spi: Introduce device-managed SPI controller allocation

[ Upstream commit 5e844cc37a5cbaa460e68f9a989d321d63088a89 ]

SPI driver probing currently comprises two steps, whereas removal
comprises only one step:

    spi_alloc_master()
    spi_register_master()

    spi_unregister_master()

That's because spi_unregister_master() calls device_unregister()
instead of device_del(), thereby releasing the reference on the
spi_master which was obtained by spi_alloc_master().

An SPI driver's private data is contained in the same memory allocation
as the spi_master struct.  Thus, once spi_unregister_master() has been
called, the private data is inaccessible.  But some drivers need to
access it after spi_unregister_master() to perform further teardown
steps.

Introduce devm_spi_alloc_master(), which releases a reference on the
spi_master struct only after the driver has unbound, thereby keeping the
memory allocation accessible.  Change spi_unregister_master() to not
release a reference if the spi_master was allocated by the new devm
function.

The present commit is small enough to be backportable to stable.
It allows fixing drivers which use the private data in their ->remove()
hook after it's been freed.  It also allows fixing drivers which neglect
to release a reference on the spi_master in the probe error path.

Long-term, most SPI drivers shall be moved over to the devm function
introduced herein.  The few that can't shall be changed in a treewide
commit to explicitly release the last reference on the master.
That commit shall amend spi_unregister_master() to no longer release
a reference, thereby completing the migration.

As a result, the behaviour will be less surprising and more consistent
with subsystems such as IIO, which also includes the private data in the
allocation of the generic iio_dev struct, but calls device_del() in
iio_device_unregister().

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/272bae2ef08abd21388c98e23729886663d19192.1605121038.git.lukas@wunner.de
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index 4b743ac35396..8470695e5dd7 100644
--- a/include/linux/spi/spi.h
+++ b/include/linux/spi/spi.h
@@ -601,6 +601,8 @@ extern void spi_finalize_current_transfer(struct spi_master *master);
 /* the spi driver core manages memory for the spi_master classdev */
 extern struct spi_master *
 spi_alloc_master(struct device *host, unsigned size);
+extern struct spi_master *
+devm_spi_alloc_master(struct device *dev, unsigned int size);
 
 extern int spi_register_master(struct spi_master *master);
 extern int devm_spi_register_master(struct device *dev,