[PATCH v3 1/2] spi: axi-spi-engine: fix stale SYNC IRQ pending

[Jonathan Santos <Jonathan.Santos@analog.com>]

spi_engine_setup() sends a SYNC(1) command and polls SYNC_ID to confirm
it was parsed by the FPGA, but never clears the corresponding interrupt
pending bit (INT_PENDING[SYNC]). When the first real SPI transfer starts
and INT_SYNC is enabled, that stale pending bit fires immediately, causing
the IRQ handler to see the leftover SYNC_ID from setup, match it against
the current transfer's ID, and prematurely signal completion before the
hardware finishes.

This race manifests at low SPI clock frequencies (~2-3 MHz), where the
FPGA takes long enough to execute the transfer that handler is parsed
before it finishes. At higher SCLK rates the transfer completes fast
enough that the issue is masked.

Fix this by clearing INT_PENDING[SYNC] after the polled SYNC, ensuring no
stale interrupt is left pending.

Reported-by: Dennis Heinzel <dennis.heinzel@irs.systems>
Closes: https://ez.analog.com/linux-software-drivers/f/q-a/604145/axi-spi-engine-stale-sync-pending-can-complete-first-transfer-early-at-low-spi-clock-2-3-mhz
Fixes: 1d0ee0c9df31 ("spi: axi-spi-engine: wait for completion in setup")
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
---
 drivers/spi/spi-axi-spi-engine.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/spi/spi-axi-spi-engine.c b/drivers/spi/spi-axi-spi-engine.c
index 02bbc5d0cfc5..87c05cb064cc 100644
--- a/drivers/spi/spi-axi-spi-engine.c
+++ b/drivers/spi/spi-axi-spi-engine.c
@@ -887,6 +887,7 @@ static int spi_engine_setup(struct spi_device *device)
 	struct spi_controller *host = device->controller;
 	struct spi_engine *spi_engine = spi_controller_get_devdata(host);
 	unsigned int reg;
+	int ret;
 
 	if (device->mode & SPI_CS_HIGH)
 		spi_engine->cs_inv |= BIT(spi_get_chipselect(device, 0));
@@ -922,8 +923,15 @@ static int spi_engine_setup(struct spi_device *device)
 	writel_relaxed(SPI_ENGINE_CMD_SYNC(1),
 		       spi_engine->base + SPI_ENGINE_REG_CMD_FIFO);
 
-	return readl_relaxed_poll_timeout(spi_engine->base + SPI_ENGINE_REG_SYNC_ID,
-					  reg, reg == 1, 1, 1000);
+	ret = readl_relaxed_poll_timeout(spi_engine->base + SPI_ENGINE_REG_SYNC_ID,
+					 reg, reg == 1, 1, 1 * USEC_PER_MSEC);
+
+	/* Clear the stale SYNC pending bit so it doesn't fire when the IRQ is later enabled. */
+	writel_relaxed(SPI_ENGINE_INT_SYNC, spi_engine->base + SPI_ENGINE_REG_INT_PENDING);
+	/* Read back to flush posted write. */
+	readl_relaxed(spi_engine->base + SPI_ENGINE_REG_INT_PENDING);
+
+	return ret;
 }
 
 static int spi_engine_transfer_one_message(struct spi_controller *host,
-- 
2.34.1