spi: gpio: Implement LSB First bitbang support

Add support for slave DT property spi-lsb-first, i.e., SPI_LSB_FIRST mode.
Duplicate the inline helpers bitbang_txrx_be_cpha{0,1} as LE versions.
Conditionally call them from all the spi-gpio txrx_word callbacks.

Some alternatives to this implementation approach were discussed back
then [0], but eventually it was considered reasonable.

[0] https://lore.kernel.org/linux-arm-kernel/20191212033952.5967-8-afaerber@suse.de/Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Christian Hewitt <christianshewitt@gmail.com>
Link: https://lore.kernel.org/r/feac3377-4ad1-77d8-9a18-3588d80fb909@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bitbang-txrx.h

@@ -41,6 +41,8 @@
  * chips need ... there may be several reasons you'd need to tweak timings
  * in these routines, not just to make it faster or slower to match a
  * particular CPU clock rate.
+ *
+ * ToDo: Maybe the bitrev macros can be used to improve the code?
  */
 
 static inline u32
@@ -106,3 +108,67 @@ bitbang_txrx_be_cpha1(struct spi_device *spi,
 	}
 	return word;
 }
+
+static inline u32
+bitbang_txrx_le_cpha0(struct spi_device *spi,
+		unsigned int nsecs, unsigned int cpol, unsigned int flags,
+		u32 word, u8 bits)
+{
+	/* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */
+
+	u32 oldbit = !(word & 1);
+	/* clock starts at inactive polarity */
+	for (; likely(bits); bits--) {
+
+		/* setup LSB (to slave) on trailing edge */
+		if ((flags & SPI_MASTER_NO_TX) == 0) {
+			if ((word & 1) != oldbit) {
+				setmosi(spi, word & 1);
+				oldbit = word & 1;
+			}
+		}
+		spidelay(nsecs);	/* T(setup) */
+
+		setsck(spi, !cpol);
+		spidelay(nsecs);
+
+		/* sample LSB (from slave) on leading edge */
+		word >>= 1;
+		if ((flags & SPI_MASTER_NO_RX) == 0)
+			word |= getmiso(spi) << (bits - 1);
+		setsck(spi, cpol);
+	}
+	return word;
+}
+
+static inline u32
+bitbang_txrx_le_cpha1(struct spi_device *spi,
+		unsigned int nsecs, unsigned int cpol, unsigned int flags,
+		u32 word, u8 bits)
+{
+	/* if (cpol == 0) this is SPI_MODE_1; else this is SPI_MODE_3 */
+
+	u32 oldbit = !(word & 1);
+	/* clock starts at inactive polarity */
+	for (; likely(bits); bits--) {
+
+		/* setup LSB (to slave) on leading edge */
+		setsck(spi, !cpol);
+		if ((flags & SPI_MASTER_NO_TX) == 0) {
+			if ((word & 1) != oldbit) {
+				setmosi(spi, word & 1);
+				oldbit = word & 1;
+			}
+		}
+		spidelay(nsecs); /* T(setup) */
+
+		setsck(spi, cpol);
+		spidelay(nsecs);
+
+		/* sample LSB (from slave) on trailing edge */
+		word >>= 1;
+		if ((flags & SPI_MASTER_NO_RX) == 0)
+			word |= getmiso(spi) << (bits - 1);
+	}
+	return word;
+}

drivers/spi/spi-gpio.c

@@ -135,25 +135,37 @@ static inline int getmiso(const struct spi_device *spi)
 static u32 spi_gpio_txrx_word_mode0(struct spi_device *spi,
 		unsigned nsecs, u32 word, u8 bits, unsigned flags)
 {
-	return bitbang_txrx_be_cpha0(spi, nsecs, 0, flags, word, bits);
+	if (unlikely(spi->mode & SPI_LSB_FIRST))
+		return bitbang_txrx_le_cpha0(spi, nsecs, 0, flags, word, bits);
+	else
+		return bitbang_txrx_be_cpha0(spi, nsecs, 0, flags, word, bits);
 }
 
 static u32 spi_gpio_txrx_word_mode1(struct spi_device *spi,
 		unsigned nsecs, u32 word, u8 bits, unsigned flags)
 {
-	return bitbang_txrx_be_cpha1(spi, nsecs, 0, flags, word, bits);
+	if (unlikely(spi->mode & SPI_LSB_FIRST))
+		return bitbang_txrx_le_cpha1(spi, nsecs, 0, flags, word, bits);
+	else
+		return bitbang_txrx_be_cpha1(spi, nsecs, 0, flags, word, bits);
 }
 
