[PATCH 5/5] spi: Document devm_spi_register_controller() gotcha

[Lukas Wunner <lukas@xxxxxxxxx>]

As a rule, devm_spi_register_controller() must not be used if the
driver's ->remove() hook performs teardown steps which shall be
performed after unbinding of slaves.

Dozens of drivers are doing it wrong.  Document this gotcha to
hopefully prevent further misuse.

Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
---
 drivers/spi/spi.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index d32bdc6cbf66..e1a35aa7eeb8 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -2714,6 +2714,13 @@ static void devm_spi_unregister(struct device *dev, void *res)
  * Register a SPI device as with spi_register_controller() which will
  * automatically be unregistered and freed.
  *
+ * Be aware that a managed SPI controller and the attached slaves are
+ * unregistered after the driver's ->remove() callback has been executed.
+ * So the SPI slaves may still access the bus during and after ->remove().
+ * Thus, devm_spi_register_controller() may only be used if there is no
+ * ->remove() callback at all or if it does not perform teardown steps
+ * which render the bus inaccessible.
+ *
  * Return: zero on success, else a negative error code.
  */
 int devm_spi_register_controller(struct device *dev,
-- 
2.26.2