spi: Add generic support for unused native cs with cs-gpios

Some SPI master controllers always drive a native chip select when
performing a transfer.  Hence when using both native and GPIO chip
selects, at least one native chip select must be left unused, to be
driven when performing transfers with slave devices using GPIO chip
selects.

Currently, to find an unused native chip select, SPI controller drivers
need to parse and process cs-gpios theirselves.  This is not only
duplicated in each driver that needs it, but also duplicates part of the
work done later at SPI controller registration time.  Note that this
cannot be done after spi_register_controller() returns, as at that time,
slave devices may have been probed already.

Hence add generic support to the SPI subsystem for finding an unused
native chip select.  Optionally, this unused native chip select, and all
other in-use native chip selects, can be validated against the maximum
number of native chip selects available on the controller hardware.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200102133822.29346-2-geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 5485ef89197c..197c9e0ac2a6 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -2464,6 +2464,8 @@ static int spi_get_gpio_descs(struct spi_controller *ctlr)
 	int nb, i;
 	struct gpio_desc **cs;
 	struct device *dev = &ctlr->dev;
+	unsigned long native_cs_mask = 0;
+	unsigned int num_cs_gpios = 0;
 
 	nb = gpiod_count(dev, "cs");
 	ctlr->num_chipselect = max_t(int, nb, ctlr->num_chipselect);
@@ -2505,7 +2507,22 @@ static int spi_get_gpio_descs(struct spi_controller *ctlr)
 			if (!gpioname)
 				return -ENOMEM;
 			gpiod_set_consumer_name(cs[i], gpioname);
+			num_cs_gpios++;
+			continue;
+		}
+
+		if (ctlr->max_native_cs && i >= ctlr->max_native_cs) {
+			dev_err(dev, "Invalid native chip select %d\n", i);
+			return -EINVAL;
 		}
+		native_cs_mask |= BIT(i);
+	}
+
+	ctlr->unused_native_cs = ffz(native_cs_mask);
+	if (num_cs_gpios && ctlr->max_native_cs &&
+	    ctlr->unused_native_cs >= ctlr->max_native_cs) {
+		dev_err(dev, "No unused native chip select available\n");
+		return -EINVAL;
 	}
 
 	return 0;