1 files changed, 382 insertions, 87 deletions

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index 6f243a90c844..b4070c0de3df 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -25,7 +25,9 @@
 #include <linux/of.h>
 #include <linux/of_address.h>
 #include <linux/of_device.h>
-#include <linux/of_gpio.h>
+#include <linux/gpio/consumer.h>
+#include <linux/gpio/machine.h> /* FIXME: using chip internals */
+#include <linux/gpio/driver.h> /* FIXME: using chip internals */
 #include <linux/of_irq.h>
 #include <linux/spi/spi.h>
 
@@ -66,6 +68,7 @@
 #define BCM2835_SPI_FIFO_SIZE		64
 #define BCM2835_SPI_FIFO_SIZE_3_4	48
 #define BCM2835_SPI_DMA_MIN_LENGTH	96
+#define BCM2835_SPI_NUM_CS		3   /* raise as necessary */
 #define BCM2835_SPI_MODE_BITS	(SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \
 				| SPI_NO_CS | SPI_3WIRE)
 
@@ -92,7 +95,8 @@ MODULE_PARM_DESC(polling_limit_us,
  * @rx_prologue: bytes received without DMA if first RX sglist entry's
  *	length is not a multiple of 4 (to overcome hardware limitation)
  * @tx_spillover: whether @tx_prologue spills over to second TX sglist entry
- * @dma_pending: whether a DMA transfer is in progress
+ * @prepare_cs: precalculated CS register value for ->prepare_message()
+ *	(uses slave-specific clock polarity and phase settings)
  * @debugfs_dir: the debugfs directory - neede to remove debugfs when
  *      unloading the module
  * @count_transfer_polling: count of how often polling mode is used
@@ -102,6 +106,19 @@ MODULE_PARM_DESC(polling_limit_us,
  *      These are counted as well in @count_transfer_polling and
  *      @count_transfer_irq
  * @count_transfer_dma: count how often dma mode is used
+ * @chip_select: SPI slave currently selected
+ *	(used by bcm2835_spi_dma_tx_done() to write @clear_rx_cs)
+ * @tx_dma_active: whether a TX DMA descriptor is in progress
+ * @rx_dma_active: whether a RX DMA descriptor is in progress
+ *	(used by bcm2835_spi_dma_tx_done() to handle a race)
+ * @fill_tx_desc: preallocated TX DMA descriptor used for RX-only transfers
+ *	(cyclically copies from zero page to TX FIFO)
+ * @fill_tx_addr: bus address of zero page
+ * @clear_rx_desc: preallocated RX DMA descriptor used for TX-only transfers
+ *	(cyclically clears RX FIFO by writing @clear_rx_cs to CS register)
+ * @clear_rx_addr: bus address of @clear_rx_cs
+ * @clear_rx_cs: precalculated CS register value to clear RX FIFO
+ *	(uses slave-specific clock polarity and phase settings)
  */
 struct bcm2835_spi {
 	void __iomem *regs;
@@ -115,13 +132,22 @@ struct bcm2835_spi {
 	int tx_prologue;
 	int rx_prologue;
 	unsigned int tx_spillover;
-	unsigned int dma_pending;
+	u32 prepare_cs[BCM2835_SPI_NUM_CS];
 
 	struct dentry *debugfs_dir;
 	u64 count_transfer_polling;
 	u64 count_transfer_irq;
 	u64 count_transfer_irq_after_polling;
 	u64 count_transfer_dma;
+
+	u8 chip_select;
+	unsigned int tx_dma_active;
+	unsigned int rx_dma_active;
+	struct dma_async_tx_descriptor *fill_tx_desc;
+	dma_addr_t fill_tx_addr;
+	struct dma_async_tx_descriptor *clear_rx_desc[BCM2835_SPI_NUM_CS];
+	dma_addr_t clear_rx_addr;
+	u32 clear_rx_cs[BCM2835_SPI_NUM_CS] ____cacheline_aligned;
 };
 
 #if defined(CONFIG_DEBUG_FS)
@@ -319,6 +345,13 @@ static void bcm2835_spi_reset_hw(struct spi_controller *ctlr)
 		BCM2835_SPI_CS_INTD |
 		BCM2835_SPI_CS_DMAEN |
 		BCM2835_SPI_CS_TA);
+	/*
+	 * Transmission sometimes breaks unless the DONE bit is written at the
+	 * end of every transfer.  The spec says it's a RO bit.  Either the
+	 * spec is wrong and the bit is actually of type RW1C, or it's a
+	 * hardware erratum.
+	 */
+	cs |= BCM2835_SPI_CS_DONE;
 	/* and reset RX/TX FIFOS */
 	cs |= BCM2835_SPI_CS_CLEAR_RX | BCM2835_SPI_CS_CLEAR_TX;
 