 static u32 spi_gpio_txrx_word_mode2(struct spi_device *spi,
 		unsigned nsecs, u32 word, u8 bits, unsigned flags)
 {
-	return bitbang_txrx_be_cpha0(spi, nsecs, 1, flags, word, bits);
+	if (unlikely(spi->mode & SPI_LSB_FIRST))
+		return bitbang_txrx_le_cpha0(spi, nsecs, 1, flags, word, bits);
+	else
+		return bitbang_txrx_be_cpha0(spi, nsecs, 1, flags, word, bits);
 }
 
 static u32 spi_gpio_txrx_word_mode3(struct spi_device *spi,
 		unsigned nsecs, u32 word, u8 bits, unsigned flags)
 {
-	return bitbang_txrx_be_cpha1(spi, nsecs, 1, flags, word, bits);
+	if (unlikely(spi->mode & SPI_LSB_FIRST))
+		return bitbang_txrx_le_cpha1(spi, nsecs, 1, flags, word, bits);
+	else
+		return bitbang_txrx_be_cpha1(spi, nsecs, 1, flags, word, bits);
 }
 
 /*
@@ -170,28 +182,40 @@ static u32 spi_gpio_spec_txrx_word_mode0(struct spi_device *spi,
 		unsigned nsecs, u32 word, u8 bits, unsigned flags)
 {
 	flags = spi->master->flags;
-	return bitbang_txrx_be_cpha0(spi, nsecs, 0, flags, word, bits);
+	if (unlikely(spi->mode & SPI_LSB_FIRST))
+		return bitbang_txrx_le_cpha0(spi, nsecs, 0, flags, word, bits);
+	else
+		return bitbang_txrx_be_cpha0(spi, nsecs, 0, flags, word, bits);
 }
 
 static u32 spi_gpio_spec_txrx_word_mode1(struct spi_device *spi,
 		unsigned nsecs, u32 word, u8 bits, unsigned flags)
 {
 	flags = spi->master->flags;
-	return bitbang_txrx_be_cpha1(spi, nsecs, 0, flags, word, bits);
+	if (unlikely(spi->mode & SPI_LSB_FIRST))
+		return bitbang_txrx_le_cpha1(spi, nsecs, 0, flags, word, bits);
+	else
+		return bitbang_txrx_be_cpha1(spi, nsecs, 0, flags, word, bits);
 }
 
 static u32 spi_gpio_spec_txrx_word_mode2(struct spi_device *spi,
 		unsigned nsecs, u32 word, u8 bits, unsigned flags)
 {
 	flags = spi->master->flags;
-	return bitbang_txrx_be_cpha0(spi, nsecs, 1, flags, word, bits);
+	if (unlikely(spi->mode & SPI_LSB_FIRST))
+		return bitbang_txrx_le_cpha0(spi, nsecs, 1, flags, word, bits);
+	else
+		return bitbang_txrx_be_cpha0(spi, nsecs, 1, flags, word, bits);
 }
 
 static u32 spi_gpio_spec_txrx_word_mode3(struct spi_device *spi,
 		unsigned nsecs, u32 word, u8 bits, unsigned flags)
 {
 	flags = spi->master->flags;
-	return bitbang_txrx_be_cpha1(spi, nsecs, 1, flags, word, bits);
+	if (unlikely(spi->mode & SPI_LSB_FIRST))
+		return bitbang_txrx_le_cpha1(spi, nsecs, 1, flags, word, bits);
+	else
+		return bitbang_txrx_be_cpha1(spi, nsecs, 1, flags, word, bits);
 }
 
 /*----------------------------------------------------------------------*/
@@ -378,7 +402,7 @@ static int spi_gpio_probe(struct platform_device *pdev)
 
 	master->bits_per_word_mask = SPI_BPW_RANGE_MASK(1, 32);
 	master->mode_bits = SPI_3WIRE | SPI_3WIRE_HIZ | SPI_CPHA | SPI_CPOL |
-			    SPI_CS_HIGH;
+			    SPI_CS_HIGH | SPI_LSB_FIRST;
 	if (!spi_gpio->mosi) {
 		/* HW configuration without MOSI pin
 		 *