Messages in this thread | | | From | Doug Anderson <> | Date | Thu, 3 Mar 2022 16:04:41 -0800 | Subject | Re: [PATCH] PM: runtime: Have devm_pm_runtime_enable() handle pm_runtime_dont_use_autosuspend() |
| |
Hi,
On Thu, Mar 3, 2022 at 3:11 PM Ulf Hansson <ulf.hansson@linaro.org> wrote: > > Hi Laurent, Doug, > > On Tue, 1 Mar 2022 at 17:29, Laurent Pinchart > <laurent.pinchart@ideasonboard.com> wrote: > > > > Hi Ulf, > > > > On Tue, Mar 01, 2022 at 12:18:02PM +0100, Ulf Hansson wrote: > > > On Tue, 1 Mar 2022 at 11:49, Laurent Pinchart wrote: > > > > On Tue, Mar 01, 2022 at 11:26:46AM +0100, Ulf Hansson wrote: > > > > > On Wed, 23 Feb 2022 at 17:35, Douglas Anderson wrote: > > > > > > > > > > > > The PM Runtime docs say: > > > > > > Drivers in ->remove() callback should undo the runtime PM changes done > > > > > > in ->probe(). Usually this means calling pm_runtime_disable(), > > > > > > pm_runtime_dont_use_autosuspend() etc. > > > > > > > > > > > > From grepping code, it's clear that many people aren't aware of the > > > > > > need to call pm_runtime_dont_use_autosuspend(). > > > > > > > > > > Well, I admit it's good practice that they should take care of this. > > > > > > > > > > However, it doesn't really matter to keep the autosuspend turned on > > > > > when runtime PM becomes disabled, I think. When the driver gets probed > > > > > again, it will most likely call pm_runtime_use_autosuspend() again, > > > > > which should work fine, right? > > > > > > > > For the probe path I agree, but are there valid use cases where, at > > > > runtime, a driver would disable runtime PM and re-enable it a bit later > > > > ? If so, we need to ensure this won't disable auto-suspend. > > > > > > I am not sure I fully understand whether there is a problem. > > > > > > Can you perhaps write the sequence of the runtime PM calls that may > > > cause an issue? > > > > Simply > > > > pm_runtime_disable(); > > /* Do something that requires runtime PM to be disabled */ > > pm_runtime_enable(); > > > > at runtime (not at probe or remove time). If probe() has enabled > > auto-suspend, we don't want the above sequence to disable it. What I'm > > not sure is if there are any valid use cases for the above sequence. > > The above sequence certainly exists already, for example during system > suspend/resume. > > So what happens is that the runtime PM auto-suspend feature gets > temporarily disabled between pm_runtime_disable() and > pm_runtime_enable(). That seems correct, right? > > > > > > > > > When brainstorming solutions, one idea that came up was to leverage > > > > > > the new-ish devm_pm_runtime_enable() function. The idea here is that: > > > > > > * When the devm action is called we know that the driver is being > > > > > > removed. It's the perfect time to undo the use_autosuspend. > > > > > > * The code of pm_runtime_dont_use_autosuspend() already handles the > > > > > > case of being called when autosuspend wasn't enabled. > > > > > > > > > > Hmm, I am hesitating to extend devm_pm_runtime_enable(), as it > > > > > currently makes it look too simple to turn off things at ->remove() > > > > > for runtime PM. While in fact it's more complicated. > > > > > > > > > > A bigger problem, for example, is that a driver calls > > > > > pm_runtime_put_sync() during ->remove(), relying on that it actually > > > > > ends up calling its ->runtime_suspend() callback to turn off various > > > > > specific resources for the device. And in fact there are no guarantees > > > > > that will happen - and when it doesn't, the next time the driver's > > > > > ->probe() runs, things are likely to be really screwed up. > > > > > > > > > > To cover this case, one could use the below code in the ->remove() callback: > > > > > > > > > > ... > > > > > pm_runtime_get_sync(); > > > > > > > > > > "turn off resources for the devices - like calling > > > > > clk_disable_unprepare(), for example" > > > > > > > > > > pm_runtime_disable(); > > > > > pm_runtime_put_noidle(); > > > > > ... > > > > > > > > > > In this example, it would be too late to call pm_runtime_disable() > > > > > through the pm_runtime_disable_action(). > > > > > > > > My experience with runtime PM is that it's hard to use, at least if you > > > > want to get it right :-) That's especially the case if a driver wants to > > > > support both CONFIG_PM and !CONFIG_PM. Here's an example at probe time: > > > > > > > > /* > > > > * We