[PATCH] spi: virtio: Use the per-transfer bits per word

[Hao-Qun Huang <alvinhuang0603@gmail.com>]

virtio_spi_transfer_one() puts spi->bits_per_word into the request
header, so a transfer that sets its own word size reaches the backend
with the device default instead. The SPI core has already copied that
default into xfer->bits_per_word when the transfer leaves it at zero,
the same way it does for xfer->speed_hz, which this function already
uses.

Per-transfer word sizes are ordinary SPI usage. mipi_dbi, for one, sends
a 9-bit command and reads the reply as 8-bit data in the same message.
With a 16-bit device default, a one-byte transfer asking for 8 bits goes
out as a partial 16-bit word, which the backend may reject.

Fixes: f98cabe3f6cf ("SPI: Add virtio SPI driver")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hao-Qun Huang <alvinhuang0603@gmail.com>
---
Reproduced with a local KUnit test, not part of this patch, that runs
spi_sync() through this driver and the real split virtqueue against a
software backend reading the request headers: 9 of 13 cases fail
without the change, all 13 pass with it. Not tested against a real
virtio-spi device or backend, so the framing on the wire was not
observed.

 drivers/spi/spi-virtio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/spi/spi-virtio.c b/drivers/spi/spi-virtio.c
index 2256dfec5407..3e181bd8bc94 100644
--- a/drivers/spi/spi-virtio.c
+++ b/drivers/spi/spi-virtio.c
@@ -168,7 +168,7 @@ static int virtio_spi_transfer_one(struct spi_controller *ctrl,
 
 	/* Fill struct spi_transfer_head */
 	th->chip_select_id = spi_get_chipselect(spi, 0);
-	th->bits_per_word = spi->bits_per_word;
+	th->bits_per_word = xfer->bits_per_word;
 	th->cs_change = xfer->cs_change;
 	th->tx_nbits = xfer->tx_nbits;
 	th->rx_nbits = xfer->rx_nbits;

base-commit: cba2348ab114391f5b1a00fa65c5b739f13f0563
-- 
2.43.0