[PATCH] spi: cadence: recover the controller after a failed transfer

[Mario Behn <mario.behn@gmail.com>]

When a transfer times out, the bytes it clocked in are still in the RX
FIFO: the interrupt that would have read them did not run in time, and
the host path never drains the FIFO between transfers (only the target
path does, in cdns_unprepare_transfer_hardware).  Every later transfer
then reads the stale bytes first and returns shifted data, and the
device behind the controller looks dead until the system is rebooted.

Seen on a Zynq-7010 with an AD9361 on spi0 under PREEMPT_RT: after one
"SPI transfer timed out" every register read returned 0xFF, the status
register showed RX FIFO not empty while the bus was idle, and a later
sample rate change failed with a calibration timeout.

Add a handle_err callback that masks the interrupts, drops the stale
bytes and brings the controller back to its initial state.  The next
message sets the clock mode, the clock rate and the chip select again
through the usual prepare_message, transfer_one and set_cs path.

Signed-off-by: Mario Behn <mario.behn@gmail.com>
---
 drivers/spi/spi-cadence.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/drivers/spi/spi-cadence.c b/drivers/spi/spi-cadence.c
index 50ef3a7871c2..474ce14a59b3 100644
--- a/drivers/spi/spi-cadence.c
+++ b/drivers/spi/spi-cadence.c
@@ -542,6 +542,30 @@ static int cdns_transfer_one(struct spi_controller *ctlr,
 	return transfer->len;
 }
 
+/**
+ * cdns_spi_handle_err - Recover the controller after a failed transfer
+ * @ctlr:	Pointer to the spi_controller structure
+ * @msg:	Pointer to the spi_message that failed
+ *
+ * A transfer that timed out leaves the bytes it clocked in sitting in the
+ * RX FIFO, because the interrupt that reads them did not run in time and
+ * the host path never drains the FIFO between transfers.  Every later
+ * transfer would then read those stale bytes first and return shifted
+ * data.  Mask the interrupts, drop the stale bytes and bring the
+ * controller back to its initial state; the next message sets the clock
+ * and the chip select again on its way through transfer_one.
+ */
+static void cdns_spi_handle_err(struct spi_controller *ctlr,
+				struct spi_message *msg)
+{
+	struct cdns_spi *xspi = spi_controller_get_devdata(ctlr);
+
+	cdns_spi_write(xspi, CDNS_SPI_IDR, CDNS_SPI_IXR_ALL);
+	xspi->tx_bytes = 0;
+	xspi->rx_bytes = 0;
+	cdns_spi_init_hw(xspi, spi_controller_is_target(ctlr));
+}
+
 /**
  * cdns_prepare_transfer_hardware - Prepares hardware for transfer.
  * @ctlr:	Pointer to the spi_controller structure which provides
@@ -727,6 +751,7 @@ static int cdns_spi_probe(struct platform_device *pdev)
 	ctlr->prepare_message = cdns_prepare_message;
 	ctlr->transfer_one = cdns_transfer_one;
 	ctlr->unprepare_transfer_hardware = cdns_unprepare_transfer_hardware;
+	ctlr->handle_err = cdns_spi_handle_err;
 	ctlr->mode_bits = SPI_CPOL | SPI_CPHA;
 	ctlr->bits_per_word_mask = SPI_BPW_MASK(8);
 	ctlr->flags = SPI_CONTROLLER_MUST_TX;
-- 
2.53.0