@@ -448,14 +481,14 @@ static void bcm2835_spi_transfer_prologue(struct spi_controller *ctlr,
 	bs->rx_prologue  = 0;
 	bs->tx_spillover = false;
 
-	if (!sg_is_last(&tfr->tx_sg.sgl[0]))
+	if (bs->tx_buf && !sg_is_last(&tfr->tx_sg.sgl[0]))
 		bs->tx_prologue = sg_dma_len(&tfr->tx_sg.sgl[0]) & 3;
 
-	if (!sg_is_last(&tfr->rx_sg.sgl[0])) {
+	if (bs->rx_buf && !sg_is_last(&tfr->rx_sg.sgl[0])) {
 		bs->rx_prologue = sg_dma_len(&tfr->rx_sg.sgl[0]) & 3;
 
 		if (bs->rx_prologue > bs->tx_prologue) {
-			if (sg_is_last(&tfr->tx_sg.sgl[0])) {
+			if (!bs->tx_buf || sg_is_last(&tfr->tx_sg.sgl[0])) {
 				bs->tx_prologue  = bs->rx_prologue;
 			} else {
 				bs->tx_prologue += 4;
@@ -477,7 +510,9 @@ static void bcm2835_spi_transfer_prologue(struct spi_controller *ctlr,
 		bcm2835_wr_fifo_count(bs, bs->rx_prologue);
 		bcm2835_wait_tx_fifo_empty(bs);
 		bcm2835_rd_fifo_count(bs, bs->rx_prologue);
-		bcm2835_spi_reset_hw(ctlr);
+		bcm2835_wr(bs, BCM2835_SPI_CS, cs | BCM2835_SPI_CS_CLEAR_RX
+						  | BCM2835_SPI_CS_CLEAR_TX
+						  | BCM2835_SPI_CS_DONE);
 
 		dma_sync_single_for_device(ctlr->dma_rx->device->dev,
 					   sg_dma_address(&tfr->rx_sg.sgl[0]),
@@ -487,6 +522,9 @@ static void bcm2835_spi_transfer_prologue(struct spi_controller *ctlr,
 		sg_dma_len(&tfr->rx_sg.sgl[0])     -= bs->rx_prologue;
 	}
 
+	if (!bs->tx_buf)
+		return;
+
 	/*
 	 * Write remaining TX prologue.  Adjust first entry in TX sglist.
 	 * Also adjust second entry if prologue spills over to it.
@@ -498,7 +536,8 @@ static void bcm2835_spi_transfer_prologue(struct spi_controller *ctlr,
 						  | BCM2835_SPI_CS_DMAEN);
 		bcm2835_wr_fifo_count(bs, tx_remaining);
 		bcm2835_wait_tx_fifo_empty(bs);
-		bcm2835_wr(bs, BCM2835_SPI_CS, cs | BCM2835_SPI_CS_CLEAR_TX);
+		bcm2835_wr(bs, BCM2835_SPI_CS, cs | BCM2835_SPI_CS_CLEAR_TX
+						  | BCM2835_SPI_CS_DONE);
 	}
 
 	if (likely(!bs->tx_spillover)) {
@@ -531,6 +570,9 @@ static void bcm2835_spi_undo_prologue(struct bcm2835_spi *bs)
 		sg_dma_len(&tfr->rx_sg.sgl[0])     += bs->rx_prologue;
 	}
 
+	if (!bs->tx_buf)
+		goto out;
+
 	if (likely(!bs->tx_spillover)) {
 		sg_dma_address(&tfr->tx_sg.sgl[0]) -= bs->tx_prologue;
 		sg_dma_len(&tfr->tx_sg.sgl[0])     += bs->tx_prologue;
@@ -539,32 +581,85 @@ static void bcm2835_spi_undo_prologue(struct bcm2835_spi *bs)
 		sg_dma_address(&tfr->tx_sg.sgl[1]) -= 4;
 		sg_dma_len(&tfr->tx_sg.sgl[1])     += 4;
 	}
+out:
+	bs->tx_prologue = 0;
 }
 
-static void bcm2835_spi_dma_done(void *data)
+/**
+ * bcm2835_spi_dma_rx_done() - callback for DMA RX channel
+ * @data: SPI master controller
+ *
+ * Used for bidirectional and RX-only transfers.
+ */
+static void bcm2835_spi_dma_rx_done(void *data)
 {
 	struct spi_controller *ctlr = data;
 	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
 
