[PATCH] spi: mux: repair mux usage

[PATCH] spi: mux: repair mux usage

[Peter Rosin]

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@axentia.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

Re: [PATCH] spi: mux: repair mux usage

[Mark Brown]

On Mon, 25 May 2020 12:43:52 +0200, Peter Rosin wrote:
> 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.
> 
> [...]

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git for-next

Thanks!

[1/1] spi: mux: repair mux usage
      commit: a2b02e4623fb127fa65a13e4ac5aa56e4ae16291

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark

Re: [PATCH] spi: mux: repair mux usage

[Chris Packham]

On 25/05/20 10:43 pm, Peter Rosin wrote:
> 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@axentia.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;


I've tested this on the 2 hardware platforms I have with the multiplexed 
CS and both work fine with this change so

Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>