spi_nxp_lpspi: Optimize TX fill for less interrupt

Optimize the TX fill algorithm to have less interrupts by filling the TX
fifo as much as possible during each interrupt handle.

Before, the algorithm was just a very simple, fill the TX fifo with as
much from only the current buffer as possible, then send it and wait for
the next interrupt. Now the algorithm is to fill the TX fifo as much as
possible, even if it means reading from multiple buffers during the
interrupt.

This has the advantage from master mode of having less interrupts. And
it is very important for slave mode because the slave mode does not
control the pacing of the transfer and so therefore should fill as much
as possible whenever possible in order not to miss a deadline.

Signed-off-by: Declan Snyder <[email protected]>

Author: decsny

drivers/spi/spi_nxp_lpspi/spi_nxp_lpspi.c

@@ -96,68 +96,105 @@ static inline void lpspi_handle_rx_irq(const struct device *dev)
 	}
 }
 
-static inline uint32_t lpspi_next_tx_word(const struct device *dev, int offset)
+/* constructs the next word from the buffer */
+static inline uint32_t lpspi_next_tx_word(const struct device *dev, const uint8_t *buf,
+					  int offset, size_t max_bytes)
 {
-	struct lpspi_data *data = dev->data;
-	struct lpspi_driver_data *lpspi_data = (struct lpspi_driver_data *)data->driver_data;
-	struct spi_context *ctx = &data->ctx;
-	const uint8_t *byte = ctx->tx_buf + offset;
-	uint32_t num_bytes = MIN(lpspi_data->word_size_bytes, ctx->tx_len);
+	const uint8_t *byte = buf + offset;
 	uint32_t next_word = 0;
 
-	for (uint8_t i = 0; i < num_bytes; i++) {
+	for (uint8_t i = 0; i < max_bytes; i++) {
 		next_word |= byte[i] << (BITS_PER_BYTE * i);
 	}
 
 	return next_word;
 }
 
-static inline void lpspi_fill_tx_fifo(const struct device *dev)
+/* fills the TX fifo with specified amount of data from the specified buffer */
+static inline void lpspi_fill_tx_fifo(const struct device *dev, const uint8_t *buf,
+				      size_t buf_len, size_t fill_len)
 {
 	LPSPI_Type *base = (LPSPI_Type *)DEVICE_MMIO_NAMED_GET(dev, reg_base);
 	struct lpspi_data *data = dev->data;
 	struct lpspi_driver_data *lpspi_data = (struct lpspi_driver_data *)data->driver_data;
-	size_t bytes_in_xfer = lpspi_data->fill_len * lpspi_data->word_size_bytes;
-	size_t offset;
-
-	for (offset = 0; offset < bytes_in_xfer; offset += lpspi_data->word_size_bytes) {
-		base->TDR = lpspi_next_tx_word(dev, offset);
+	uint8_t word_size = lpspi_data->word_size_bytes;
+	size_t buf_remaining_bytes = buf_len * word_size;
+	size_t offset = 0;
+	uint32_t next_word;
+	uint32_t next_word_bytes;
+
+	for (int word_count = 0; word_count < fill_len; word_count++) {
+		next_word_bytes = MIN(word_size, buf_remaining_bytes);
+		next_word = lpspi_next_tx_word(dev, buf, offset, next_word_bytes);
+		base->TDR = next_word;
+		offset += word_size;
+		buf_remaining_bytes -= word_size;
 	}
 
 	LOG_DBG("Filled TX FIFO to %d words (%d bytes)", lpspi_data->fill_len, offset);
 }
 
-static void lpspi_fill_tx_fifo_nop(const struct device *dev)
+/* just fills TX fifo with the specified amount of NOPS */
+static void lpspi_fill_tx_fifo_nop(const struct device *dev, size_t fill_len)
 {
 	LPSPI_Type *base = (LPSPI_Type *)DEVICE_MMIO_NAMED_GET(dev, reg_base);
-	struct lpspi_data *data = dev->data;
-	struct lpspi_driver_data *lpspi_data = (struct lpspi_driver_data *)data->driver_data;
 
-	for (int i = 0; i < lpspi_data->fill_len; i++) {
+	for (int i = 0; i < fill_len; i++) {
 		base->TDR = 0;
 	}
 
