[PATCH v2 1/5] spi: spi-mux: release mux when parent setup fails

[Joseph CT Chan <jctchan@gmail.com>]

From: Joseph CT Chan <josephctchan@google.com>

When spi_mux_select() switches to a different child, it locks the mux
with mux_control_select(), copies the child's settings into the spi-mux
device and calls spi_setup() on the parent controller. If spi_setup()
fails, spi_mux_select() returns the error without calling
mux_control_deselect(), so the mux stays locked. The next message on
any child of this mux then blocks forever in mux_control_select(), and
so does every other user of the same mux control.

spi_mux_select() also records the new chip select in current_cs before
spi_setup() runs. After a failure, the next message for the same child
skips the copy and spi_setup(), and is sent with a parent that was
never set up for it.

On spi_setup() failure, set current_cs to SPI_MUX_NO_CS, so the next
message sets up the parent again, and release the mux. Record the new
chip select only after spi_setup() succeeds.

Tested with KUnit (spi_mux: spi_mux_error_test, added later in this
series) on ARCH=um; that case also needs "spi: spi-mux: finalize
message when forwarding fails" to see the error status. Builds with
W=1 on x86_64. Not tested on hardware.

Developed with help from the Claude and Gemini AI assistants.

Fixes: 5a5fc308418a ("spi: spi-mux: Fix coverity issue, unchecked return value")
Cc: stable@vger.kernel.org
Signed-off-by: Joseph CT Chan <josephctchan@google.com>
---
 drivers/spi/spi-mux.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spi-mux.c b/drivers/spi/spi-mux.c
index 08fe1fa32dea..28d7718138a5 100644
--- a/drivers/spi/spi-mux.c
+++ b/drivers/spi/spi-mux.c
@@ -66,9 +66,16 @@ static int spi_mux_select(struct spi_device *spi)
 	priv->spi->mode = spi->mode;
 	priv->spi->bits_per_word = spi->bits_per_word;
 
+	ret = spi_setup(priv->spi);
+	if (ret) {
+		priv->current_cs = SPI_MUX_NO_CS;
+		mux_control_deselect(priv->mux);
+		return ret;
+	}
+
 	priv->current_cs = spi_get_chipselect(spi, 0);
 
-	return spi_setup(priv->spi);
+	return 0;
 }
 
 static int spi_mux_setup(struct spi_device *spi)
-- 
2.56.0.385.gac671c819a-goog