Re: [RFCv1 PATCH 7/7] tuner-core: s_tuner should not change tuner mode.

[Andy Walls <awalls@md.metrocast.net>]

On Sun, 2011-06-12 at 15:23 +0200, Hans Verkuil wrote:
> On Sunday, June 12, 2011 14:53:06 Andy Walls wrote:
> > On Sun, 2011-06-12 at 14:30 +0200, Hans Verkuil wrote:
> > > On Sunday, June 12, 2011 14:13:30 Mauro Carvalho Chehab wrote:
> > > > Em 12-06-2011 08:59, Mauro Carvalho Chehab escreveu:
> > > > > Em 12-06-2011 08:36, Hans Verkuil escreveu:
> > > > >>>> What about this:
> > > > >>>>
> > > > >>>> Opening /dev/radio effectively starts the radio mode. So if there is TV
> > > > >>>> capture in progress, then the open should return -EBUSY. Otherwise it
> > > > >>>> switches the tuner to radio mode. And it stays in radio mode until the
> > > > >>>> last filehandle of /dev/radio is closed. At that point it will automatically
> > > > >>>> switch back to TV mode (if there is one, of course).
> > > > >>>
> > > > >>> No. This would break existing applications. The mode switch should be done
> > > > >>> at S_FREQUENCY (e. g. when the radio application is tuning into a channel).
> > > > >>
> > > > >> This is not what happens today as the switch to radio occurs as soon as you open
> > > > >> the radio node. It's the reason for the s_radio op.
> > > > > 
> > > > > The s_radio op is something that I wanted to remove. It was there in the past to feed
> > > > > the TV/radio hint logic. I wrote a patch for it, but I ended by discarding from my
> > > > > final queue (I can't remember why).
> > > > > 
> > > > > I think that the hint logic were completely removed, but we may need to take a look
> > > > > on the callers for s_radio. I'll check it right now.
> > > > > 
> > > > 
> > > > The s_radio callback requires some care, as it is used on several places. It is probably
> > > > safe to remove it from tuner, but a few sub-drivers like msp3400 needs it. The actual
> > > > troubles seem to happen at the bridge drivers that call it during open(). It should be
> > > > called only at s_frequency. I opted to keep the callback just to avoid having a bridge
> > > > driver switching its registers to radio mode, and not having the tuner following it.
> > > > 
> > > > If we move the radio mode switch at the bridge drivers to s_frequency only, we can just
> > > > remove this callback from tuner, letting it to be implemented only at the audio decoders.
> > > 
> > > Why would the audio decoders need it? If we do the mode switch when s_freq is
> > > called, then the audio decoders can do the same and s_radio can disappear completely.
> > > 
> > > I would like that, but I'm a bit afraid of application breakage since we're changing
> > > the behavior of /dev/radio. It seems that pretty much every video driver with radio
> > > capability is calling s_radio during open(): bttv, ivtv, saa7134, usbvision, em28xx,
> > > cx18, cx88, cx231xx and tm6000.
> > 
> > I think ivtvhopper relies on it:
> > 
> > http://www.gateways-home.org/wb/pages/mycoding/--ivtvhopper-java.php
> > 
> > Also, per my recommendation, ivtvhopper changes radio freq by
> > using /dev/video24, since V4L2 priorities got in the way:
> > 
> > http://ivtvdriver.org/pipermail/ivtv-users/2010-December/010097.html
> 
> Well, radio support for ivtv is weird and we really need a ivtv-alsa (easier
> said than done). Because it is so non-standard, I am not terribly concerned
> about it.

I use /dev/radio & /dev/video24 for FM radio using ivtv-radio, myself.

BTW, the cx18-alsa module annoys me as a developer.  PulseAudio holds
the device nodes open, pinning the cx18-alsa and cx18 modules in kernel.
When killed, PulseAudio respawns rapidly and reopens the nodes.
Unloading cx18 for development purposes is a real pain when the
cx18-alsa module exists.


> BTW, one problem with /dev/radio and ivtv (and I think cx18 might have the same
> problem) is that /dev/radio can be opened only once. A second attempt to open
> it will result in -EBUSY. That's a driver bug. I wonder if that's really the
> problem described in the link above instead of priority handling.

Gah, I think you are right.  It probably was a multiple open() problem
on /dev/radio for the app author.

I do remember researching that cx18 and ivtv are single open()
on /dev/radio.

I also remember finding that the V4L2 spec doesn't require multiple
opens, and implies drivers need not support it in at least two places:

        "Multiple Opens
        
        In general, V4L2 devices can be opened more than once. When this
        is supported by the driver, ..."


        "Name
        v4l2-open — Open a V4L2 device
        
        ...
        
        EBUSY
                The driver does not support multiple opens and the
                device is already in use.
        ..."
        

Regards,
Andy