spi: bcm2835aux: warn in dmesg that native cs is not really supported

From personal bad experience (even as the author of the original driver)
it shows that native-cs is "somewhat" supported by the spi bus driver
when using a buggy device tree.

So make sure that the driver is warning in dmesg about this fact
that we are running in a not supported mode that may have surprizing
limitations.

Fixes: 1ea29b39 ("spi: bcm2835aux: add bcm2835 auxiliary spi device...")
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm2835aux.c

@@ -396,6 +396,38 @@ static void bcm2835aux_spi_handle_err(struct spi_master *master,
 	bcm2835aux_spi_reset_hw(bs);
 }
 
+static int bcm2835aux_spi_setup(struct spi_device *spi)
+{
+	int ret;
+
+	/* sanity check for native cs */
+	if (spi->mode & SPI_NO_CS)
+		return 0;
+	if (gpio_is_valid(spi->cs_gpio))
+		return 0;
+
+	/* for dt-backwards compatibility: only support native on CS0
+	 * known things not supported with broken native CS:
+	 * * multiple chip-selects: cs0-cs2 are all
+	 *     simultaniously asserted whenever there is a transfer
+	 *     this even includes SPI_NO_CS
+	 * * SPI_CS_HIGH: cs are always asserted low
+	 * * cs_change: cs is deasserted after each spi_transfer
+	 * * cs_delay_usec: cs is always deasserted one SCK cycle
+	 *     after the last transfer
+	 * probably more...
+	 */
+	dev_warn(&spi->dev,
+		 "Native CS is not supported - please configure cs-gpio in device-tree\n");
+
+	if (spi->chip_select == 0)
+		return 0;
+
+	dev_warn(&spi->dev, "Native CS is not working for cs > 0\n");
+
+	return -EINVAL;
+}
+
 static int bcm2835aux_spi_probe(struct platform_device *pdev)
 {
 	struct spi_master *master;
@@ -425,6 +457,7 @@ static int bcm2835aux_spi_probe(struct platform_device *pdev)
 	 *     a spi_transfer
 	 */
 	master->num_chipselect = 1;
+	master->setup = bcm2835aux_spi_setup;
 	master->transfer_one = bcm2835aux_spi_transfer_one;
 	master->handle_err = bcm2835aux_spi_handle_err;
 	master->prepare_message = bcm2835aux_spi_prepare_message;