On 10/17/2011 03:49 PM, Guennadi Liakhovetski wrote: > On Mon, 17 Oct 2011, Sylwester Nawrocki wrote: >> On 10/17/2011 10:06 AM, Guennadi Liakhovetski wrote: >>> On Sun, 9 Oct 2011, Sakari Ailus wrote: >>>> On Mon, Oct 03, 2011 at 12:57:10PM +0200, Guennadi Liakhovetski wrote: ... >>>> The bridge driver can't (nor should) know about the power management >>>> requirements of random subdevs. The name of the s_power op is rather >>>> poitless in its current state. >>>> >>>> The power state of the subdev probably even never matters to the bridge --- >>> >>> Exactly, that's the conclusion I come to in this RFC too. >>> >>>> or do we really have an example of that? >>>> >>>> In my opinion the bridge driver should instead tell the bridge drivers what >>>> they can expect to hear from the bridge --- for example that the bridge can >>>> issue set / get controls or fmt ops to the subdev. The subdev may or may not >>>> need to be powered for those: only the subdev driver knows. >>> >>> Hm, why should the bridge driver tell the subdev driver (I presume, that's >>> a typo in your above sentence) what to _expect_? Isn't just calling those >>> operations enough? >>> >>>> This is analogous to opening the subdev node from user space. Anything else >>>> except streaming is allowed. And streaming, which for sure requires powering >>>> on the subdev, is already nicely handled by the s_stream op. >>>> >>>> What do you think? >>>> >>>> In practice the name of s_power should change, as well as possible >>>> implementatio on subdev drivers. >>> >>> But why do we need it at all? >> >> AFAICS in some TV card drivers it is used to put the analog tuner into low >> power state. >> So core.s_power op provides the mans to suspend/resume a sub-device. >> >> If the bridge driver only implements a user space interface for the subdev, >> it may want to bring a subdev up in some specific moment, before video.s_stream, >> e.g. in some ioctl or at device open(), etc. >> >> Let's imagine bringing the sensor up takes appr. 700 ms, often we don't want >> the sensor driver to be doing this before every s_stream(). > > Sorry, I still don't understand, how the bridge driver knows better, than > the subdev driver, whether the user will resume streaming in 500ms or in > 20s? Maybe there's some such information available with tuners, which I'm > just unaware about? What I meant was that if the bridge driver assumes in advance that enabling sensor's power and getting it fully operational takes long time, it can enable sensor's power earlier than it's really necessary, to avoid excessive latencies during further actions. The bridge driver could also choose to keep the sensor powered on, whenever it sees appropriate, to avoid re-enabling the sensor to often. And I'm not convinced the subdev driver has all prerequisites for implementing the power control policy. Regards, -- Sylwester Nawrocki Samsung Poland R&D Center -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
