[PATCH v2] spi: microchip-core: prevent RX overflows when transmit size > FIFO size

[Conor Dooley <conor.dooley@xxxxxxxxxxxxx>]

When the size of a transfer exceeds the size of the FIFO (32 bytes), RX
overflows will be generated and receive data will be corrupted and
warnings will be produced. For example, here's an error generated by a
transfer of 36 bytes:

  spi_master spi0: mchp_corespi_interrupt: RX OVERFLOW: rxlen: 4, txlen: 0

The driver is currently split between handling receiving in the
interrupt handler, and sending outside of it. Move all handling out of
the interrupt handling, and explicitly link the number of bytes read of
of the RX FIFO to the number written into the TX one. This both resolves
the overflow problems as well as simplifying the flow of the driver.

CC: stable@xxxxxxxxxxxxxxx
Fixes: 9ac8d17694b6 ("spi: add support for microchip fpga spi controllers")
Signed-off-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
---

Contrary to last time, the reporter did actually manage to this this
version, and it resolved their probably apparently.

Changes in v2:
- Move all FIFO interactions out of the interrupt handler
- Disable the non-error interrupts since they're dealt with out of the
  isr now

v1: https://lore.kernel.org/all/20250114-easiness-pregame-d1d2d4b57e7b@spud/

CC: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
CC: Daire McNamara <daire.mcnamara@xxxxxxxxxxxxx>
CC: Mark Brown <broonie@xxxxxxxxxx>
CC: linux-spi@xxxxxxxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
---
 drivers/spi/spi-microchip-core.c | 41 ++++++++++++++------------------
 1 file changed, 18 insertions(+), 23 deletions(-)

diff --git a/drivers/spi/spi-microchip-core.c b/drivers/spi/spi-microchip-core.c
index 5b6af55855efc..62ba0bd9cbb7e 100644
--- a/drivers/spi/spi-microchip-core.c
+++ b/drivers/spi/spi-microchip-core.c
@@ -70,8 +70,7 @@
 #define INT_RX_CHANNEL_OVERFLOW		BIT(2)
 #define INT_TX_CHANNEL_UNDERRUN		BIT(3)
 
-#define INT_ENABLE_MASK (CONTROL_RX_DATA_INT | CONTROL_TX_DATA_INT | \
-			 CONTROL_RX_OVER_INT | CONTROL_TX_UNDER_INT)
+#define INT_ENABLE_MASK (CONTROL_RX_OVER_INT | CONTROL_TX_UNDER_INT)
 
 #define REG_CONTROL		(0x00)
 #define REG_FRAME_SIZE		(0x04)
@@ -133,10 +132,15 @@ static inline void mchp_corespi_disable(struct mchp_corespi *spi)
 	mchp_corespi_write(spi, REG_CONTROL, control);
 }
 
-static inline void mchp_corespi_read_fifo(struct mchp_corespi *spi)
+static inline void mchp_corespi_read_fifo(struct mchp_corespi *spi, int fifo_max)
 {
-	while (spi->rx_len >= spi->n_bytes && !(mchp_corespi_read(spi, REG_STATUS) & STATUS_RXFIFO_EMPTY)) {
-		u32 data = mchp_corespi_read(spi, REG_RX_DATA);
+	for (int i = 0; i < fifo_max; i++) {
+		u32 data;
+
+		while (mchp_corespi_read(spi, REG_STATUS) & STATUS_RXFIFO_EMPTY)
+			;
+
+		data = mchp_corespi_read(spi, REG_RX_DATA);
 
 		spi->rx_len -= spi->n_bytes;
 
@@ -211,11 +215,10 @@ static inline void mchp_corespi_set_xfer_size(struct mchp_corespi *spi, int len)
 	mchp_corespi_write(spi, REG_FRAMESUP, len);
 }
 
-static inline void mchp_corespi_write_fifo(struct mchp_corespi *spi)
+static inline void mchp_corespi_write_fifo(struct mchp_corespi *spi, int fifo_max)
 {
-	int fifo_max, i = 0;
+	int i = 0;
 
-	fifo_max = DIV_ROUND_UP(min(spi->tx_len, FIFO_DEPTH), spi->n_bytes);
 	mchp_corespi_set_xfer_size(spi, fifo_max);
 
 	while ((i < fifo_max) && !(mchp_corespi_read(spi, REG_STATUS) & STATUS_TXFIFO_FULL)) {
@@ -413,19 +416,6 @@ static irqreturn_t mchp_corespi_interrupt(int irq, void *dev_id)
 	if (intfield == 0)
 		return IRQ_NONE;
 
-	if (intfield & INT_TXDONE)
-		mchp_corespi_write(spi, REG_INT_CLEAR, INT_TXDONE);
-
-	if (intfield & INT_RXRDY) {
-		mchp_corespi_write(spi, REG_INT_CLEAR, INT_RXRDY);
-
-		if (spi->rx_len)
-			mchp_corespi_read_fifo(spi);
-	}
-
-	if (!spi->rx_len && !spi->tx_len)
-		finalise = true;
-
 	if (intfield & INT_RX_CHANNEL_OVERFLOW) {
 		mchp_corespi_write(spi, REG_INT_CLEAR, INT_RX_CHANNEL_OVERFLOW);
 		finalise = true;
@@ -512,9 +502,14 @@ static int mchp_corespi_transfer_one(struct spi_controller *host,
 
 	mchp_corespi_write(spi, REG_SLAVE_SELECT, spi->pending_slave_select);
 
-	while (spi->tx_len)
-		mchp_corespi_write_fifo(spi);
+	while (spi->tx_len) {
+		int fifo_max = DIV_ROUND_UP(min(spi->tx_len, FIFO_DEPTH), spi->n_bytes);
 
+		mchp_corespi_write_fifo(spi, fifo_max);
+		mchp_corespi_read_fifo(spi, fifo_max);
+	}
+
+	spi_finalize_current_transfer(host);
 	return 1;
 }
 
-- 
2.48.1