-	/* reset fifo and HW */
-	bcm2835_spi_reset_hw(ctlr);
-
-	/* and terminate tx-dma as we do not have an irq for it
+	/* terminate tx-dma as we do not have an irq for it
 	 * because when the rx dma will terminate and this callback
 	 * is called the tx-dma must have finished - can't get to this
 	 * situation otherwise...
 	 */
-	if (cmpxchg(&bs->dma_pending, true, false)) {
-		dmaengine_terminate_async(ctlr->dma_tx);
-		bcm2835_spi_undo_prologue(bs);
-	}
+	dmaengine_terminate_async(ctlr->dma_tx);
+	bs->tx_dma_active = false;
+	bs->rx_dma_active = false;
+	bcm2835_spi_undo_prologue(bs);
+
+	/* reset fifo and HW */
+	bcm2835_spi_reset_hw(ctlr);
 
 	/* and mark as completed */;
 	complete(&ctlr->xfer_completion);
 }
 
+/**
+ * bcm2835_spi_dma_tx_done() - callback for DMA TX channel
+ * @data: SPI master controller
+ *
+ * Used for TX-only transfers.
+ */
+static void bcm2835_spi_dma_tx_done(void *data)
+{
+	struct spi_controller *ctlr = data;
+	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
+
+	/* busy-wait for TX FIFO to empty */
+	while (!(bcm2835_rd(bs, BCM2835_SPI_CS) & BCM2835_SPI_CS_DONE))
+		bcm2835_wr(bs, BCM2835_SPI_CS,
+			   bs->clear_rx_cs[bs->chip_select]);
+
+	bs->tx_dma_active = false;
+	smp_wmb();
+
+	/*
+	 * In case of a very short transfer, RX DMA may not have been
+	 * issued yet.  The onus is then on bcm2835_spi_transfer_one_dma()
+	 * to terminate it immediately after issuing.
+	 */
+	if (cmpxchg(&bs->rx_dma_active, true, false))
+		dmaengine_terminate_async(ctlr->dma_rx);
+
+	bcm2835_spi_undo_prologue(bs);
+	bcm2835_spi_reset_hw(ctlr);
+	complete(&ctlr->xfer_completion);
+}
+
+/**
+ * bcm2835_spi_prepare_sg() - prepare and submit DMA descriptor for sglist
+ * @ctlr: SPI master controller
+ * @spi: SPI slave
+ * @tfr: SPI transfer
+ * @bs: BCM2835 SPI controller
+ * @is_tx: whether to submit DMA descriptor for TX or RX sglist
+ *
+ * Prepare and submit a DMA descriptor for the TX or RX sglist of @tfr.
+ * Return 0 on success or a negative error number.
+ */
 static int bcm2835_spi_prepare_sg(struct spi_controller *ctlr,
+				  struct spi_device *spi,
 				  struct spi_transfer *tfr,
+				  struct bcm2835_spi *bs,
 				  bool is_tx)
 {
 	struct dma_chan *chan;
@@ -581,8 +676,7 @@ static int bcm2835_spi_prepare_sg(struct spi_controller *ctlr,
 		chan  = ctlr->dma_tx;
 		nents = tfr->tx_sg.nents;
 		sgl   = tfr->tx_sg.sgl;
-		flags = 0 /* no  tx interrupt */;
-
+		flags = tfr->rx_buf ? 0 : DMA_PREP_INTERRUPT;
 	} else {
 		dir   = DMA_DEV_TO_MEM;
 		chan  = ctlr->dma_rx;
@@ -595,10 +689,17 @@ static int bcm2835_spi_prepare_sg(struct spi_controller *ctlr,
 	if (!desc)
 		return -EINVAL;
 
-	/* set callback for rx */
+	/*
+	 * Completion is signaled by the RX channel for bidirectional and
+	 * RX-only transfers; else by the TX channel for TX-only transfers.
+	 */
 	if (!is_tx) {
-		desc->callback = bcm2835_spi_dma_done;
+		desc->callback = bcm2835_spi_dma_rx_done;
+		desc->callback_param = ctlr;
+	} else if (!tfr->rx_buf) {
+		desc->callback = bcm2835_spi_dma_tx_done;
 		desc->callback_param = ctlr;
+		bs->chip_select = spi->chip_select;
 	}
 
 	/* submit it to DMA-engine */
@@ -607,12 +708,60 @@ static int bcm2835_spi_prepare_sg(struct spi_controller *ctlr,
 	return dma_submit_error(cookie);
 }
 
