spi: bcm2835: Increase the CS limit to 24

Increase the maximum number of CS lines to 24, and ensure this limit is
not exceeded.

See: #4281

Suggested-by: Joe Burmeister <joe.burmeister@devtank.co.uk>
Signed-off-by: Phil Elwell <phil@raspberrypi.com>

drivers/spi/spi-bcm2835.c

@@ -28,6 +28,7 @@
 #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_gpio.h>
 #include <linux/of_irq.h>
 #include <linux/spi/spi.h>
 
@@ -68,7 +69,7 @@
 #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		4   /* raise as necessary */
+#define BCM2835_SPI_NUM_CS		24  /* more than enough */
 #define BCM2835_SPI_MODE_BITS	(SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \
 				| SPI_NO_CS | SPI_3WIRE)
 
@@ -1292,6 +1293,11 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
 	struct bcm2835_spi *bs;
 	int err;
 
+	if (of_gpio_named_count(pdev->dev.of_node, "cs-gpios") >
+	    BCM2835_SPI_NUM_CS)
+		return dev_err_probe(&pdev->dev, -EINVAL,
+				     "too many chip selects\n");
+
 	ctlr = devm_spi_alloc_master(&pdev->dev, ALIGN(sizeof(*bs),
 						  dma_get_cache_alignment()));
 	if (!ctlr)