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

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

Em Fri, 11 Sep 2020 09:18:20 -0300
"Daniel W. S. Almeida" <dwlsalmeida@gmail.com> escreveu:

> Hey Mauro,
> 
> > Thanks for all the hard work on it. Very much appreciated!
> > 
> > I finally found some time to test it. For now, just a quick
> > test from my side, without passing any arguments to the
> > driver.
> >   
> 
> That's nice!
> 
> > My plan is to write some patches on the top of yours, in order to
> > address the problems I'll find on it. If not something more critical
> > won't be solved in time, we may still add it at staging/media. 
> > Let's see.  
> 
> OK
> 
> > 	3. dvbv5-zap wrote an empty audio file (without -P flag).
> > 	   Probably there are still some issues at the program
> > 	   channel descriptor or service;  
> 
> I don't remember whether I tried this. I tried dumping the stream to a
> file with dvbzap, which should work. By the way, I guess we should be
> comparing the output to this
> 
> $ ffmpeg -f lavfi -i sine=frequency=1000:duration=5 -ac 2 -c:a s302m
> -strict -2 out.ts
> 
> Since it produces a playable transport stream file that actually sounds
> like a sine tone.
> 
> Inspecting ffmpeg & vidtv output side by side in dvbinspector, you'll
> see that they're mostly the same. I have a separate PID for the PCR and
> some other minor differences.

The problem is here:

	$ dvbv5-zap -c dvb_channel.conf "S302m: Sine Wave PCM Audio" -t 30 -o pcm_audio.ts -P
	using demux 'dvb0.demux0'
	reading channels from file 'dvb_channel.conf'
	service has pid type 06:  273

See, it identified the EL type ID as type 6, which is handled by
dvbv5 library here:

		case 0x06: /* private data */
			/*
			* Those can be used by sub-titling, teletext and/or
			* DVB AC-3. So, need to seek for the AC-3 descriptors
			*/
			dvb_desc_find(struct dvb_desc_service, desc, stream, AC_3_descriptor)
				has_ac3 = 1;

			dvb_desc_find(struct dvb_desc_service, desc, stream, enhanced_AC_3_descriptor)
				has_ac3 = 1;

			if (has_ac3) {
				entry->audio_pid = realloc(entry->audio_pid,
							   sizeof(*entry->audio_pid) *
							   (audio_len + 1));
				entry->audio_pid[audio_len] = pid;
				audio_len++;
			} else {
				entry->other_el_pid = realloc(entry->other_el_pid,
							   sizeof(*entry->other_el_pid) *
							   (other_len + 1));
				entry->other_el_pid[other_len].type = stream->type;
				entry->other_el_pid[other_len].pid = pid;
				other_len++;
			}
			break;

Basically, it is not recognizing the stream as an audio PID, but
as some other random data. Due to that, the output of
dvb_channel.conf will be wrong.

As type 6 seems to be the correct one for SMPTE 302M, we need to fix
dvbv5-tools (and likely other tools like kaffeine), in order to
recognize it as audio as well.

> > 	4. The provider service field is null. Perhaps we could
> > 	   add some string there, like "linuxtv.org".
> > 	5. Maybe we could also add a simple NIT table, just to
> > 	   avoid dvbv5-scan to wait for it until timeout.
> > 
> > Also, it probably makes sense to add a debugfs interface in
> > order to allow injecting errors at the stream at runtime.  
> 
> Sure. This is fun, sign me up for it.

Well, if you have some spare time, you could try to write
a debugfs binding for vidtv. The best would be to have it
on a separate file. Failing to bind debugfs should not
prevent loading the bridge driver.

> As I said in a previous email, I think the buffer in vidtv_s302m.c is
> not exactly what we want. It sounds like noise.

Yeah, after changing dvb_channel.conf to:

    [S302m: Sine Wave PCM Audio]
	SERVICE_ID = 2176
	AUDIO_PID = 273
	FREQUENCY = 330000000
	MODULATION = QAM/AUTO
	INVERSION = AUTO
	SYMBOL_RATE = 6940000
	INNER_FEC = AUTO
	DELIVERY_SYSTEM = DVBC/ANNEX_A

   (e. g. changing "PID_06" to "AUDIO_PID")

I was able to record and play it.

Anyway, the actual sound doesn't matter much here, at least for
the first version, as this could be changed later on. 

> I got it from here:
> https://www.daycounter.com/Calculators/Sine-Generator-Calculator.phtml

On a quick look at the s302m_sin_lut table, it is indeed a sinal
waveform, with a DC offset on it (in order to convert it to unsigned),
as "0" is 0x8000. 

The "noise" is actually looks like "humm". This is periodic, it seems. 

> By the way, after some time trying out stuff, I guess this is actually
> what we need in vidtv_s302m_write_frame:
> 
> static u32 vidtv_s302m_write_frame(struct vidtv_encoder *e,
> 				   u16 sample)
> {
> 	u32 nbytes = 0;
> 	struct vidtv_s302m_frame_16 f = {};
> 	struct vidtv_s302m_ctx *ctx = e->ctx;
> 
> 	/* from ffmpeg: see s302enc.c */
> 
> 	u8 vucf = ctx->frame_index == 0 ? 1 : 0;
> 
> 	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;

doing that didn't work. Yet, this check inside the driver:
 
        #ifdef __LITTLE_ENDIAN
        f.data[0] = reverse[f.data[0]];
        f.data[1] = reverse[f.data[1]];
        f.data[2] = reverse[f.data[2]];
        f.data[3] = reverse[f.data[3]];
        f.data[4] = reverse[f.data[4]];
        #endif

Seems plain wrong, as you're already ensuring the endiannes
it is needed when you're doing things like "sample & 0xff".

Thanks,
Mauro
_______________________________________________
Linux-kernel-mentees mailing list
Linux-kernel-mentees@lists.linuxfoundation.org
https://lists.linuxfoundation.org/mailman/listinfo/linux-kernel-mentees