linux-kernel - Re: [PATCH v2 00/26] Improve DVB documentation and reduce its gap

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Date:   Mon, 4 Sep 2017 06:06:29 -0300
From:   Mauro Carvalho Chehab <mchehab@...pensource.com>
To:     Honza Petrouš <jpetrous@...il.com>
Cc:     Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...radead.org>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v2 00/26] Improve DVB documentation and reduce its gap

Em Mon, 4 Sep 2017 09:12:49 +0200
Honza Petrouš <jpetrous@...il.com> escreveu:

> 2017-09-04 2:54 GMT+02:00 Mauro Carvalho Chehab <mchehab@...pensource.com>:
> > Em Sun, 3 Sep 2017 22:05:23 +0200
> > Honza Petrouš <jpetrous@...il.com> escreveu:
> >  
> >> 1) #define CA_SET_DESCR      _IOW('o', 134, ca_descr_t)
> >> ============================================
> >>
> >> CA_SET_DESCR is used for feeding descrambler device
> >> with correct keys (called here "control words") what
> >> allows to get services unscrambled.
> >>
> >> The best docu is:
> >>
> >> "Digital Video Broadcasting (DVB);
> >> Support for use of the DVB Scrambling Algorithm version 3
> >> within digital broadcasting systems"
> >>
> >> Defined as DVB Document A125 and publicly
> >> available here:
> >>
> >> https://www.dvb.org/resources/public/standards/a125_dvb-csa3.pdf
> >>
> >>
> >> typedef struct ca_descr {
> >>         unsigned int index;
> >>         unsigned int parity;    /* 0 == even, 1 == odd */
> >>         unsigned char cw[8];
> >> } ca_descr_t;
> >>
> >> The 'index' is adress of the descrambler instance, as there exist
> >> limited number of them (retieved by CA_GET_DESCR_INFO).  
> >
> > Thanks for the info. If I understood well, the enclosed patch should
> > be documenting it.
> >
> >
> > Thanks,
> > Mauro
> >
> > [PATCH] media: ca docs: document CA_SET_DESCR ioctl and structs
> >
> > The av7110 driver uses CA_SET_DESCR to store the descrambler
> > control words at the CA descrambler slots.
> >
> > Document it.
> >
> > Thanks-to: Honza Petrouš <jpetrous@...il.com>
> > Signed-off-by: Mauro Carvalho Chehab <mchehab@...pensource.com>
> >
> > diff --git a/Documentation/media/uapi/dvb/ca-set-descr.rst b/Documentation/media/uapi/dvb/ca-set-descr.rst
> > index 9c484317d55c..a6c47205ffd8 100644
> > --- a/Documentation/media/uapi/dvb/ca-set-descr.rst
> > +++ b/Documentation/media/uapi/dvb/ca-set-descr.rst
> > @@ -28,22 +28,11 @@ Arguments
> >  ``msg``
> >    Pointer to struct :c:type:`ca_descr`.
> >
> > -.. c:type:: ca_descr
> > -
> > -.. code-block:: c
> > -
> > -    struct ca_descr {
> > -       unsigned int index;
> > -       unsigned int parity;
> > -       unsigned char cw[8];
> > -    };
> > -
> > -
> >  Description
> >  -----------
> >
> > -.. note:: This ioctl is undocumented. Documentation is welcome.
> > -
> > +CA_SET_DESCR is used for feeding descrambler CA slots with descrambling
> > +keys (refered as control words).
> >
> >  Return Value
> >  ------------
> > diff --git a/include/uapi/linux/dvb/ca.h b/include/uapi/linux/dvb/ca.h
> > index f66ed53f4dc7..a62ddf0cebcd 100644
> > --- a/include/uapi/linux/dvb/ca.h
> > +++ b/include/uapi/linux/dvb/ca.h
> > @@ -109,9 +109,16 @@ struct ca_msg {
> >         unsigned char msg[256];
> >  };
> >
> > +/**
> > + * struct ca_descr - CA descrambler control words info
> > + *
> > + * @index: CA Descrambler slot
> > + * @parity: control words parity, where 0 means even and 1 means odd
> > + * @cw: CA Descrambler control words
> > + */
> >  struct ca_descr {
> >         unsigned int index;
> > -       unsigned int parity;    /* 0 == even, 1 == odd */
> > +       unsigned int parity;
> >         unsigned char cw[8];
> >  };
> >
> >  
> 
> Yeh, it should be that way.

Good! I'll add this patch to the series.

> BTW, the only issue I have in mind is how to link particular
> descrambler with the PID
> after your removal of the CA_SET_PID. And yes, I know that currently we have
> no any user of such ioctl in our driver base :)

Well, I don't think that an ioctl like CA_SET_PID would solve it.

On a generic case with is quite common nowadays on embedded hardware,
We have K demods and M CIs (where K may be different than M).

Also, You may need to route N PIDs to O descramblers.

As user switch channels, the N PIDs should be unset, and another
set of N' pids will be routed.

CA_SET_PID allows to set just one PID, without identifying from
what demod it would be received, and doesn't have a "reset" 
function to undo.

So, IMHO, the interface is broken by design. Perhaps that's
the reason why no upstream driver uses it.

What seems to be a much better design would be to use the demux
set filter ioctls and route the PIDs to the right CA.


Thanks,
Mauro