send data to a UART at any point in the program for debugging purposes, without blocking while the data is sent. I'm using DMA to try to minimise the cpu time used for this.
Currently I'm filling the data into a FIFO queue, and then initiating a DMA request to send the data directly from the FIFO queue to the UART.
The issue with this is I can't set up the DMA to read from both the start and end of the FIFO buffer, in the case where the middle of the FIFO is unused and a message wraps from the end of the buffer to the start.
The two solutions to this would be to set up the first DMA request to read from the head of the FIFO to the end of the buffer, and then once that is complete, read from the start of the buffer to the tail of the FIFO.
The other way to do it would be to memcpy() out the bytes to be sent to another buffer, where they are all sequential, then initiate a single DMA request to send all the data at once.
Both of these would probably work but I'm looking for insight on what the best approach would be here.

Answer 1

The implementation I've usually chosen is similar to what you have proposed:
The logging functions creates a text and adds it to circular buffer.
DMA is used for the UART transmission. DMA is setup to send a contiguous chunk of data.
Whenever the DMA finishes, an interrupt is triggered. It first frees up the transmitted data in the circular buffer. Then it checks if more data needs to be transmitted. If so, it is immediately started again with new data.
Pseudo code:
tx_len = 0;
void log_message(const char* msg)
{
circ_buf_add(msg);
start_tx();
}
void start_tx()
{
if (tx_len > 0)
return; // already transmitting
const char* start;
int len;
circ_buf_get_chunk(&start, &tx_len);
if (tx_len == 0)
return;
uart_tx_dma(start, tx_len);
}
void dma_interrupt_handler()
{
circ_buf_remove(tx_len);
tx_len = 0;
start_tx();
}
It usually makes sense to limit the length of the transmitted chunk. The shorter it is, the sooner space is freed up in the circular buffer.

Answer 2

The examples proposed so far are fire and forget. In the case where your code needs to know if the data has been send. We have used the following structure in which the fifo holds structs pointing to the data.
This way your data is held by the code sending it. It is able to monitor the transmission but it is also responsible for not using the data until the transmission is complete.
A different advantage is that you do not have to allocate a buffer in advance. Only two pointers are required to point to the start en end of the linked list structure.
Some meta code:
enum transmission_state {
Unused,
WaitToBeSend,
Sending,
Done,
Error // Optional but handy
}
struct data_to_send
{
// Point to your data.
data* data_pointer;
// Set the length of your data.
int length;
// What is the current state of this transmission.
transmission_state state;
// Pointer to the next data to be send creating a linked list.
// Only have the send and dma functions use this.
data_to_send* next;
};
// Definition of the fifo.
data_to_send* fifo_first = null;
data_to_send* fifo_end = null;
// Use this function in your code to add data to be send.
void send(data_to_send* dts)
{
if(null == fifo_first) {
fifo_first = dts;
fifo_end = dts;
dts.next = null;
start_dma_transfer(fifo_first);
}
else {
fifo_end.next = dts;
fifo_end = dts;
dts.state = WaitToBeSend;
dts.next = null;
}
};
// Start a transfer.
void start_dma_transfer(data_to_send* dts)
{
dts->state = Sending;
// Do some DMA stuff to start the transmission.
dma_transfer(dts->data, dts->length)
}
// The interrupt handler called when the dma is done.
void dma_interrupt_handler()
{
fifo_first->state = Done;
if(null != fifo_first->next) {
// Send the next data.
fifo_first = fifo_first->next;
start_dma_transfer(fifo_first);
}
else {
// No new data to be send.
fifo_first = null;
fifo_end = null;
}
}
int main()
{
// Setup a transmission
byte data[3] = {x,y,z};
data_to_send transmission = default_dts; // Set to some default.
transmission.data = data;
transmission.length = 3;
send(&transmission);
// Do other important things.
// Later periodically check the transmission.
if(Done == transmission.status) {
// You could use the data for something else or send new data.
}
}
This structure can also be used for I2C and SPI in which case you can add responses to the data_to_send struct and check for a response and act upon it.