Re: RFC tuner-core: how to set vt->type in g_tuner?

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

Em 25-06-2011 11:02, Hans Verkuil escreveu:
> Hi all,
> 
> The tuner-core.c implementation does this at the start of g_tuner:
> 
>         if (check_mode(t, vt->type) == -EINVAL)
>                 return 0;
>         vt->type = t->mode;
> 
> The idea is that the vt->type is set depending on whether the VIDIOC_G_TUNER
> ioctl is called from a radio device node or a video device node. If we have a
> tuner that can do both radio and TV, then the type is set to whatever the
> current tuner mode is.
> 
> This seems reasonable, but it will actually run into problems when g_tuner
> is called for audio demodulators like msp3400 or cx25840. These need to know
> the correct vt->type in order to fill in the right fields.
> 
> The problem here is that the tuner subdevices are not necessarily called first.
> In fact, the msp3400/cx25840 are actually called first by ivtv.
> 
> So the msp3400 will get called with type TV, and later the tuner may change that
> to type RADIO. This causes inconsistencies. This has actually been observed when
> testing with ivtv and a PVR-500.
> 
> There are two solutions:
> 
> 1) Audit the drivers and ensure that the tuner subdevices are registered first.
> 
> 2) Do not allow the tuner to switch the type.

(2) is the right thing to do. A VIDIOC_GET_foo should not change anything. They are
supposed to be read only access.

> 
> The problem with 1 is that this will be hard to enforce in the long term. Another
> problem with 1 is that I do think it is a bit unexpected from an application PoV
> that the type is suddenly inconsistent with the node the ioctl is called from.
> 
> The problem with 2 is that some sensible defaults need to be filled in if the
> a radio/TV tuner is called with vt->type set to a different mode than the current
> mode.
> 
> I do not think that is very hard though: afc/signal can be 0, ditto for rxsubchans.
> The audmode field should just report the value last set with s_tuner.
> 
> g_frequency has the same problem as g_tuner. I believe g_frequency shouldn't change
> the type either. Since it already has the last set frequency for tv or radio it can
> just report it.
> 
> I think the second solution is the easiest to implement and the most intuitive as
> well.
> 
> Comments?
> 
> 	Hans
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