linux-kernel - [PATCH] spi: mux: repair mux usage

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Message-Id: <20200525104352.26807-1-peda@axentia.se>
Date:   Mon, 25 May 2020 12:43:52 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Rosin <peda@...ntia.se>, Mark Brown <broonie@...nel.org>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-spi@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: [PATCH] spi: mux: repair mux usage

It is not valid to cache/short out selection of the mux.

mux_control_select() only locks the mux until mux_control_deselect()
is called. mux_control_deselect() may put the mux in some low power
state or some other user of the mux might select it for other purposes.
These things are probably not happening in the original setting where
this driver was developed, but it is said to be a generic SPI mux.

Also, the mux framework will short out the actual low level muxing
operation when/if that is possible.

Fixes: e9e40543ad5b ("spi: Add generic SPI multiplexer")
Signed-off-by: Peter Rosin <peda@...ntia.se>
---
 drivers/spi/spi-mux.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-mux.c b/drivers/spi/spi-mux.c
index 4f94c9127fc1..cc9ef371db14 100644
--- a/drivers/spi/spi-mux.c
+++ b/drivers/spi/spi-mux.c
@@ -51,6 +51,10 @@ static int spi_mux_select(struct spi_device *spi)
 	struct spi_mux_priv *priv = spi_controller_get_devdata(spi->controller);
 	int ret;
 
+	ret = mux_control_select(priv->mux, spi->chip_select);
+	if (ret)
+		return ret;
+
 	if (priv->current_cs == spi->chip_select)
 		return 0;
 
@@ -62,10 +66,6 @@ static int spi_mux_select(struct spi_device *spi)
 	priv->spi->mode = spi->mode;
 	priv->spi->bits_per_word = spi->bits_per_word;
 
-	ret = mux_control_select(priv->mux, spi->chip_select);
-	if (ret)
-		return ret;
-
 	priv->current_cs = spi->chip_select;
 
 	return 0;
-- 
2.20.1