need the driver to work in the event that CONFIG_PM is disabled in > > > > * the kernel, so power up and verify the chip now. In the event that > > > > * CONFIG_PM is disabled this will leave the chip on, so that streaming > > > > * will work. > > > > */ > > > > ret = ov5693_sensor_powerup(ov5693); > > > > if (ret) > > > > goto err_media_entity_cleanup; > > > > > > > > ret = ov5693_detect(ov5693); > > > > if (ret) > > > > goto err_powerdown; > > > > > > > > pm_runtime_set_active(&client->dev); > > > > pm_runtime_get_noresume(&client->dev); > > > > pm_runtime_enable(&client->dev); > > > > > > > > ret = v4l2_async_register_subdev_sensor(&ov5693->sd); > > > > if (ret) { > > > > dev_err(&client->dev, "failed to register V4L2 subdev: %d", > > > > ret); > > > > goto err_pm_runtime; > > > > } > > > > > > > > pm_runtime_set_autosuspend_delay(&client->dev, 1000); > > > > pm_runtime_use_autosuspend(&client->dev); > > > > pm_runtime_put_autosuspend(&client->dev); > > > > > > > > And the corresponding code at remove time: > > > > > > > > /* > > > > * Disable runtime PM. In case CONFIG_PM is disabled in the kernel, > > > > * make sure to turn power off manually. > > > > */ > > > > pm_runtime_disable(&client->dev); > > > > if (!pm_runtime_status_suspended(&client->dev)) > > > > ov5693_sensor_powerdown(ov5693); > > > > pm_runtime_set_suspended(&client->dev); > > > > > > > > And of course there's no documentation that explains all this, so there > > > > are endless variations of patterns originating from cargo-cult > > > > programming. > > > > > > > > I don't know what the right solution is, but we need to move towards an > > > > easier to use API if we want drivers to get it right. Any step in that > > > > direction would be welcome. > > > > > > Yep, I fully agree with you, while it's not an easy task. At least the > > > example above looks fine to me. :-) > > > > It took me several days to figure out how to get it right. Most > > developers don't bother, so we end up with drivers broken in different > > ways :-S > > Yes, it's definitely non-trivial. > > Power management in general relies on cross-interaction of several > different frameworks, so one really needs a decent overview too, > before adding PM support in a driver. > > > > > > Recently I noticed that some drivers are calling > > > pm_runtime_force_suspend() at ->remove(). This works fine in quite > > > many cases, but it wouldn't solve the case when CONFIG_PM is unset. > > > > > > Perhaps we should explore adding a new API, along the lines of > > > pm_runtime_force_suspend(), but make it specific for the ->remove() > > > path, and in some way make it work for when CONFIG_PM is unset too. > > > > I'm all for an improved API for drivers that would make the above > > simpler. And documentation too, Documentation/power/runtime_pm.rst is > > more of a documentation of the runtime PM core than the driver API. > > There are some useful tips for drivers, but they're lost in a sea of > > difficult to understand and/or irrelevant information (and there's also > > a tiny bit of information in Documentation/driver-api/pm/devices.rst). > > We're missing a document targetted at driver authors. > > Yes, I agree - the docs can certainly be improved! I will add it to my > TODO list and try to put some time on it, not too far ahead I hope. I > was actually planning for a blog-post/LWN article, maybe I should > spend some time on this instead - or both. :-) > > When it comes to the improved API for the ->remove() case, we need to > explore this a bit more. I will think about it. > > About $subject patch, if you or Doug insist that you want to move > forward on it, I will not object - even if I think we need something > entirely different, in the long run.
Hrm, I guess the question is how far away are we from the long run. ;-) If this doesn't impede the long term solution and the long term solution is still a ways out, it feels worth landing this or something similar to it just to help make the existing code a little more robust, even if it doesn't solve every problem. That being said, I don't have any code that depends on this patch and I myself will likely not mess this up again in code that I write. ;-) Just for context, the patch was originally suggested by Laurent [1] in response to me fixing some code that I wrote.
[1] https://lore.kernel.org/r/YhZY+FLTlv7V5BIB@pendragon.ideasonboard.com
-Doug
| |