PM: runtime: Capture device status before disabling runtime PM

In some cases (for example, during system-wide suspend and resume of
devices) it is useful to know whether or not runtime PM has ever been
enabled for a given device and, if so, what the runtime PM status of
it had been right before runtime PM was disabled for it last time.

For this reason, introduce a new struct dev_pm_info field called
last_status that will be used for capturing the runtime PM status of
the device when its power.disable_depth counter changes from 0 to 1.

The new field will be set to RPM_INVALID to start with and whenever
power.disable_depth changes from 1 to 0, so it will be valid only
when runtime PM of the device is currently disabled, but it has been
enabled at least once.

Immediately use power.last_status in rpm_resume() to make it handle
the case when PM runtime is disabled for the device, but its runtime
PM status is RPM_ACTIVE more consistently.  Namely, make it return 1
if power.last_status is also equal to RPM_ACTIVE in that case (the
idea being that if the status was RPM_ACTIVE last time when
power.disable_depth was changing from 0 to 1 and it is still
RPM_ACTIVE, it can be assumed to reflect what happened to the device
last time when it was using runtime PM) and -EACCES otherwise.

Update the documentation to provide a description of last_status and
change the description of pm_runtime_resume() in it to reflect the
new behavior of rpm_active().

While at it, rearrange the code in pm_runtime_enable() to be more
straightforward and replace the WARN() macro in it with a pr_warn()
invocation which is less disruptive.

Link: https://lore.kernel.org/linux-pm/20211026222626.39222-1-ulf.hansson@linaro.org/t/#u
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 10 insertions, 4 deletions

diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
index d6bf84f061f4..65b86e487afe 100644
--- a/Documentation/power/runtime_pm.rst
+++ b/Documentation/power/runtime_pm.rst
@@ -265,6 +265,10 @@ defined in include/linux/pm.h:
       RPM_SUSPENDED, which means that each device is initially regarded by the
       PM core as 'suspended', regardless of its real hardware status
 
+  `enum rpm_status last_status;`
+    - the last runtime PM status of the device captured before disabling runtime
+      PM for it (invalid initially and when disable_depth is 0)
+
   `unsigned int runtime_auto;`
     - if set, indicates that the user space has allowed the device driver to
       power manage the device at run time via the /sys/devices/.../power/control
@@ -333,10 +337,12 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h:
 
   `int pm_runtime_resume(struct device *dev);`
     - execute the subsystem-level resume callback for the device; returns 0 on
-      success, 1 if the device's runtime PM status was already 'active' or
-      error code on failure, where -EAGAIN means it may be safe to attempt to
-      resume the device again in future, but 'power.runtime_error' should be
-      checked additionally, and -EACCES means that 'power.disable_depth' is
+      success, 1 if the device's runtime PM status is already 'active' (also if
+      'power.disable_depth' is nonzero, but the status was 'active' when it was
+      changing from 0 to 1) or error code on failure, where -EAGAIN means it may
+      be safe to attempt to resume the device again in future, but
+      'power.runtime_error' should be checked additionally, and -EACCES means
+      that the callback could not be run, because 'power.disable_depth' was
       different from 0
 
   `int pm_runtime_resume_and_get(struct device *dev);`