linux-kernel - Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable

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Date:	Wed, 20 May 2015 11:03:23 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Kevin Hilman <khilman@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] PM / Runtime: Add pm_runtime_enable_recursive

On 19 May 2015 at 19:49, Alan Stern <stern@...land.harvard.edu> wrote:
> On Tue, 19 May 2015, Tomeu Vizoso wrote:
>
>> This function makes less cumbersome to enable runtime PM in a device and
>> its descendants.
>>
>> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@...labora.com>
>
> I don't see the point of this.  In the scenario you have in mind, are
> the device and all its descendants registered by the same
> subsystem/driver?

Not quite, the scenario here is a driver (uvcvideo) that deals with a
specific piece of hardware and knows that all the descendants of the
device it's bound to are virtual.

The subtree is:

1-1:1.0 (bound to uvcvideo)
        ep_87
        input4
                event4
        media0
        video0

I liked how the force_direct_complete flag played out here, but I
agree with Rafael that it can be abused as the PM domain or the bus
type weren't able to prevent going directly to complete.

This is my testing branch, btw:

https://git.collabora.com/cgit/user/tomeu/linux.git/log/?h=fast-resume-v5

> If they are, can't the subsystem/driver code enable
> runtime PM for each of them when they are registered, by adding a
> single call in the right spot?
>
> If they don't all belong to the same subsystem/driver, who is going to
> call your pm_runtime_enable_recursive routine?  No single caller will
> have the right to enable runtime PM for all these devices.

Yeah, I was thinking that uvcvideo might be able to decide that, but
I'm not really sure about it.

Thanks,

Tomeu

> Alan Stern
>
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