[PATCH] PM: runtime: Update runtime_pm.rst for non-fatal resume errors

[Stanley Jhu]

Since commit eef1d74bf88f ("PM: runtime: Only set runtime_error on
suspend callback failures"), rpm_callback() no longer sets
dev->power.runtime_error when ->runtime_resume() returns an error.
However, Documentation/power/runtime_pm.rst still states that the PM
core treats any ->runtime_resume() error as a fatal device error and
refuses to run runtime PM helper functions until the device status is
manually reset.

Update the resume callback semantics bullet to state that on error the
PM core resets the runtime PM status to 'suspended', returns the error
code from synchronous resume helpers, and leaves power.runtime_error
unset so a subsequent resume request can invoke ->runtime_resume()
again. Also drop ->runtime_resume() from the ->runtime_idle()
comparison of callbacks whose negative return codes are treated as fatal
device errors.

Fixes: eef1d74bf88f ("PM: runtime: Only set runtime_error on suspend callback failures")
Signed-off-by: Stanley Jhu <stanleyjhu@xxxxxxxxxx>
---
This updates Documentation/power/runtime_pm.rst as discussed in the v3
thread for commit eef1d74bf88f:
https://lore.kernel.org/r/CAJZ5v0ga8qqYg79+hQVtO8T=FOCj8XLmTz3HpEKoOykV_K-HSQ@xxxxxxxxxxxxxx

Two .runtime_resume() paths assumed the old latching behavior:
- blk_pre_runtime_resume() (q->pm_only underflow):
https://lore.kernel.org/r/20261029040001.2100001-1-stanleyjhu@xxxxxxxxxx
- __ufshcd_wl_resume() in UFS, where after a failed resume the retried
  resume releases the clock-gating hold a second time:
https://lore.kernel.org/r/20261029040002.2100002-1-stanleyjhu@xxxxxxxxxx

Should other .runtime_resume() callbacks that undo suspend-time state
on failure, assuming no retry, be audited?

 Documentation/power/runtime_pm.rst | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst
index 40a5c23d5259..04dc2a538352 100644
--- a/Documentation/power/runtime_pm.rst
+++ b/Documentation/power/runtime_pm.rst
@@ -222,11 +222,12 @@ what to do to handle the device).
     I/O operations as needed.  The runtime PM status of the device is then
     'active'.
 
-  * If the resume callback returns an error code, the PM core regards this as a
-    fatal error and will refuse to run the helper functions described in
-    `Section 4`_ for the device, until its status is directly set to either
-    'active', or 'suspended' (by means of special helper functions provided by
-    the PM core for this purpose).
+  * If the resume callback returns an error code, the PM core sets the
+    device's runtime PM status back to 'suspended' and returns the error
+    code from synchronous resume helpers.  As with a suspend callback
+    returning -EBUSY or -EAGAIN, this does not set power.runtime_error, so
+    the helper functions described in `Section 4`_ are not blocked and a
+    subsequent resume request can run the resume callback again.
 
 The idle callback (a subsystem-level one, if present, or the driver one) is
 executed by the PM core whenever the device appears to be idle, which is
@@ -250,9 +251,9 @@ call to pm_runtime_autosuspend().
 To prevent this suspension (for example, if the callback routine has scheduled
 a delayed suspend or determined the device cannot be idle), the routine must
 return a non-zero value (typically -EBUSY or -EAGAIN).  Unlike
-->runtime_suspend() and ->runtime_resume(), the PM core does not treat negative
-return codes from ->runtime_idle() as a fatal device error; any non-zero value
-simply stops the PM core from suspending the device.
+->runtime_suspend(), the PM core does not treat negative return codes from
+->runtime_idle() as a fatal device error, and any non-zero value simply stops
+the PM core from suspending the device.
 
 Core Guarantees and Synchronization Rules
 -----------------------------------------
-- 
2.56.0.313.g2479322e68-goog