Re: [git:v4l-dvb/for_v3.7] [media] add LNA support for DVB API

[Mauro Carvalho Chehab <mchehab@redhat.com>]

Em Fri, 28 Sep 2012 16:21:35 +0200
Hans Verkuil <hverkuil@xs4all.nl> escreveu:

> On Fri September 28 2012 14:43:32 Hans Verkuil wrote:
> > Hi Antti,
> > 
> > Mauro asked me to look into LNA as well, in particular how this could be done
> > on the analog side as well.

Thanks for that!

> > 
> > While reading through this patch I noticed that the new property was added to
> > dtv_property_process_set, but not to dtv_property_process_get. Can you look
> > into that and add 'get' support for this property?
> 
> It's a sign of insanity when you start replying to your own email, but so
> be it :-)

:)

> 
> I've been thinking how this can be implemented in such a way that it works
> well in all the various board/tuner configurations.
> 
> First, the property should be cached in dtv_frontend_properties, that way
> tuners with a built-in LNA can use it. The initial value should be AUTO,
> I guess.
> 
> The property_process_set code changes to:
> 
> 	case DTV_LNA:
> 		if (fe->ops.set_lna)
> 			r = fe->ops.set_lna(fe, tvp->u.data);
> 		if (!r)
> 			c->lna = tvp->u.data;
> 		break;
> 
> Tuners can now check the c->lna value when set_params is called and take the
> appropriate steps if they have a built-in LNA.
>
> To be able to access the LNA from the V4L2 side you would need to add two
> new tuner ops in include/media/v4l2-subdev.h:
> 
> 	int (*g_lna)(struct v4l2_subdev *sd, u32 *lna);
> 	int (*s_lna)(struct v4l2_subdev *sd, u32 lna);
> 
> The tuner-core.c driver can implement these ops to get and set c->lna accordingly.
> 
> A menu control would be needed to actually change the LNA.
> 
> The code that handles that control would have to call a board-specific function
> if necessary (if the LNA is on the board), and call the tuner's s_lna op otherwise.
> 

Makes sense.

> > The only question is whether the digital and analog APIs should share the same
> LNA setting or keep different LNA states for each.
> 
> So if I call DTV_LNA to set the LNA, and then check the LNA control value from V4L2,
> should the two match? Or should we keep separate states and whenever you select
> digital or analog mode the LNA is updated with the corresponding LNA value for that
> mode.
> 
> The latter is a bit more work (struct analog_parameters should probably be extended
> with an LNA value), but I do think it is a cleaner solution.

I think they both should share the same LNA state, as this depends on the physical
connection (e. g. if the antenna has LNA; if the signal reception is weak or strong
with that particular antenna).

> I am not sure if the LNA work on the analog side should be done without having
> hardware that actually uses it, but at least the LNA support on the digital side
> should be done in such a way that it can be extended for analog as well.

There are several saa7134 hardware with LNA support. I have one of such boards
here, although never needed to dig into the LNA stuff on it.

> 
> Anyway, I think this approach would work for analog LNA support. Comments are
> welcome!

Regards,
Mauro