spi: spi-fsl-dspi: Reset SR flags before sending a new message

If, in a previous transfer, the controller sends more data than expected
by the DSPI target, SR.RFDF (RX FIFO is not empty) will remain asserted.
When flushing the FIFOs at the beginning of a new transfer (writing 1
into MCR.CLR_TXF and MCR.CLR_RXF), SR.RFDF should also be cleared.
Otherwise, when running in target mode with DMA, if SR.RFDF remains
asserted, the DMA callback will be fired before the controller sends any
data.

Take this opportunity to reset all Status Register fields.

Fixes: 5ce3cc5 ("spi: spi-fsl-dspi: Provide support for DSPI slave mode operation (Vybryd vf610)")
Signed-off-by: Larisa Grigore <[email protected]>
Signed-off-by: James Clark <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Mark Brown <[email protected]>

Author: Larisa Grigore

drivers/spi/spi-fsl-dspi.c

@@ -975,6 +975,8 @@ static int dspi_transfer_one_message(struct spi_controller *ctlr,
 				   SPI_MCR_CLR_TXF | SPI_MCR_CLR_RXF,
 				   SPI_MCR_CLR_TXF | SPI_MCR_CLR_RXF);
 
+		regmap_write(dspi->regmap, SPI_SR, SPI_SR_CLEAR);
+
 		spi_take_timestamp_pre(dspi->ctlr, dspi->cur_transfer,
 				       dspi->progress, !dspi->irq);