+/**
+ * bcm2835_spi_transfer_one_dma() - perform SPI transfer using DMA engine
+ * @ctlr: SPI master controller
+ * @spi: SPI slave
+ * @tfr: SPI transfer
+ * @cs: CS register
+ *
+ * For *bidirectional* transfers (both tx_buf and rx_buf are non-%NULL), set up
+ * the TX and RX DMA channel to copy between memory and FIFO register.
+ *
+ * For *TX-only* transfers (rx_buf is %NULL), copying the RX FIFO's contents to
+ * memory is pointless.  However not reading the RX FIFO isn't an option either
+ * because transmission is halted once it's full.  As a workaround, cyclically
+ * clear the RX FIFO by setting the CLEAR_RX bit in the CS register.
+ *
+ * The CS register value is precalculated in bcm2835_spi_setup().  Normally
+ * this is called only once, on slave registration.  A DMA descriptor to write
+ * this value is preallocated in bcm2835_dma_init().  All that's left to do
+ * when performing a TX-only transfer is to submit this descriptor to the RX
+ * DMA channel.  Latency is thereby minimized.  The descriptor does not
+ * generate any interrupts while running.  It must be terminated once the
+ * TX DMA channel is done.
+ *
+ * Clearing the RX FIFO is paced by the DREQ signal.  The signal is asserted
+ * when the RX FIFO becomes half full, i.e. 32 bytes.  (Tuneable with the DC
+ * register.)  Reading 32 bytes from the RX FIFO would normally require 8 bus
+ * accesses, whereas clearing it requires only 1 bus access.  So an 8-fold
+ * reduction in bus traffic and thus energy consumption is achieved.
+ *
+ * For *RX-only* transfers (tx_buf is %NULL), fill the TX FIFO by cyclically
+ * copying from the zero page.  The DMA descriptor to do this is preallocated
+ * in bcm2835_dma_init().  It must be terminated once the RX DMA channel is
+ * done and can then be reused.
+ *
+ * The BCM2835 DMA driver autodetects when a transaction copies from the zero
+ * page and utilizes the DMA controller's ability to synthesize zeroes instead
+ * of copying them from memory.  This reduces traffic on the memory bus.  The
+ * feature is not available on so-called "lite" channels, but normally TX DMA
+ * is backed by a full-featured channel.
+ *
+ * Zero-filling the TX FIFO is paced by the DREQ signal.  Unfortunately the
+ * BCM2835 SPI controller continues to assert DREQ even after the DLEN register
+ * has been counted down to zero (hardware erratum).  Thus, when the transfer
+ * has finished, the DMA engine zero-fills the TX FIFO until it is half full.
+ * (Tuneable with the DC register.)  So up to 9 gratuitous bus accesses are
+ * performed at the end of an RX-only transfer.
+ */
 static int bcm2835_spi_transfer_one_dma(struct spi_controller *ctlr,
 					struct spi_device *spi,
 					struct spi_transfer *tfr,
 					u32 cs)
 {
 	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
+	dma_cookie_t cookie;
 	int ret;
 
 	/* update usage statistics */
@@ -625,16 +774,15 @@ static int bcm2835_spi_transfer_one_dma(struct spi_controller *ctlr,
 	bcm2835_spi_transfer_prologue(ctlr, tfr, bs, cs);
 
 	/* setup tx-DMA */
-	ret = bcm2835_spi_prepare_sg(ctlr, tfr, true);
+	if (bs->tx_buf) {
+		ret = bcm2835_spi_prepare_sg(ctlr, spi, tfr, bs, true);
+	} else {
+		cookie = dmaengine_submit(bs->fill_tx_desc);
+		ret = dma_submit_error(cookie);
+	}
 	if (ret)
 		goto err_reset_hw;
 
-	/* start TX early */
-	dma_async_issue_pending(ctlr->dma_tx);
-
-	/* mark as dma pending */
-	bs->dma_pending = 1;
-
 	/* set the DMA length */
 	bcm2835_wr(bs, BCM2835_SPI_DLEN, bs->tx_len);
 
@@ -642,20 +790,43 @@ static int bcm2835_spi_transfer_one_dma(struct spi_controller *ctlr,
 	bcm2835_wr(bs, BCM2835_SPI_CS,
 		   cs | BCM2835_SPI_CS_TA | BCM2835_SPI_CS_DMAEN);
 
