Re: [PATCH 0/5] Driver support for cards based on Digital Devices bridge (ddbridge)

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

Em 16-07-2011 11:16, Antti Palosaari escreveu:
> On 07/16/2011 03:25 PM, Mauro Carvalho Chehab wrote:
>> Em 15-07-2011 20:41, Antti Palosaari escreveu:
>>> On 07/15/2011 08:01 PM, Andreas Oberritter wrote:
>>>> On 15.07.2011 15:25, Mauro Carvalho Chehab wrote:
>>>>> Em 15-07-2011 05:26, Ralph Metzler escreveu:
>>>>>> At the same time I want to add delivery system properties to
>>>>>> support everything in one frontend device.
>>>>>> Adding a parameter to select C or T as default should help in most
>>>>>> cases where the application does not support switching yet.
>>>>>
>>>>> If I understood well, creating a multi-delivery type of frontend for
>>>>> devices like DRX-K makes sense for me.
>>>>>
>>>>> We need to take some care about how to add support for them, to avoid
>>>>> breaking userspace, or to follow kernel deprecating rules, by adding
>>>>> some legacy compatibility glue for a few kernel versions. So, the sooner
>>>>> we add such support, the better, as less drivers will need to support
>>>>> a "fallback" mechanism.
>>>>>
>>>>> The current DVB version 5 API doesn't prevent some userspace application
>>>>> to change the delivery system[1] for a given frontend. This feature is
>>>>> actually used by DVB-T2 and DVB-S2 drivers. This actually improved the
>>>>> DVB API multi-fe support, by avoiding the need of create of a secondary
>>>>> frontend for T2/S2.
>>>>>
>>>>> Userspace applications can detect that feature by using FE_CAN_2G_MODULATION
>>>>> flag, but this mechanism doesn't allow other types of changes like
>>>>> from/to DVB-T/DVB-C or from/to DVB-T/ISDB-T. So, drivers that allow such
>>>>> type of delivery system switch, using the same chip ended by needing to
>>>>> add two frontends.
>>>>>
>>>>> Maybe we can add a generic FE_CAN_MULTI_DELIVERY flag to fe_caps_t, and
>>>>> add a way to query the type of delivery systems supported by a driver.
>>>>>
>>>>> [1] http://linuxtv.org/downloads/v4l-dvb-apis/FE_GET_SET_PROPERTY.html#DTV-DELIVERY-SYSTEM
>>>>
>>>> I don't think it's necessary to add a new flag. It should be sufficient
>>>> to add a property like "DTV_SUPPORTED_DELIVERY_SYSTEMS", which should be
>>>> read-only and return an array of type fe_delivery_system_t.
>>>>
>>>> Querying this new property on present kernels hopefully fails with a
>>>> non-zero return code. in which case FE_GET_INFO should be used to query
>>>> the delivery system.
>>>>
>>>> In future kernels we can provide a default implementation, returning
>>>> exactly one fe_delivery_system_t for unported drivers. Other drivers
>>>> should be able to override this default implementation in their
>>>> get_property callback.
>>>
>>> One thing I want to say is that consider about devices which does have MFE using two different *physical* demods, not integrated to same silicon.
>>>
>>> If you add such FE delsys switch mechanism it needs some more glue to bind two physical FEs to one virtual FE. I see much easier to keep all FEs as own - just register those under the same adapter if FEs are shared.
>>
>> In this case, the driver should just create two frontends, as currently.
>>
>> There's a difference when there are two physical FE's and just one FE:
>> with 2 FE's, the userspace application can just keep both opened at
>> the same time. Some applications (like vdr) assumes that all multi-fe
>> are like that.
> 
> Does this mean demod is not sleeping (.init() called)?
> 
>> When there's just a single FE, but the driver needs to "fork" it in two
>> due to the API troubles, the driver needs to prevent the usage of both
>> fe's, either at open or at the ioctl level. So, applications like vdr
>> will only use the first frontend.
> 
> Lets take example. There is shared MFE having DVB-S, DVB-T and DVB-C. DVB-T and DVB-C are integrated to one chip whilst DVB-S have own.
> 
> Currently it will shown as:

Let me name the approaches:

Approach 1)
> * adapter0
> ** frontend0 (DVB-S)
> ** frontend1 (DVB-T)
> ** frontend2 (DVB-C)

Approach 2)
> Your new "ideal" solution will be:
> * adapter0
> ** frontend0 (DVB-S/T/C)

Approach 3)
> What really happens (mixed old and new):
> * adapter0
> ** frontend0 (DVB-S)
> ** frontend1 (DVB-T/C)

What I've said before is that approach 3 is the "ideal" solution.

> It does not look very good to offer this kind of mixed solution, since it is possible to offer only one solution for userspace, new or old, but not mixing.

Good point. 

There's an additional aspect to handle: if a driver that uses approach 1, a conversion
to either approach 2 or 3 would break existing applications that can't handle with
the new approach.

There's a 4th posibility: always offering fe0 with MFE capabilities, and creating additional fe's
for old applications that can't cope with the new mode.
For example, on a device that supports DVB-S/DVB-S2/DVB-T/DVB-T2/DVB-C/ISDB-T, it will be shown as:

Approach 4) fe0 is a frontend "superset"

*adapter0
*frontend0 (DVB-S/DVB-S2/DVB-T/DVB-T2/DVB-C/ISDB-T) - aka: FE superset
*frontend1 (DVB-S/DVB-S2)
*frontend2 (DVB-T/DVB-T2)
*frontend3 (DVB-C)
*frontend4 (ISDB-T)

fe0 will need some special logic to allow redirecting a FE call to the right fe, if
there are more than one physical frontend bound into the FE API.

I'm starting to think that (4) is the better approach, as it won't break legacy
applications, and it will provide an easier way for new applications to control
the frontend with just one frontend.


Cheers,
Mauro