In SoundWire mode leave hard RESET asserted when exiting probe,
and wait for an UNATTACHED notification before deasserting RESET.
If the boot state of the reset GPIO was deasserted it is possible
that the SoundWire core had already enumerated the CS42L42 before
cs42l42_sdw_probe() is called. When cs42l42_common_probe() hard
resets the CS42L42 it triggers a race condition:
1) After cs42l42_sdw_probe() returns the thread that called it
will call cs42l42_sdw_update_status() to report the last
status recorded by the SoundWire core.
2) The SoundWire bus master will see a PING with the CS42L42
now reporting as unenumerated and will trigger the core
SoundWire code to start enumerating CS42L42.
These two threads are racing against each other. If (1)
happens before (2) a stale ATTACHED notification will be
reported to the cs42l42 driver when in fact the status of
cs42l42 is now unattached.
To avoid this race condition:
- Leave RESET asserted on exit from cs42l42_sdw_probe().
This ensures that an UNATTACHED notification must be
sent to the cs42l42 driver. If cs42l42 was already
enumerated it will be seen to drop off the bus, causing
an UNATTACH notification. If it was never enumerated the
status is already UNATTACHED and this will be reported
by thread (1).
- When the UNATTACH notification is received, release RESET.
This will cause CS42L42 to be enumerated and eventually
report an ATTACHED notification.
- The ATTACHED notification is now valid.
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Signed-off-by: Stefan Binding <sbinding@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230913150012.604775-4-sbinding@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>
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