[PATCH] arm: spi: atmel_qspi: fix race condition in transfer completion check

[Ramin Moussavi]

In atmel_qspi_transfer(), the status register is polled with:

  imr = QSPI_SR_INSTRE | QSPI_SR_CSR;
  return readl_poll_timeout(aq->regs + QSPI_SR, sr,
                            (sr & imr) == imr,
                            ATMEL_QSPI_TIMEOUT);

However, this is racy: QSPI_SR_INSTRE can be set before QSPI_SR_CSR,
and will then be cleared by the read. If that happens, the condition
"(sr & imr) == imr" can never be true, and the function times out.

This race condition is avoided in at91bootstrap by accumulating the
status bits across reads until both bits have been observed:

  /* Poll INSTruction End and Chip Select Rise flags. */
  imr = (QSPI_SR_INSTRE | QSPI_SR_CSR);
  sr = 0;
  do {
    udelay(1);
    sr |= qspi_readl(qspi, QSPI_SR) & imr;
  } while ((--timeout) && (sr != imr));

Update U-Boot's atmel_qspi_transfer() to use the same pattern,
ensuring that both flags are observed even if they are not set
simultaneously.

Signed-off-by: Ramin Moussavi <lordrasmus at gmail.com>
---
 drivers/spi/atmel-quadspi.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/spi/atmel-quadspi.c b/drivers/spi/atmel-quadspi.c
index 8aa7a83aef4..0f9ce409a0c 100644
--- a/drivers/spi/atmel-quadspi.c
+++ b/drivers/spi/atmel-quadspi.c
@@ -616,6 +616,8 @@ static int atmel_qspi_transfer(struct atmel_qspi *aq,
 			       const struct spi_mem_op *op, u32 offset)
 {
 	u32 sr, imr;
+	u32 val = 0;
+	unsigned long timeout;
 
 	/* Skip to the final steps if there is no data */
 	if (op->data.nbytes) {
@@ -636,8 +638,16 @@ static int atmel_qspi_transfer(struct atmel_qspi *aq,
 
 	/* Poll INSTruction End and Chip Select Rise flags. */
 	imr = QSPI_SR_INSTRE | QSPI_SR_CSR;
-	return readl_poll_timeout(aq->regs + QSPI_SR, sr, (sr & imr) == imr,
-				  ATMEL_QSPI_TIMEOUT);
+
+	timeout = timer_get_us() + ATMEL_QSPI_TIMEOUT;
+	while (1) {
+		val |= readl(aq->regs + QSPI_SR) & imr;
+		if ((val & imr) == imr)
+			return 0;
+
+		if (time_after(timer_get_us(), timeout))
+			return -ETIMEDOUT;
+	}
 }
 
 static int atmel_qspi_sama7g5_set_cfg(struct atmel_qspi *aq,
-- 
2.50.1