+	bs->tx_dma_active = true;
+	smp_wmb();
+
+	/* start TX early */
+	dma_async_issue_pending(ctlr->dma_tx);
+
 	/* setup rx-DMA late - to run transfers while
 	 * mapping of the rx buffers still takes place
 	 * this saves 10us or more.
 	 */
-	ret = bcm2835_spi_prepare_sg(ctlr, tfr, false);
+	if (bs->rx_buf) {
+		ret = bcm2835_spi_prepare_sg(ctlr, spi, tfr, bs, false);
+	} else {
+		cookie = dmaengine_submit(bs->clear_rx_desc[spi->chip_select]);
+		ret = dma_submit_error(cookie);
+	}
 	if (ret) {
 		/* need to reset on errors */
 		dmaengine_terminate_sync(ctlr->dma_tx);
-		bs->dma_pending = false;
+		bs->tx_dma_active = false;
 		goto err_reset_hw;
 	}
 
 	/* start rx dma late */
 	dma_async_issue_pending(ctlr->dma_rx);
+	bs->rx_dma_active = true;
+	smp_mb();
+
+	/*
+	 * In case of a very short TX-only transfer, bcm2835_spi_dma_tx_done()
+	 * may run before RX DMA is issued.  Terminate RX DMA if so.
+	 */
+	if (!bs->rx_buf && !bs->tx_dma_active &&
+	    cmpxchg(&bs->rx_dma_active, true, false)) {
+		dmaengine_terminate_async(ctlr->dma_rx);
+		bcm2835_spi_reset_hw(ctlr);
+	}
 
 	/* wait for wakeup in framework */
 	return 1;
@@ -678,26 +849,52 @@ static bool bcm2835_spi_can_dma(struct spi_controller *ctlr,
 	return true;
 }
 
-static void bcm2835_dma_release(struct spi_controller *ctlr)
+static void bcm2835_dma_release(struct spi_controller *ctlr,
+				struct bcm2835_spi *bs)
 {
+	int i;
+
 	if (ctlr->dma_tx) {
 		dmaengine_terminate_sync(ctlr->dma_tx);
+
+		if (bs->fill_tx_desc)
+			dmaengine_desc_free(bs->fill_tx_desc);
+
+		if (bs->fill_tx_addr)
+			dma_unmap_page_attrs(ctlr->dma_tx->device->dev,
+					     bs->fill_tx_addr, sizeof(u32),
+					     DMA_TO_DEVICE,
+					     DMA_ATTR_SKIP_CPU_SYNC);
+
 		dma_release_channel(ctlr->dma_tx);
 		ctlr->dma_tx = NULL;
 	}
+
 	if (ctlr->dma_rx) {
 		dmaengine_terminate_sync(ctlr->dma_rx);
+
+		for (i = 0; i < BCM2835_SPI_NUM_CS; i++)
+			if (bs->clear_rx_desc[i])
+				dmaengine_desc_free(bs->clear_rx_desc[i]);
+
+		if (bs->clear_rx_addr)
+			dma_unmap_single(ctlr->dma_rx->device->dev,
+					 bs->clear_rx_addr,
+					 sizeof(bs->clear_rx_cs),
+					 DMA_TO_DEVICE);
+
 		dma_release_channel(ctlr->dma_rx);
 		ctlr->dma_rx = NULL;
 	}
 }
 
-static void bcm2835_dma_init(struct spi_controller *ctlr, struct device *dev)
+static void bcm2835_dma_init(struct spi_controller *ctlr, struct device *dev,
+			     struct bcm2835_spi *bs)
 {
 	struct dma_slave_config slave_config;
 	const __be32 *addr;
 	dma_addr_t dma_reg_base;
-	int ret;
+	int ret, i;
 
 	/* base address in dma-space */
 	addr = of_get_address(ctlr->dev.of_node, 0, NULL, NULL);
@@ -719,7 +916,11 @@ static void bcm2835_dma_init(struct spi_controller *ctlr, struct device *dev)
 		goto err_release;
 	}
 
-	/* configure DMAs */
+	/*
+	 * The TX DMA channel either copies a transfer's TX buffer to the FIFO
+	 * or, in case of an RX-only transfer, cyclically copies from the zero
+	 * page to the FIFO using a preallocated, reusable descriptor.
+	 */
 	slave_config.dst_addr = (u32)(dma_reg_base + BCM2835_SPI_FIFO);
 	slave_config.dst_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
 
@@ -727,17 +928,74 @@ static void bcm2835_dma_init(struct spi_controller *ctlr, struct device *dev)
 	if (ret)
 		goto err_config;
 
