From 5442dcaa0d90fc376bdfc179a018931a8f43dea4 Mon Sep 17 00:00:00 2001
From: Chris Lesiak <chris.lesiak@licor.com>
Date: Thu, 7 Mar 2019 20:39:00 +0000
Subject: [PATCH] spi: Fix zero length xfer bug
Git-commit: 5442dcaa0d90fc376bdfc179a018931a8f43dea4
Patch-mainline: v5.2-rc1
References: bsc#1051510

This fixes a bug for messages containing both zero length and
unidirectional xfers.

The function spi_map_msg will allocate dummy tx and/or rx buffers
for use with unidirectional transfers when the hardware can only do
a bidirectional transfer.  That dummy buffer will be used in place
of a NULL buffer even when the xfer length is 0.

Then in the function __spi_map_msg, if he hardware can dma,
the zero length xfer will have spi_map_buf called on the dummy
buffer.

Eventually, __sg_alloc_table is called and returns -EINVAL
because nents == 0.

This fix prevents the error by not using the dummy buffer when
the xfer length is zero.

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/spi/spi.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -977,6 +977,8 @@ static int spi_map_msg(struct spi_contro
 		if (max_tx || max_rx) {
 			list_for_each_entry(xfer, &msg->transfers,
 					    transfer_list) {
+				if (!xfer->len)
+					continue;
 				if (!xfer->tx_buf)
 					xfer->tx_buf = ctlr->dummy_tx;
 				if (!xfer->rx_buf)