spi: microchip-core: ensure TX and RX FIFOs are empty at start of a transfer

While transmitting with rx_len == 0, the RX FIFO is not going to be
emptied in the interrupt handler. A subsequent transfer could then
read crap from the previous transfer out of the RX FIFO into the
start RX buffer. The core provides a register that will empty the RX and
TX FIFOs, so do that before each transfer.

Fixes: 9ac8d176 ("spi: add support for microchip fpga spi controllers")
Signed-off-by: Steve Wilkins <steve.wilkins@raymarine.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20240715-flammable-provoke-459226d08e70@wendySigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-microchip-core.c

@@ -91,6 +91,8 @@
 #define REG_CONTROL2		(0x28)
 #define REG_COMMAND		(0x2c)
 #define  COMMAND_CLRFRAMECNT	BIT(4)
+#define  COMMAND_TXFIFORST		BIT(3)
+#define  COMMAND_RXFIFORST		BIT(2)
 #define REG_PKTSIZE		(0x30)
 #define REG_CMD_SIZE		(0x34)
 #define REG_HWSTATUS		(0x38)
@@ -496,6 +498,8 @@ static int mchp_corespi_transfer_one(struct spi_controller *host,
 	mchp_corespi_set_xfer_size(spi, (spi->tx_len > FIFO_DEPTH)
 				   ? FIFO_DEPTH : spi->tx_len);
 
+	mchp_corespi_write(spi, REG_COMMAND, COMMAND_RXFIFORST | COMMAND_TXFIFORST);
+
 	mchp_corespi_write(spi, REG_SLAVE_SELECT, spi->pending_slave_select);
 
 	while (spi->tx_len)