Re: [PATCH 2/5] media: v4l2-core changes to use media tuner token api

[Hans Verkuil]

On 09/24/2014 05:57 PM, Shuah Khan wrote:
> On 09/24/2014 06:25 AM, Hans Verkuil wrote:
>> Hi Shuah,
>>
>>
>> Let's be realistic: if an application doesn't test for error codes,
>> then that's the problem of the application. Also, EBUSY will only be
>> returned if someone else is holding the tuner in a different mode.
>> And that didn't work anyway (and probably in worse ways too if the
>> driver would forcefully change the tuner mode).
>>
>> So I really don't see a problem with this.
>>
> 
> I didn't have high hopes you would agree to the simpler approach. :)
> 
>>>
>>> Compared to the current approach, holding lock in open and releasing
>>> it in close is cleaner with predictable failure conditions. It is lot
>>> easier to maintain. In addition, this approach keeps it same as the
>>> dvb core token hold approach as outlined below.
>>
>> Absolutely not an option for v4l2. You should always be able to open
>> a v4l2 device, regardless of the tuner mode or any other mode.
>>
>> The only exception is a radio tuner device: that should take the tuner
>> on open. I think this is a very unfortunate design and I wish that that
>> could be dropped.
> 
> Right this is another problem that needs to be addressed in the
> user-space.

This is however out-of-scope for your project. For radio devices you can
just take the tuner token on open.

BTW, the tuner token in v4l2 should *only* be taken if there actually is
a tuner. Luckily that's very easy to test for: struct v4l2_ioctl_ops will
have a non-NULL vidioc_s_tuner field.

> 
>>
>> One thing that we haven't looked at at all is what should be done if
>> the current input is not a tuner but a composite or S-Video input.
>>
>> It is likely (I would have to test this to be sure) that you can capture
>> from a DVB tuner and at the same time from an S-Video input without any
>> problems. By taking the tuner token unconditionally this would become
>> impossible. But doing this right will require more work.
>>
>> BTW, what happens if the analog video part of a hybrid board doesn't have
>> a tuner but only S-Video/Composite inputs? I think I've seen such boards
>> actually. I would have to dig through my collection though.
>>
> 
> I would recommend trying to bound the problems that need to be solved
> for the first phase of this media token feature. If we don't we will
> never be done with it. :)

I want to think about this myself (i.e. non-tuner inputs) and do some
testing as well. I'll get back to this when I know more myself.

> 
> I would propose the first step as addressing dvb and v4l2 conflicts
> and include snd-usb-audio so there is confidence that the media token
> approach can span non-media drivers.

Certainly.

> I am currently testing with tvtime, xawtv, vlc, and kaffeine. I am
> planning to add kradio for snd-usb-audio work for the next round of
> patches.
> 
>>
>> S_PRIORITY has nothing to do with tuner ownership. If there is a real need
>> to release the token at another time than on close (and I don't see
>> such a need), then make a new ioctl. Let's not overload S_PRIO with an
>> unrelated action.
> 
> This is not an issue for fine grained approach since simpler approach
> is nacked. i.e Mauro suggested changing S_PRIORITY as another place
> to release it if we were to go with simple appraoch (open/close).
> 
>>
>>>
>>> Devin recommended testing on devices that have an encoder to cover
>>> the cases where there are multiple /dev/videoX nodes tied to the
>>> same tuner.
>>
>> Good examples are cx23885 (already vb2) and cx88 (the vb2 patches have
>> been posted, but not yet merged). It shouldn't be too hard to find
>> hardware based on those chipsets.
>>
> 
> Please see my bounding the problem comment. Can these devices wait until
> the second phase. We have multiple combinations with hardware features,
> applications. The way I am designing the media token is if driver
> doesn't create the token, no change in the dvb-core, v4l2-core behavior.
> It is not required that driver must create a token to allow evolving
> driver support and hardware support as we go.

>From what I know I don't think these types of devices will pose any problem
for this approach. However, in general you should consider all sorts of
combinations and see if the design will still work for those. You don't
want to have to redesign it later just because you ignored a particular type
of device. Which I why I want to see the snd-usb-audio work first, since I
think that's probably the most complex part of the whole design.

And I do think you should try to get one of those cards (cx88 or cx23885
based). They are common, they are complex and have hybrid tuners, sometimes
with radio support as well. You can probably get some of them fairly cheaply
on ebay. These are good cards to test with, if not now then in the near
future. My philosophy is that you can never have too much hardware :-)

Regards,

	Hans
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