[PATCH V2] SPI: BCM2835: allow arbitrary GPIO to act as SPI-chip_select

[kernel@xxxxxxxxxxxxxxxx]

From: Martin Sperl <kernel@xxxxxxxxxxxxxxxx>

use cs-gpios for the spi bus master and standard gpio operation
instead of relying on the HW with just 2 chip_selects.

Signed-off-by: Martin Sperl <kernel@xxxxxxxxxxxxxxxx>
---
 drivers/spi/spi-bcm2835.c |   65 ++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 58 insertions(+), 7 deletions(-)

diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
index e1dea40..5fbad64 100644
--- a/drivers/spi/spi-bcm2835.c
+++ b/drivers/spi/spi-bcm2835.c
@@ -30,6 +30,7 @@
 #include <linux/of.h>
 #include <linux/of_irq.h>
 #include <linux/of_device.h>
+#include <linux/of_gpio.h>
 #include <linux/spi/spi.h>

 /* SPI register offsets */
@@ -116,6 +117,16 @@ static inline void bcm2835_wr_fifo(struct bcm2835_spi *bs, int len)
 	}
 }

+/* ideally spi_set_cs would be exported by spi-core */
+static inline void bcm2835_set_cs(struct spi_device *spi, bool enable)
+{
+	if (spi->mode & SPI_CS_HIGH)
+		enable = !enable;
+
+	if (gpio_is_valid(spi->cs_gpio))
+		gpio_set_value(spi->cs_gpio, !enable);
+}
+
 static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)
 {
 	struct spi_master *master = dev_id;
@@ -207,12 +218,24 @@ static int bcm2835_spi_start_transfer(
 		cs |= BCM2835_SPI_CS_CPHA;

 	if (!(spi->mode & SPI_NO_CS)) {
-		if (spi->mode & SPI_CS_HIGH) {
-			cs |= BCM2835_SPI_CS_CSPOL;
-			cs |= BCM2835_SPI_CS_CSPOL0 << spi->chip_select;
-		}
+		if (gpio_is_valid(spi->cs_gpio)) {
+			bcm2835_set_cs(spi, 1);
+		} else {
+			/* Legacy mode using HW chip selects
+			 * note that there is a bug in this when there are
+			 * multiple devices on the bus with at least one
+			 * having SPI_CS_HIGH set - the other CS_CSPOLX get
+			 * reset to 0 when any other device starts a transfer
+			 * this is due to a shared register for the polarity
+			 */
+			if (spi->mode & SPI_CS_HIGH) {
+				cs |= BCM2835_SPI_CS_CSPOL;
+				cs |= BCM2835_SPI_CS_CSPOL0
+					<< spi->chip_select;
+			}

-		cs |= spi->chip_select;
+			cs |= spi->chip_select;
+		}
 	}

 	reinit_completion(&bs->done);
@@ -248,9 +271,12 @@ static int bcm2835_spi_finish_transfer(
 	if (tfr->delay_usecs)
 		udelay(tfr->delay_usecs);

-	if (cs_change)
+	if (cs_change) {
+		/* reset CS */
+		bcm2835_set_cs(spi, 0);
 		/* Clear TA flag */
 		bcm2835_wr(bs, BCM2835_SPI_CS, cs & ~BCM2835_SPI_CS_TA);
+	}

 	return 0;
 }
@@ -290,15 +316,39 @@ static int bcm2835_spi_transfer_one(
 	}

 out:
-	/* Clear FIFOs, and disable the HW block */
+	/* Clear FIFOs, reset CS and disable the HW block */
 	bcm2835_wr(bs, BCM2835_SPI_CS,
 		   BCM2835_SPI_CS_CLEAR_RX | BCM2835_SPI_CS_CLEAR_TX);
+	bcm2835_set_cs(spi, 0);
 	mesg->status = err;
 	spi_finalize_current_message(master);

 	return 0;
 }

+static int bcm2835_spi_setup(struct spi_device *spi)
+{
+	/* do not care when in SPI_NO_CS mode */
+	if (spi->mode & SPI_NO_CS)
+		return 0;
+
+	/* setting up CS right from the start */
+	if (gpio_is_valid(spi->cs_gpio)) {
+		bcm2835_set_cs(spi, 0);
+		return 0;
+	}
+
+	/* Legacy mode using HW chipselects */
+	if (spi->chip_select > 2) {
+		dev_err(&spi->dev,
+			"setup: only three native chip-selects are supported\n"
+			);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 static int bcm2835_spi_probe(struct platform_device *pdev)
 {
 	struct spi_master *master;
@@ -318,6 +368,7 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
 	master->bits_per_word_mask = SPI_BPW_MASK(8);
 	master->num_chipselect = 3;
 	master->transfer_one_message = bcm2835_spi_transfer_one;
+	master->setup = bcm2835_spi_setup;
 	master->dev.of_node = pdev->dev.of_node;

 	bs = spi_master_get_devdata(master);
--
1.7.10.4

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