[PATCH v1 RFC 1/2] spi: introduce fallback to pio

[Robin Gong <yibin.gong@nxp.com>]

Add SPI_CONTROLLER_FALLBACK to fallback to pio mode in case dma transfer
failed.
If spi client driver want to enable this feature please set master->flags
with SPI_MASTER_FALLBACK and add master->fallback checking in its can_dma()
as spi-imx.c

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
---
 drivers/spi/spi.c       | 11 +++++++++++
 include/linux/spi/spi.h |  4 ++++
 2 files changed, 15 insertions(+)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 8158e28..3bf860c 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -982,6 +982,8 @@ static int __spi_unmap_msg(struct spi_controller *ctlr, struct spi_message *msg)
 		spi_unmap_buf(ctlr, tx_dev, &xfer->tx_sg, DMA_TO_DEVICE);
 	}
 
+	ctlr->cur_msg_mapped = false;
+
 	return 0;
 }
 #else /* !CONFIG_HAS_DMA */
@@ -1234,8 +1236,16 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
 		if (xfer->tx_buf || xfer->rx_buf) {
 			reinit_completion(&ctlr->xfer_completion);
 
+fallback_pio:
 			ret = ctlr->transfer_one(ctlr, msg->spi, xfer);
 			if (ret < 0) {
+				if (ctlr->cur_msg_mapped &&
+				   (ctlr->flags & SPI_CONTROLLER_FALLBACK)) {
+					__spi_unmap_msg(ctlr, msg);
+					ctlr->fallback = true;
+					goto fallback_pio;
+				}
+
 				SPI_STATISTICS_INCREMENT_FIELD(statm,
 							       errors);
 				SPI_STATISTICS_INCREMENT_FIELD(stats,
@@ -1693,6 +1703,7 @@ void spi_finalize_current_message(struct spi_controller *ctlr)
 	spin_lock_irqsave(&ctlr->queue_lock, flags);
 	ctlr->cur_msg = NULL;
 	ctlr->cur_msg_prepared = false;
+	ctlr->fallback = false;
 	kthread_queue_work(&ctlr->kworker, &ctlr->pump_messages);
 	spin_unlock_irqrestore(&ctlr->queue_lock, flags);
 
diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index aac57b5..95291fe4 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -506,6 +506,8 @@ struct spi_controller {
 #define SPI_CONTROLLER_MUST_TX		BIT(4)	/* requires tx */
 
 #define SPI_MASTER_GPIO_SS		BIT(5)	/* GPIO CS must select slave */
+/* fallback to PIO if DMA failed */
+#define SPI_CONTROLLER_FALLBACK		BIT(6)
 
 	/* flag indicating this is an SPI slave controller */
 	bool			slave;
@@ -602,6 +604,7 @@ struct spi_controller {
 	bool				auto_runtime_pm;
 	bool                            cur_msg_prepared;
 	bool				cur_msg_mapped;
+	bool				fallback;
 	struct completion               xfer_completion;
 	size_t				max_dma_len;
 
@@ -1517,6 +1520,7 @@ of_find_spi_device_by_node(struct device_node *node)
 #define SPI_MASTER_NO_TX		SPI_CONTROLLER_NO_TX
 #define SPI_MASTER_MUST_RX		SPI_CONTROLLER_MUST_RX
 #define SPI_MASTER_MUST_TX		SPI_CONTROLLER_MUST_TX
+#define SPI_MASTER_FALLBACK		SPI_CONTROLLER_FALLBACK
 
 #define spi_master_get_devdata(_ctlr)	spi_controller_get_devdata(_ctlr)
 #define spi_master_set_devdata(_ctlr, _data)	\
-- 
2.7.4