spi: atmel: Fix interrupt setup for PDC transfers

Additionally to the current DMA transfer the PDC allows to set up a next DMA
transfer. This is useful for larger SPI transfers.

The driver currently waits for ENDRX as end of the transfer. But ENDRX is set
when the current DMA transfer is done (RCR = 0), i.e. it doesn't include the
next DMA transfer.
Thus a subsequent SPI transfer could be started although there is currently a
transfer in progress. This can cause invalid accesses to the SPI slave devices
and to SPI transfer errors.

This issue has been observed on a hardware with a M25P128 SPI NOR flash.

So instead of ENDRX we should wait for RXBUFF. This flag is set if there is
no more DMA transfer in progress (RCR = RNCR = 0).

Signed-off-by: Torsten Fleischer <torfl6749@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org

drivers/spi/spi-atmel.c

@@ -764,17 +764,17 @@ static void atmel_spi_pdc_next_xfer(struct spi_master *master,
 			(unsigned long long)xfer->rx_dma);
 	}
 
-	/* REVISIT: We're waiting for ENDRX before we start the next
+	/* REVISIT: We're waiting for RXBUFF before we start the next
 	 * transfer because we need to handle some difficult timing
-	 * issues otherwise. If we wait for ENDTX in one transfer and
-	 * then starts waiting for ENDRX in the next, it's difficult
-	 * to tell the difference between the ENDRX interrupt we're
-	 * actually waiting for and the ENDRX interrupt of the
+	 * issues otherwise. If we wait for TXBUFE in one transfer and
+	 * then starts waiting for RXBUFF in the next, it's difficult
+	 * to tell the difference between the RXBUFF interrupt we're
+	 * actually waiting for and the RXBUFF interrupt of the
 	 * previous transfer.
 	 *
 	 * It should be doable, though. Just not now...
 	 */
-	spi_writel(as, IER, SPI_BIT(ENDRX) | SPI_BIT(OVRES));
+	spi_writel(as, IER, SPI_BIT(RXBUFF) | SPI_BIT(OVRES));
 	spi_writel(as, PTCR, SPI_BIT(TXTEN) | SPI_BIT(RXTEN));
 }