[PATCH v2 2/4] spi: stm32: fix Overrun issue at < 8bpw

[Alain Volmat]

From: Deepak Kumar <deepak.kumar01 at st.com>

When SPI communication is suspended by hardware automatically, it could
happen that few bits of next frame are already clocked out due to
internal synchronization delay.

To achieve a safe suspension, we need to ensure that each word must be
at least 8 SPI clock cycles long. That's why, if bpw is less than 8
bits, we need to use midi to reach 8 SPI clock cycles at least.

This will ensure that each word achieve safe suspension and prevent
overrun condition.

Signed-off-by: Deepak Kumar <deepak.kumar01 at st.com>
Signed-off-by: Alain Volmat <alain.volmat at foss.st.com>
---
 drivers/spi/spi-stm32.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-stm32.c b/drivers/spi/spi-stm32.c
index 40651b6050f6..ced6d7b215b9 100644
--- a/drivers/spi/spi-stm32.c
+++ b/drivers/spi/spi-stm32.c
@@ -1906,11 +1906,12 @@ static void stm32h7_spi_data_idleness(struct stm32_spi *spi, struct spi_transfer
 	cfg2_clrb |= STM32H7_SPI_CFG2_MIDI;
 	if ((len > 1) && (spi->cur_midi > 0)) {
 		u32 sck_period_ns = DIV_ROUND_UP(NSEC_PER_SEC, spi->cur_speed);
-		u32 midi = min_t(u32,
-				 DIV_ROUND_UP(spi->cur_midi, sck_period_ns),
-				 FIELD_GET(STM32H7_SPI_CFG2_MIDI,
-				 STM32H7_SPI_CFG2_MIDI));
+		u32 midi = DIV_ROUND_UP(spi->cur_midi, sck_period_ns);
 
+		if ((spi->cur_bpw + midi) < 8)
+			midi = 8 - spi->cur_bpw;
+
+		midi = min_t(u32, midi, FIELD_MAX(STM32H7_SPI_CFG2_MIDI));
 
 		dev_dbg(spi->dev, "period=%dns, midi=%d(=%dns)\n",
 			sck_period_ns, midi, midi * sck_period_ns);

-- 
2.34.1