spi: stm32: add a delay before SPI disable

As explained in errata sheet, in section "2.14.5 Truncation of SPI output
signals after EOT event":
On STM32MP1x, EOT interrupt can be thrown before the true end of
communication.

So we add a delay of a half period to wait the real end of the
transmission.

Link: https://www.st.com/resource/en/errata_sheet/es0539-stm32mp131x3x5x-device-errata-stmicroelectronics.pdf
Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20230906132735.748174-1-valentin.caron@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-stm32.c

@@ -277,6 +277,7 @@ struct stm32_spi_cfg {
  * @fifo_size: size of the embedded fifo in bytes
  * @cur_midi: master inter-data idleness in ns
  * @cur_speed: speed configured in Hz
+ * @cur_half_period: time of a half bit in us
  * @cur_bpw: number of bits in a single SPI data frame
  * @cur_fthlv: fifo threshold level (data frames in a single data packet)
  * @cur_comm: SPI communication mode
@@ -304,6 +305,7 @@ struct stm32_spi {
 
 	unsigned int cur_midi;
 	unsigned int cur_speed;
+	unsigned int cur_half_period;
 	unsigned int cur_bpw;
 	unsigned int cur_fthlv;
 	unsigned int cur_comm;
@@ -468,6 +470,8 @@ static int stm32_spi_prepare_mbr(struct stm32_spi *spi, u32 speed_hz,
 
 	spi->cur_speed = spi->clk_rate / (1 << mbrdiv);
 
+	spi->cur_half_period = DIV_ROUND_CLOSEST(USEC_PER_SEC, 2 * spi->cur_speed);
+
 	return mbrdiv - 1;
 }
 
@@ -709,6 +713,10 @@ static void stm32h7_spi_disable(struct stm32_spi *spi)
 		return;
 	}
 
+	/* Add a delay to make sure that transmission is ended. */
+	if (spi->cur_half_period)
+		udelay(spi->cur_half_period);
+
 	if (spi->cur_usedma && spi->dma_tx)
 		dmaengine_terminate_async(spi->dma_tx);
 	if (spi->cur_usedma && spi->dma_rx)