spi: bitbang: Fix lsb-first Rx

Shifting the recieved bit by "bits" inserts it at the top of the
*currently remaining* Tx data, so we end up accumulating the whole
transfer into bit 0 of the output word. Oops.

For the algorithm to work as intended, we need to remember where the
top of the *original* word was, and shift Rx to there.

Fixes: 1847e3046c52 ("spi: gpio: Implement LSB First bitbang support")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/28324d8622da80461cce35a82859b003d6f6c4b0.1659538737.git.robin.murphy@arm.com
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --git a/drivers/spi/spi-bitbang-txrx.h b/drivers/spi/spi-bitbang-txrx.h
index 267342d..2dcbe16 100644 (file)
--- a/drivers/spi/spi-bitbang-txrx.h
+++ b/drivers/spi/spi-bitbang-txrx.h
@@ -116,6 +116,7 @@ bitbang_txrx_le_cpha0(struct spi_device *spi,
 {
        /* if (cpol == 0) this is SPI_MODE_0; else this is SPI_MODE_2 */
 
+       u8 rxbit = bits - 1;
        u32 oldbit = !(word & 1);
        /* clock starts at inactive polarity */
        for (; likely(bits); bits--) {
@@ -135,7 +136,7 @@ bitbang_txrx_le_cpha0(struct spi_device *spi,
                /* sample LSB (from slave) on leading edge */
                word >>= 1;
                if ((flags & SPI_MASTER_NO_RX) == 0)
-                       word |= getmiso(spi) << (bits - 1);
+                       word |= getmiso(spi) << rxbit;
                setsck(spi, cpol);
        }
        return word;
@@ -148,6 +149,7 @@ bitbang_txrx_le_cpha1(struct spi_device *spi,
 {
        /* if (cpol == 0) this is SPI_MODE_1; else this is SPI_MODE_3 */
 
+       u8 rxbit = bits - 1;
        u32 oldbit = !(word & 1);
        /* clock starts at inactive polarity */
        for (; likely(bits); bits--) {
@@ -168,7 +170,7 @@ bitbang_txrx_le_cpha1(struct spi_device *spi,
                /* sample LSB (from slave) on trailing edge */
                word >>= 1;
                if ((flags & SPI_MASTER_NO_RX) == 0)
-                       word |= getmiso(spi) << (bits - 1);
+                       word |= getmiso(spi) << rxbit;
        }
        return word;
 }