+	bs->fill_tx_addr = dma_map_page_attrs(ctlr->dma_tx->device->dev,
+					      ZERO_PAGE(0), 0, sizeof(u32),
+					      DMA_TO_DEVICE,
+					      DMA_ATTR_SKIP_CPU_SYNC);
+	if (dma_mapping_error(ctlr->dma_tx->device->dev, bs->fill_tx_addr)) {
+		dev_err(dev, "cannot map zero page - not using DMA mode\n");
+		bs->fill_tx_addr = 0;
+		goto err_release;
+	}
+
+	bs->fill_tx_desc = dmaengine_prep_dma_cyclic(ctlr->dma_tx,
+						     bs->fill_tx_addr,
+						     sizeof(u32), 0,
+						     DMA_MEM_TO_DEV, 0);
+	if (!bs->fill_tx_desc) {
+		dev_err(dev, "cannot prepare fill_tx_desc - not using DMA mode\n");
+		goto err_release;
+	}
+
+	ret = dmaengine_desc_set_reuse(bs->fill_tx_desc);
+	if (ret) {
+		dev_err(dev, "cannot reuse fill_tx_desc - not using DMA mode\n");
+		goto err_release;
+	}
+
+	/*
+	 * The RX DMA channel is used bidirectionally:  It either reads the
+	 * RX FIFO or, in case of a TX-only transfer, cyclically writes a
+	 * precalculated value to the CS register to clear the RX FIFO.
+	 */
 	slave_config.src_addr = (u32)(dma_reg_base + BCM2835_SPI_FIFO);
 	slave_config.src_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
+	slave_config.dst_addr = (u32)(dma_reg_base + BCM2835_SPI_CS);
+	slave_config.dst_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
 
 	ret = dmaengine_slave_config(ctlr->dma_rx, &slave_config);
 	if (ret)
 		goto err_config;
 
+	bs->clear_rx_addr = dma_map_single(ctlr->dma_rx->device->dev,
+					   bs->clear_rx_cs,
+					   sizeof(bs->clear_rx_cs),
+					   DMA_TO_DEVICE);
+	if (dma_mapping_error(ctlr->dma_rx->device->dev, bs->clear_rx_addr)) {
+		dev_err(dev, "cannot map clear_rx_cs - not using DMA mode\n");
+		bs->clear_rx_addr = 0;
+		goto err_release;
+	}
+
+	for (i = 0; i < BCM2835_SPI_NUM_CS; i++) {
+		bs->clear_rx_desc[i] = dmaengine_prep_dma_cyclic(ctlr->dma_rx,
+					   bs->clear_rx_addr + i * sizeof(u32),
+					   sizeof(u32), 0,
+					   DMA_MEM_TO_DEV, 0);
+		if (!bs->clear_rx_desc[i]) {
+			dev_err(dev, "cannot prepare clear_rx_desc - not using DMA mode\n");
+			goto err_release;
+		}
+
+		ret = dmaengine_desc_set_reuse(bs->clear_rx_desc[i]);
+		if (ret) {
+			dev_err(dev, "cannot reuse clear_rx_desc - not using DMA mode\n");
+			goto err_release;
+		}
+	}
+
 	/* all went well, so set can_dma */
 	ctlr->can_dma = bcm2835_spi_can_dma;
-	/* need to do TX AND RX DMA, so we need dummy buffers */
-	ctlr->flags = SPI_CONTROLLER_MUST_RX | SPI_CONTROLLER_MUST_TX;
 
 	return;
 
@@ -745,7 +1003,7 @@ err_config:
 	dev_err(dev, "issue configuring dma: %d - not using DMA mode\n",
 		ret);
 err_release:
-	bcm2835_dma_release(ctlr);
+	bcm2835_dma_release(ctlr, bs);
 err:
 	return;
 }
@@ -812,7 +1070,7 @@ static int bcm2835_spi_transfer_one(struct spi_controller *ctlr,
 	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
 	unsigned long spi_hz, clk_hz, cdiv, spi_used_hz;
 	unsigned long hz_per_byte, byte_limit;
-	u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
+	u32 cs = bs->prepare_cs[spi->chip_select];
 
 	/* set clock */
 	spi_hz = tfr->speed_hz;
@@ -834,17 +1092,8 @@ static int bcm2835_spi_transfer_one(struct spi_controller *ctlr,
 	bcm2835_wr(bs, BCM2835_SPI_CLK, cdiv);
 
 	/* handle all the 3-wire mode */
-	if ((spi->mode & SPI_3WIRE) && (tfr->rx_buf))
+	if (spi->mode & SPI_3WIRE && tfr->rx_buf)
 		cs |= BCM2835_SPI_CS_REN;
-	else
-		cs &= ~BCM2835_SPI_CS_REN;
-
-	/*
-	 * The driver always uses software-controlled GPIO Chip Select.
-	 * Set the hardware-controlled native Chip Select to an invalid
-	 * value to prevent it from interfering.
-	 */
-	cs |= BCM2835_SPI_CS_CS_10 | BCM2835_SPI_CS_CS_01;
 
