spi: stm32: properly handle 0 byte transfer

stable inclusion
from stable-5.10.20
commit 03a422ecf30d50487f723f283f0ecc38f1a70e63
bugzilla: 50608

--------------------------------

[ Upstream commit 2269f5a8 ]

On 0 byte transfer request, return straight from the
xfer function after finalizing the transfer.

Fixes: dcbe0d84 ("spi: add driver for STM32 SPI controller")
Signed-off-by: NAlain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/1612551572-495-2-git-send-email-alain.volmat@foss.st.comSigned-off-by: NMark Brown <broonie@kernel.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

drivers/spi/spi-stm32.c

@@ -1677,6 +1677,10 @@ static int stm32_spi_transfer_one(struct spi_master *master,
 	struct stm32_spi *spi = spi_master_get_devdata(master);
 	int ret;
 
+	/* Don't do anything on 0 bytes transfers */
+	if (transfer->len == 0)
+		return 0;
+
 	spi->tx_buf = transfer->tx_buf;
 	spi->rx_buf = transfer->rx_buf;
 	spi->tx_len = spi->tx_buf ? transfer->len : 0;