Messages in this thread | | | Subject | Re: [PATCH] PM: runtime: document common mistake with pm_runtime_get_sync() | From | Krzysztof Kozlowski <> | Date | Fri, 23 Apr 2021 17:03:29 +0200 |
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On 23/04/2021 16:08, Rafael J. Wysocki wrote: > On Thu, Apr 22, 2021 at 6:46 PM Krzysztof Kozlowski > <krzysztof.kozlowski@canonical.com> wrote: >> >> pm_runtime_get_sync(), contradictory to intuition, does not drop the >> runtime PM usage counter on errors which lead to several wrong usages in >> drivers (missing the put). pm_runtime_resume_and_get() was added as a >> better implementation so document the preference of using it, hoping it >> will stop bad patterns. >> >> Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com> >> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com> >> --- >> Documentation/power/runtime_pm.rst | 4 +++- >> include/linux/pm_runtime.h | 3 +++ >> 2 files changed, 6 insertions(+), 1 deletion(-) >> >> diff --git a/Documentation/power/runtime_pm.rst b/Documentation/power/runtime_pm.rst >> index 18ae21bf7f92..478f08942811 100644 >> --- a/Documentation/power/runtime_pm.rst >> +++ b/Documentation/power/runtime_pm.rst >> @@ -378,7 +378,9 @@ drivers/base/power/runtime.c and include/linux/pm_runtime.h: >> >> `int pm_runtime_get_sync(struct device *dev);` >> - increment the device's usage counter, run pm_runtime_resume(dev) and >> - return its result >> + return its result; >> + be aware that it does not drop the device's usage counter on errors so >> + usage of pm_runtime_resume_and_get(dev) usually results in cleaner code > > Whether or not it results in cleaner code depends on what that code does. > > If the code is > > pm_runtime_get_sync(dev); > > <Do something that will fail if the device is in a low-power state, > but there is no way to handle the failure gracefully anyway> > > pm_runtime_put(dev); > > then having to use pm_runtime_resume_and_get() instead of the > pm_runtime_get_sync() would be a nuisance. > > However, if the code wants to check the return value, that is: > > error = pm_runtime_resume_and_get(dev); > if (error) > return error; > > <Do something that will crash and burn the system if the device is in > a low-power state> > > pm_runtime_put(dev); > > then obviously pm_runtime_resume_and_get() should be your choice. > > The rule of thumb seems to be whether or not the return value is going > to be used.
Yes, you're right. What I wanted to point is that there is a pattern of missing put when using pm_runtime_get_sync() all over the kernel. It's quite common mistake because the interface is non-intuitive.
Therefore I find worth to warn users of the API: usually, for simple cases, one should use the pm_runtime_resume_and_get(). This only a hint, matching common cases, but not every case. I am not claiming that one is better than other, just that old interface mislead in the past.
Maybe you wish to rephrase the comment to: "be aware that it does not drop the device's usage counter on errors so check if pm_runtime_resume_and_get(dev) would result in a cleaner code"
Best regards, Krzysztof
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