 	/* set transmit buffers and length */
 	bs->tx_buf = tfr->tx_buf;
@@ -881,7 +1130,6 @@ static int bcm2835_spi_prepare_message(struct spi_controller *ctlr,
 {
 	struct spi_device *spi = msg->spi;
 	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
-	u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
 	int ret;
 
 	if (ctlr->can_dma) {
@@ -896,14 +1144,11 @@ static int bcm2835_spi_prepare_message(struct spi_controller *ctlr,
 			return ret;
 	}
 
-	cs &= ~(BCM2835_SPI_CS_CPOL | BCM2835_SPI_CS_CPHA);
-
-	if (spi->mode & SPI_CPOL)
-		cs |= BCM2835_SPI_CS_CPOL;
-	if (spi->mode & SPI_CPHA)
-		cs |= BCM2835_SPI_CS_CPHA;
-
-	bcm2835_wr(bs, BCM2835_SPI_CS, cs);
+	/*
+	 * Set up clock polarity before spi_transfer_one_message() asserts
+	 * chip select to avoid a gratuitous clock signal edge.
+	 */
+	bcm2835_wr(bs, BCM2835_SPI_CS, bs->prepare_cs[spi->chip_select]);
 
 	return 0;
 }
@@ -914,11 +1159,12 @@ static void bcm2835_spi_handle_err(struct spi_controller *ctlr,
 	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
 
 	/* if an error occurred and we have an active dma, then terminate */
-	if (cmpxchg(&bs->dma_pending, true, false)) {
-		dmaengine_terminate_sync(ctlr->dma_tx);
-		dmaengine_terminate_sync(ctlr->dma_rx);
-		bcm2835_spi_undo_prologue(bs);
-	}
+	dmaengine_terminate_sync(ctlr->dma_tx);
+	bs->tx_dma_active = false;
+	dmaengine_terminate_sync(ctlr->dma_rx);
+	bs->rx_dma_active = false;
+	bcm2835_spi_undo_prologue(bs);
+
 	/* and reset */
 	bcm2835_spi_reset_hw(ctlr);
 }
@@ -930,14 +1176,50 @@ static int chip_match_name(struct gpio_chip *chip, void *data)
 
 static int bcm2835_spi_setup(struct spi_device *spi)
 {
-	int err;
+	struct spi_controller *ctlr = spi->controller;
+	struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
 	struct gpio_chip *chip;
+	enum gpio_lookup_flags lflags;
+	u32 cs;
+
+	/*
+	 * Precalculate SPI slave's CS register value for ->prepare_message():
+	 * The driver always uses software-controlled GPIO chip select, hence
+	 * set the hardware-controlled native chip select to an invalid value
+	 * to prevent it from interfering.
+	 */
+	cs = BCM2835_SPI_CS_CS_10 | BCM2835_SPI_CS_CS_01;
+	if (spi->mode & SPI_CPOL)
+		cs |= BCM2835_SPI_CS_CPOL;
+	if (spi->mode & SPI_CPHA)
+		cs |= BCM2835_SPI_CS_CPHA;
+	bs->prepare_cs[spi->chip_select] = cs;
+
+	/*
+	 * Precalculate SPI slave's CS register value to clear RX FIFO
+	 * in case of a TX-only DMA transfer.
+	 */
+	if (ctlr->dma_rx) {
+		bs->clear_rx_cs[spi->chip_select] = cs |
+						    BCM2835_SPI_CS_TA |
+						    BCM2835_SPI_CS_DMAEN |
+						    BCM2835_SPI_CS_CLEAR_RX;
+		dma_sync_single_for_device(ctlr->dma_rx->device->dev,
+					   bs->clear_rx_addr,
+					   sizeof(bs->clear_rx_cs),
+					   DMA_TO_DEVICE);
+	}
+
 	/*
 	 * sanity checking the native-chipselects
 	 */
 	if (spi->mode & SPI_NO_CS)
 		return 0;
-	if (gpio_is_valid(spi->cs_gpio))
+	/*
+	 * The SPI core has successfully requested the CS GPIO line from the
+	 * device tree, so we are done.
+	 */
+	if (spi->cs_gpiod)
 		return 0;
 	if (spi->chip_select > 1) {
 		/* error in the case of native CS requested with CS > 1
@@ -948,29 +1230,43 @@ static int bcm2835_spi_setup(struct spi_device *spi)
 			"setup: only two native chip-selects are supported\n");
 		return -EINVAL;
 	}
-	/* now translate native cs to GPIO */
+
+	/*
+	 * Translate native CS to GPIO
+	 *
+	 * FIXME: poking around in the gpiolib internals like this is
+	 * not very good practice. Find a way to locate the real problem
+	 * and fix it. Why is the GPIO descriptor in spi->cs_gpiod
+	 * sometimes not assigned correctly? Erroneous device trees?
+	 */
 
