spi: stm32: perform small transfer in polling mode

In case of interrupt based transfer, when the transfer is very
small, relying on interrupts leads to lower performances than if
the transfer were done using polling on the registers.

Add a module parameter "polling_limit_us" to indicate the threshold
in us from which a transfer would be done polling the registers rather
than relying on interrupts.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://patch.msgid.link/20251218-stm32-spi-enhancements-v2-3-3b69901ca9fe@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: Alain Volmat

drivers/spi/spi-stm32.c

@@ -202,6 +202,10 @@
 #define STM32_SPI_HOST_MODE(stm32_spi) (!(stm32_spi)->device_mode)
 #define STM32_SPI_DEVICE_MODE(stm32_spi) ((stm32_spi)->device_mode)
 
+static unsigned int polling_limit_us = 30;
+module_param(polling_limit_us, uint, 0664);
+MODULE_PARM_DESC(polling_limit_us, "maximum time in us to run a transfer in polling mode\n");
+
 /**
  * struct stm32_spi_reg - stm32 SPI register & bitfield desc
  * @reg:		register offset
@@ -266,6 +270,7 @@ struct stm32_spi;
  * @dma_rx_cb: routine to call after DMA RX channel operation is complete
  * @dma_tx_cb: routine to call after DMA TX channel operation is complete
  * @transfer_one_irq: routine to configure interrupts for driver
+ * @transfer_one_poll: routine to perform a transfer via register polling
  * @irq_handler_event: Interrupt handler for SPI controller events
  * @irq_handler_thread: thread of interrupt handler for SPI controller
  * @baud_rate_div_min: minimum baud rate divisor
@@ -291,6 +296,7 @@ struct stm32_spi_cfg {
 	void (*dma_rx_cb)(void *data);
 	void (*dma_tx_cb)(void *data);
 	int (*transfer_one_irq)(struct stm32_spi *spi);
+	int (*transfer_one_poll)(struct stm32_spi *spi);
 	irqreturn_t (*irq_handler_event)(int irq, void *dev_id);
 	irqreturn_t (*irq_handler_thread)(int irq, void *dev_id);
 	unsigned int baud_rate_div_min;
@@ -1355,6 +1361,55 @@ static int stm32fx_spi_transfer_one_irq(struct stm32_spi *spi)
 	return 1;
 }
 
+/**
+ * stm32h7_spi_transfer_one_poll - transfer a single spi_transfer by direct
+ *				   register access without interrupt usage
+ * @spi: pointer to the spi controller data structure
+ *
+ * It must returns 0 if the transfer is finished or 1 if the transfer is still
+ * in progress.
+ */
+static int stm32h7_spi_transfer_one_poll(struct stm32_spi *spi)
+{
+	unsigned long flags;
+	u32 sr;
+
+	spin_lock_irqsave(&spi->lock, flags);
+
+	stm32_spi_enable(spi);
+
+	/* Be sure to have data in fifo before starting data transfer */
+	if (spi->tx_buf)
+		stm32h7_spi_write_txfifo(spi);
+
+	if (STM32_SPI_HOST_MODE(spi))
+		stm32_spi_set_bits(spi, STM32H7_SPI_CR1, STM32H7_SPI_CR1_CSTART);
+
+	sr = readl_relaxed(spi->base + STM32H7_SPI_SR);
+	/* Keep writing / reading while waiting for the end of transfer */
+	while (spi->tx_len || spi->rx_len || !(sr & STM32H7_SPI_SR_EOT)) {
+		if (spi->rx_len && (sr & (STM32H7_SPI_SR_RXP | STM32H7_SPI_SR_RXWNE |
+					  STM32H7_SPI_SR_RXPLVL)))
+			stm32h7_spi_read_rxfifo(spi);
+
+		if (spi->tx_len && (sr & STM32H7_SPI_SR_TXP))
+			stm32h7_spi_write_txfifo(spi);
+
+		sr = readl_relaxed(spi->base + STM32H7_SPI_SR);
+
+		/* Clear suspension bit if necessary */
+		if (sr & STM32H7_SPI_SR_SUSP)
+			writel_relaxed(sr & STM32H7_SPI_SR_SUSP, spi->base + STM32H7_SPI_IFCR);
+	}
+
+	spin_unlock_irqrestore(&spi->lock, flags);
+
+	stm32h7_spi_disable(spi);
+	spi_finalize_current_transfer(spi->ctrl);
+
+	return 0;
+}
+
 /**
  * stm32h7_spi_transfer_one_irq - transfer a single spi_transfer using
  *				  interrupts
@@ -2026,6 +2081,24 @@ static int stm32_spi_transfer_one_setup(struct stm32_spi *spi,
 	return ret;
 }
 
+/**
+ * stm32_spi_can_poll - detect if poll based transfer is appropriate
+ * @spi: pointer to the spi controller data structure
+ *
+ * Returns true is poll is more appropriate, false otherwise.
+ */
+static bool stm32_spi_can_poll(struct stm32_spi *spi)
+{
+	unsigned long hz_per_byte, byte_limit;
+
+	/* Evaluate the transfer time and use polling if applicable */
+	hz_per_byte = polling_limit_us ?
+		      DIV_ROUND_UP(8 * USEC_PER_SEC, polling_limit_us) : 0;
+	byte_limit = hz_per_byte ? spi->cur_speed / hz_per_byte : 1;
+
+	return (spi->cur_xferlen < byte_limit) ? true : false;
+}
+
 /**
  * stm32_spi_transfer_one - transfer a single spi_transfer
  * @ctrl: controller interface
@@ -2058,6 +2131,8 @@ static int stm32_spi_transfer_one(struct spi_controller *ctrl,
 
 	if (spi->cur_usedma)
 		return stm32_spi_transfer_one_dma(spi, transfer);
+	else if (spi->cfg->transfer_one_poll && stm32_spi_can_poll(spi))
+		return spi->cfg->transfer_one_poll(spi);
 	else
 		return spi->cfg->transfer_one_irq(spi);
 }
@@ -2216,6 +2291,7 @@ static const struct stm32_spi_cfg stm32h7_spi_cfg = {
 	 * SPI access hence handling is performed within the SPI interrupt
 	 */
 	.transfer_one_irq = stm32h7_spi_transfer_one_irq,
+	.transfer_one_poll = stm32h7_spi_transfer_one_poll,
 	.irq_handler_thread = stm32h7_spi_irq_thread,
 	.baud_rate_div_min = STM32H7_SPI_MBR_DIV_MIN,
 	.baud_rate_div_max = STM32H7_SPI_MBR_DIV_MAX,
@@ -2245,6 +2321,7 @@ static const struct stm32_spi_cfg stm32mp25_spi_cfg = {
 	 * SPI access hence handling is performed within the SPI interrupt
 	 */
 	.transfer_one_irq = stm32h7_spi_transfer_one_irq,
+	.transfer_one_poll = stm32h7_spi_transfer_one_poll,
 	.irq_handler_thread = stm32h7_spi_irq_thread,
 	.baud_rate_div_min = STM32H7_SPI_MBR_DIV_MIN,
 	.baud_rate_div_max = STM32H7_SPI_MBR_DIV_MAX,