[Linux-kernel-mentees] [v10 0/4] media: vidtv: Implement a virtual DVB driver

Daniel W. S. Almeida dwlsalmeida at gmail.com
Sat Sep 12 14:49:01 UTC 2020

Hi Hans and Mauro & all

>Why the dvb_ prefix? All virtual drivers just start with 'vi'.
>And wouldn't it make more sense to call dvb_vidtv_bridge.ko just vidtv.ko?
>Just like the other virtual media drivers?

I guess Mauro was the one to come up with the dvb_* prefix for the
kernel modules for the reasons he explicited up in this thread. 

As far as dvb_vidtv_bridge.ko and vidtv_bridge.c, I just wanted to be
verbose so that people would look at this and see that it is the code
for a bridge driver, since this is also supposed to be a template. 

Also because I had some trouble myself figuring out what was what when
first browsing through other dvb drivers. That said, I am 100% onboard
with renaming this to vidtv.ko or whatever seems more appropiate :)

>Yet, I agree with you that at least an alias is needed.
>earlier today, I wrote a patch with such purpose:

If you all would like to just leave this at that ^ I am also ok with it.

>For regression testing of vidtv during the daily build it would be
great if
>the contrib/test/test-media script can be enhanced to include vidtv.

Sure, I can do that if you'd like. Can you provide some tips on how to
get started? :)

>Note that this script relies on the /dev/mediaX devices to run the
tests. I
>noticed that vidtv doesn't appear to create a /dev/mediaX device, even though
>CONFIG_MEDIA_CONTROLLER_DVB=y. This is definitely something that would
be good
>to support in vidtv.

I didn't write any code for this specifically. I was under the
impression that the dvb core would create any/all the necessary files. Mauro?

>I'm thinking on something like:
>echo 1 >/sys/kernel/debug/vidtv/discontinuity
>to generate a frame numbering discontinuity
>and other things like that, changing S/N ratio and other parameters
>and injecting errors, in order to test the effects on user apps.

Can you provide an initial list of things you would like to see? I can
then implement these and we can go from there

When you say 'frame numbering discontinuity', do you mean having a gap
in the the TS continuity counter for a given pid?

For the s302m encoder, maybe we can add some noise to the samples?

> changing S/N ratio

Btw 'lose lock if signal goes bad' code in vidtv_tuner.c and
vidtv_demod.c does not work currently. Mainly because they expect an
array of valid frequencies to compare to and as of now there's no default.

I mean these, just to be clear:

static unsigned int vidtv_valid_dvb_t_freqs[NUM_VALID_TUNER_FREQS];
module_param_array(vidtv_valid_dvb_t_freqs, uint, NULL, 0);
  		 "Valid DVB-T frequencies to simulate");

static unsigned int vidtv_valid_dvb_c_freqs[NUM_VALID_TUNER_FREQS];
module_param_array(vidtv_valid_dvb_c_freqs, uint, NULL, 0);
		 "Valid DVB-C frequencies to simulate");

static unsigned int vidtv_valid_dvb_s_freqs[NUM_VALID_TUNER_FREQS];
module_param_array(vidtv_valid_dvb_s_freqs, uint, NULL, 0);
		 "Valid DVB-C frequencies to simulate");

static unsigned int max_frequency_shift_hz;
module_param(max_frequency_shift_hz, uint, 0);
		 "Maximum shift in HZ allowed when tuning in a channel")

> No. I meant the audio was not a sinusoidal wave after such change.

This encoder is dead simple and yet I have been struggling :/

Here is the relevant decoding code in libavcodec/s302m.c:

        for (; buf_size > 4; buf_size -= 5) {
            *o++ = (ff_reverse[buf[1]]        <<  8) |
            *o++ = (ff_reverse[buf[4] & 0xf0] << 12) |
                   (ff_reverse[buf[3]]        <<  4) |
                   (ff_reverse[buf[2]]        >>  4);
            buf += 5;

I am somewhat confident that the opposite to the above is precisely what
I sent you last:

> f.data[0] = reverse[sample & 0xff];
> f.data[1] = reverse[(sample & 0xff00) >> 8];
> f.data[2] = (vucf << 4) | (reverse[(sample & 0x0f)] >> 4);
> f.data[3] = reverse[(sample & 0x0ff0) >> 4];
> f.data[4] = reverse[(sample & 0xf000) >> 12] >> 4;

Their encoder, which works perfectly of course, is:


        for (c = 0; c < frame->nb_samples; c++) {
            uint8_t vucf = s->framing_index == 0 ? 0x10 : 0;

            for (channels = 0; channels < avctx->channels; channels +=
2) {
                o[0] = ff_reverse[ samples[0] & 0xFF];
                o[1] = ff_reverse[(samples[0] & 0xFF00) >>  8];
                o[2] = ff_reverse[(samples[1] & 0x0F)   <<  4] | vucf;
                o[3] = ff_reverse[(samples[1] & 0x0FF0) >>  4];
                o[4] = ff_reverse[(samples[1] & 0xF000) >> 12];
                o += 5;
                samples += 2;


            if (s->framing_index >= 192)
                s->framing_index = 0;

In which samples[0] and samples[1] are the same for stereo audio.

If you feel like we should only address this later I can drop this
subject but as we've both established, the encoder in v10 is flat out
wrong. Your call :)

-- thanks
-- Daniel

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