spi: core: Ignore unsupported Dual/Quad Transfer Mode bits

The availability of SPI Dual or Quad Transfer Mode as indicated by the
"spi-tx-bus-width" and "spi-rx-bus-width" properties in the device tree is
a hardware property of the SPI master, SPI slave, and board wiring.  Hence
the SPI core should not reject an SPI slave because an SPI master driver
doesn't (yet) support Dual or Quad Transfer Mode.

Change the lack of Dual or Quad Transfer Mode support in the SPI master
driver from an error condition to a warning condition, and ignore the
unsupported mode bits, falling back to Single Transfer Mode, to avoid
breakages when running old kernels with new device trees.

Fixes: f477b7fb (spi: DUAL and QUAD support)
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org

drivers/spi/spi.c

@@ -1756,7 +1756,7 @@ EXPORT_SYMBOL_GPL(spi_busnum_to_master);
  */
 int spi_setup(struct spi_device *spi)
 {
-	unsigned	bad_bits;
+	unsigned	bad_bits, ugly_bits;
 	int		status = 0;
 
 	/* check mode to prevent that DUAL and QUAD set at the same time
@@ -1776,6 +1776,15 @@ int spi_setup(struct spi_device *spi)
 	 * that aren't supported with their current master
 	 */
 	bad_bits = spi->mode & ~spi->master->mode_bits;
+	ugly_bits = bad_bits &
+		    (SPI_TX_DUAL | SPI_TX_QUAD | SPI_RX_DUAL | SPI_RX_QUAD);
+	if (ugly_bits) {
+		dev_warn(&spi->dev,
+			 "setup: ignoring unsupported mode bits %x\n",
+			 ugly_bits);
+		spi->mode &= ~ugly_bits;
+		bad_bits &= ~ugly_bits;
+	}
 	if (bad_bits) {
 		dev_err(&spi->dev, "setup: unsupported mode bits %x\n",
 			bad_bits);