[PATCH v2 2/5] spi: spi-mux: re-apply settings after child calls spi_setup()

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

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

spi_mux_select() copies a child's max_speed_hz, mode and bits_per_word
into the spi-mux device and sets up the parent controller only when
the mux switches to a different child (current_cs differs). If the
child that is already selected changes its settings and calls
spi_setup(), spi_mux_setup() only calls spi_setup() on the spi-mux
device with its old settings, and current_cs still matches. The next
transfers of that child then run with the old speed and mode.

Set current_cs to SPI_MUX_NO_CS in spi_mux_setup(), so the next
transfer copies the child's settings and sets up the parent again.
spi_mux_setup() runs under the spi-mux controller's io_mutex, which the
message pump holds while a message is forwarded, so this does not race
with spi_mux_select(). The cost is one extra parent spi_setup() on the
next transfer.

Tested with KUnit (spi_mux: spi_mux_settings_lifecycle_test, added
later in this series) on ARCH=um; builds with W=1 on x86_64. Not tested
on hardware.

Developed with help from the Claude and Gemini AI assistants.

Fixes: e9e40543ad5b ("spi: Add generic SPI multiplexer")
Cc: stable@vger.kernel.org
Signed-off-by: Joseph CT Chan <josephctchan@google.com>
---
 drivers/spi/spi-mux.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/spi/spi-mux.c b/drivers/spi/spi-mux.c
index 28d7718138a5..a7ea6585b374 100644
--- a/drivers/spi/spi-mux.c
+++ b/drivers/spi/spi-mux.c
@@ -86,7 +86,13 @@ static int spi_mux_setup(struct spi_device *spi)
 	 * can be called multiple times, won't do a valid setup now but we will
 	 * change the settings when we do a transfer (necessary because we
 	 * can't predict from which device it will be anyway)
+	 *
+	 * The child may have changed its settings, so make the next transfer
+	 * copy them again even if this child is the current one. This runs
+	 * under our io_mutex, so no message is being forwarded right now.
 	 */
+	priv->current_cs = SPI_MUX_NO_CS;
+
 	return spi_setup(priv->spi);
 }
 
-- 
2.56.0.385.gac671c819a-goog