On 10/17/2011 05:23 PM, Guennadi Liakhovetski wrote: > On Mon, 17 Oct 2011, Sylwester Nawrocki wrote: > >> 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. > > Where would a bridge driver get this information from? And how would it > know in advance, when power would be "really needed" to enable it > "earlier?"...
I don't think this information could now be retrieved from a subdev in standard way. If a bridge driver wants low latencies it can simply be coded to issue s_power(1) before any other op call, and s_power(0) when it's done with a subdev. > >> The bridge driver could also choose to keep the sensor powered on, whenever >> it >> sees appropriate, to avoid re-enabling the sensor to often. > > On what basis would the bridge driver make these decisions? How would it > know in advance, when it'll have to re-enable the subdev next time? Re-enabling by allowing a subdev driver to entirely control the power state. The sensor might implement "lowest power consumption" policy, while the user might want "highest performance". I'm referring only to camera sensor subdevs, as I don't have much experience with other ones. Also there are some devices where you want to model power control explicitly, and it is critical to overall system operation. The s5p-tv driver is one example of these. The host driver knows exactly how the power state of its subdevs should be handled. The situation with single subdev and a bridge driver may look like it could get away without s_power(), but where multiple subdevs are involved I expect the driver writers will want to be able to control the power state. It seems s_power() is now used not only in suspend/resume paths so removing it for new core.suspend/resume callbacks IMHO would leave some functionality not covered. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
