On Wednesday, June 15, 2011 22:09:45 Devin Heitmueller wrote: > On Wed, Jun 15, 2011 at 3:55 PM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote: > > Why would that violate the spec? If the last filehandle is closed, then > > you can safely poweroff the tuner. The only exception is when you have a radio > > tuner whose audio output is hooked up to some line-in: there you can't power off > > the tuner. > > The use case that some expect to work is: > > v4l2-ctl <set standard> > v4l2-ctl <set frequency> > cat /dev/video0 > out.mpg > > By powering off the tuner after v4l2-ctl closes the device node, the > cat won't work as expected because the tuner will be powered down. > > >> We've been forced to choose between the purist perspective, which is > >> properly preserving state, never powering down the tuner and sucking > >> up 500ma on the USB port when not using the tuner, versus powering > >> down the tuner when the last party closes the filehandle, which > >> preserves power but breaks v4l2 conformance and in some cases is > >> highly noticeable with tuners that require firmware to be reloaded > >> when being powered back up. > > > > Seems fine to me. What isn't fine is that a driver like e.g. em28xx powers off > > the tuner but doesn't power it on again on the next open. It won't do that > > until the first S_FREQUENCY/S_TUNER/S_STD call. > > You don't want to power up the tuner unless you know the user intends > to use it. For example, you don't want to power up the tuner if the > user intends to capture on composite/s-video input (as this consumes > considerably more power). But the driver has that information, so it should act accordingly. So on first open you can check whether the current input has a tuner and power on the tuner in that case. On S_INPUT you can also poweron/off accordingly (bit iffy against the spec, though). So in that case the first use case would actually work. It does require that tuner-core.c supports s_power(1), of course. BTW, I noticed in tuner-core.c that the g_tuner op doesn't wake-up the tuner when called. I think it should be added, even though most (all?) tuners will need time before they can return anything sensible. BTW2: it's not a good idea to just broadcast s_power to all subdevs. That should be done to the tuner(s) only since other subdevs might also implement s_power. For now it's pretty much just tuners and some sensors, though. You know, this really needs to be a standardized module option and/or sysfs entry: 'always on', 'wake up on first open', 'wake up on first use'. Regards, Hans -- 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