-	LOG_DBG("Filled TX fifo with %d NOPs", lpspi_data->fill_len);
+	LOG_DBG("Filled TX fifo with %d NOPs", fill_len);
 }
 
+/* handles refilling the TX fifo from empty */
 static void lpspi_next_tx_fill(const struct device *dev)
 {
 	const struct lpspi_config *config = dev->config;
 	struct lpspi_data *data = dev->data;
 	struct lpspi_driver_data *lpspi_data = (struct lpspi_driver_data *)data->driver_data;
 	struct spi_context *ctx = &data->ctx;
-	size_t max_chunk;
+	size_t fill_len;
+	size_t actual_filled = 0;
 
 	/* Convert bytes to words for this xfer */
-	max_chunk = DIV_ROUND_UP(ctx->tx_len, lpspi_data->word_size_bytes);
-	max_chunk = MIN(max_chunk, config->tx_fifo_size);
-	lpspi_data->fill_len = max_chunk;
-
-	if (spi_context_tx_buf_on(ctx)) {
-		lpspi_fill_tx_fifo(dev);
-	} else {
-		lpspi_fill_tx_fifo_nop(dev);
+	fill_len = DIV_ROUND_UP(spi_context_tx_len_left(ctx), lpspi_data->word_size_bytes);
+	fill_len = MIN(fill_len, config->tx_fifo_size);
+
+	const struct spi_buf *current_buf = ctx->current_tx;
+	const uint8_t *cur_buf_pos = ctx->tx_buf;
+	size_t cur_buf_len_left = ctx->tx_len;
+	size_t bufs_left = ctx->tx_count;
+
+	while (fill_len > 0) {
+		size_t next_buf_fill = MIN(cur_buf_len_left, fill_len);
+
+		if (cur_buf_pos == NULL) {
+			lpspi_fill_tx_fifo_nop(dev, next_buf_fill);
+		} else {
+			lpspi_fill_tx_fifo(dev, cur_buf_pos,
+						current_buf->len, next_buf_fill);
+		}
+
+		fill_len -= next_buf_fill;
+		cur_buf_pos += next_buf_fill;
+
+		/* in the case where we just filled as much as we could from the current buffer,
+		 * this logic while wrong should have no effect, since fill_len will be 0,
+		 * so I choose not to make the code extra complex
+		 */
+		bufs_left--;
+		if (bufs_left > 0) {
+			current_buf += 1;
+			cur_buf_len_left = current_buf->len;
+			cur_buf_pos = current_buf->buf;
+		} else {
+			fill_len = 0;
+		}
+
+		actual_filled += next_buf_fill;
 	}
+
+	lpspi_data->fill_len = actual_filled;
 }
 
 static inline void lpspi_handle_tx_irq(const struct device *dev)
@@ -167,7 +204,12 @@ static inline void lpspi_handle_tx_irq(const struct device *dev)
 	struct lpspi_driver_data *lpspi_data = (struct lpspi_driver_data *)data->driver_data;
 	struct spi_context *ctx = &data->ctx;
 
-	spi_context_update_tx(ctx, lpspi_data->word_size_bytes, lpspi_data->fill_len);
+	while (spi_context_tx_on(ctx) && lpspi_data->fill_len > 0) {
+		size_t this_buf_words_sent = MIN(lpspi_data->fill_len, ctx->tx_len);
+
+		spi_context_update_tx(ctx, lpspi_data->word_size_bytes, this_buf_words_sent);
+		lpspi_data->fill_len -= this_buf_words_sent;
+	}
 
 	base->SR = LPSPI_SR_TDF_MASK;
 
@@ -206,10 +248,11 @@ static void lpspi_isr(const struct device *dev)
 		size_t max_fill = MIN(expected_rx_left, config->rx_fifo_size);
 		size_t tx_current_fifo_len = tx_fifo_cur_len(base);
 
-		lpspi_data->fill_len = tx_current_fifo_len < ctx->rx_len ?
+		size_t fill_len = tx_current_fifo_len < ctx->rx_len ?
 					max_fill - tx_current_fifo_len : 0;
 
-		lpspi_fill_tx_fifo_nop(dev);
+		lpspi_fill_tx_fifo_nop(dev, fill_len);
+		lpspi_data->fill_len = fill_len;
 	}
 
 	if (spi_context_rx_len_left(ctx) == 1) {