 	/* get the gpio chip for the base */
 	chip = gpiochip_find("pinctrl-bcm2835", chip_match_name);
 	if (!chip)
 		return 0;
 
-	/* and calculate the real CS */
-	spi->cs_gpio = chip->base + 8 - spi->chip_select;
+	/*
+	 * Retrieve the corresponding GPIO line used for CS.
+	 * The inversion semantics will be handled by the GPIO core
+	 * code, so we pass GPIOS_OUT_LOW for "unasserted" and
+	 * the correct flag for inversion semantics. The SPI_CS_HIGH
+	 * on spi->mode cannot be checked for polarity in this case
+	 * as the flag use_gpio_descriptors enforces SPI_CS_HIGH.
+	 */
+	if (of_property_read_bool(spi->dev.of_node, "spi-cs-high"))
+		lflags = GPIO_ACTIVE_HIGH;
+	else
+		lflags = GPIO_ACTIVE_LOW;
+	spi->cs_gpiod = gpiochip_request_own_desc(chip, 8 - spi->chip_select,
+						  DRV_NAME,
+						  lflags,
+						  GPIOD_OUT_LOW);
+	if (IS_ERR(spi->cs_gpiod))
+		return PTR_ERR(spi->cs_gpiod);
 
 	/* and set up the "mode" and level */
-	dev_info(&spi->dev, "setting up native-CS%i as GPIO %i\n",
-		 spi->chip_select, spi->cs_gpio);
-
-	/* set up GPIO as output and pull to the correct level */
-	err = gpio_direction_output(spi->cs_gpio,
-				    (spi->mode & SPI_CS_HIGH) ? 0 : 1);
-	if (err) {
-		dev_err(&spi->dev,
-			"could not set CS%i gpio %i as output: %i",
-			spi->chip_select, spi->cs_gpio, err);
-		return err;
-	}
+	dev_info(&spi->dev, "setting up native-CS%i to use GPIO\n",
+		 spi->chip_select);
 
 	return 0;
 }
@@ -979,18 +1275,19 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
 {
 	struct spi_controller *ctlr;
 	struct bcm2835_spi *bs;
-	struct resource *res;
 	int err;
 
-	ctlr = spi_alloc_master(&pdev->dev, sizeof(*bs));
+	ctlr = spi_alloc_master(&pdev->dev, ALIGN(sizeof(*bs),
+						  dma_get_cache_alignment()));
 	if (!ctlr)
 		return -ENOMEM;
 
 	platform_set_drvdata(pdev, ctlr);
 
+	ctlr->use_gpio_descriptors = true;
 	ctlr->mode_bits = BCM2835_SPI_MODE_BITS;
 	ctlr->bits_per_word_mask = SPI_BPW_MASK(8);
-	ctlr->num_chipselect = 3;
+	ctlr->num_chipselect = BCM2835_SPI_NUM_CS;
 	ctlr->setup = bcm2835_spi_setup;
 	ctlr->transfer_one = bcm2835_spi_transfer_one;
 	ctlr->handle_err = bcm2835_spi_handle_err;
@@ -999,8 +1296,7 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
 
 	bs = spi_controller_get_devdata(ctlr);
 
-	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
-	bs->regs = devm_ioremap_resource(&pdev->dev, res);
+	bs->regs = devm_platform_ioremap_resource(pdev, 0);
 	if (IS_ERR(bs->regs)) {
 		err = PTR_ERR(bs->regs);
 		goto out_controller_put;
@@ -1015,14 +1311,13 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
 
 	bs->irq = platform_get_irq(pdev, 0);
 	if (bs->irq <= 0) {
-		dev_err(&pdev->dev, "could not get IRQ: %d\n", bs->irq);
 		err = bs->irq ? bs->irq : -ENODEV;
 		goto out_controller_put;
 	}
 
 	clk_prepare_enable(bs->clk);
 
-	bcm2835_dma_init(ctlr, &pdev->dev);
+	bcm2835_dma_init(ctlr, &pdev->dev, bs);
 
 	/* initialise the hardware with the default polarities */
 	bcm2835_wr(bs, BCM2835_SPI_CS,
@@ -1066,7 +1361,7 @@ static int bcm2835_spi_remove(struct platform_device *pdev)
 
 	clk_disable_unprepare(bs->clk);
 
-	bcm2835_dma_release(ctlr);
+	bcm2835_dma_release(ctlr, bs);
 
 	return 0;
 }