[PATCH v2 1/2] spi: introduce fallback to pio

[Robin Gong]

Add fallback to pio mode in case dma transfer failed with error status
SPI_TRANS_FAIL_NO_START.
If spi client driver want to enable this feature please set xfer->error in
the proper place such as dmaengine_prep_slave_sg() failure detect(but no
any data put into spi bus yet). Besides, add master->fallback checking in
its can_dma() so that spi core could switch to pio next time. Please refer
to spi-imx.c.

Signed-off-by: Robin Gong <yibin.gong@xxxxxxx>
---
 drivers/spi/spi.c       | 12 ++++++++++++
 include/linux/spi/spi.h |  7 +++++++
 2 files changed, 19 insertions(+)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 8158e28..6fa5659 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 +1236,17 @@ 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 &&
+				   (xfer->error & SPI_TRANS_FAIL_NO_START)) {
+					__spi_unmap_msg(ctlr, msg);
+					ctlr->fallback = true;
+					xfer->error &= ~SPI_TRANS_FAIL_NO_START;
+					goto fallback_pio;
+				}
+
 				SPI_STATISTICS_INCREMENT_FIELD(statm,
 							       errors);
 				SPI_STATISTICS_INCREMENT_FIELD(stats,
@@ -1693,6 +1704,7 @@ 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..b4917df 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -447,6 +447,8 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  *	If the driver does not set this, the SPI core takes the snapshot as
  *	close to the driver hand-over as possible.
  * @irq_flags: Interrupt enable state during PTP system timestamping
+ * @fallback: fallback to pio if dma transfer return failure with
+ *	SPI_TRANS_FAIL_NO_START.
  *
  * Each SPI controller can communicate with one or more @spi_device
  * children.  These make a small bus, sharing MOSI, MISO and SCK signals
@@ -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;
 
@@ -847,6 +850,7 @@ extern void spi_res_release(struct spi_controller *ctlr,
  *	back unset and they need the better resolution.
  * @timestamped_post: See above. The reason why both exist is that these
  *	booleans are also used to keep state in the core SPI logic.
+ * @error: Error status logged by spi controller driver.
  *
  * SPI transfers always write the same number of bytes as they read.
  * Protocol drivers should always provide @rx_buf and/or @tx_buf.
@@ -940,6 +944,9 @@ struct spi_transfer {
 	bool		timestamped;
 
 	struct list_head transfer_list;
+
+#define SPI_TRANS_FAIL_NO_START	BIT(0)
+	u16		error;
 };
 
 /**
-- 
2.7.4