[alsa-devel] [RFC - AAF PCM plugin 3/5] aaf: Implement Playback mode support
Guedes, Andre
andre.guedes at intel.com
Sat Sep 1 01:18:04 CEST 2018
Hi Takashi,
On Fri, 2018-08-31 at 13:33 +0900, Takashi Sakamoto wrote:
> Hi Guedes,
>
> On Aug 29 2018 10:00, Guedes, Andre wrote:
> > On Sat, 2018-08-25 at 17:13 +0900, Takashi Sakamoto wrote:
> > > On Aug 24 2018 03:32, Guedes, Andre wrote:
> > > > On Wed, 2018-08-22 at 21:25 -0500, Pierre-Louis Bossart wrote:
> > > > > On 08/22/2018 07:46 PM, Guedes, Andre wrote:
> > > > > > On Tue, 2018-08-21 at 17:51 -0500, Pierre-Louis Bossart
> > > > > > wrote:
> > > > > > > > > > +static int aaf_mclk_start_playback(snd_pcm_aaf_t
> > > > > > > > > > *aaf)
> > > > > > > > > > +{
> > > > > > > > > > + int res;
> > > > > > > > > > + struct timespec now;
> > > > > > > > > > + struct itimerspec itspec;
> > > > > > > > > > + snd_pcm_ioplug_t *io = &aaf->io;
> > > > > > > > > > +
> > > > > > > > > > + res = clock_gettime(CLOCK_REF, &now);
> > > > > > > > > > + if (res < 0) {
> > > > > > > > > > + SNDERR("Failed to get time from
> > > > > > > > > > clock");
> > > > > > > > > > + return -errno;
> > > > > > > > > > + }
> > > > > > > > > > +
> > > > > > > > > > + aaf->mclk_period = (NSEC_PER_SEC * aaf-
> > > > > > > > > > > frames_per_pkt) /
> > > > > > > > > >
> > > > > > > > > > io->rate;
> > > > > > > > >
> > > > > > > > > is this always an integer? If not, don't you have a
> > > > > > > > > systematic
> > > > > > > > > arithmetic error?
> > > > > > > >
> > > > > > > > NSEC_PER_SEC is 64-bit so I don't see an arithmetic
> > > > > > > > error
> > > > > > > > during
> > > > > > > > calculation (e.g. integer overflow). Not sure this was
> > > > > > > > your
> > > > > > > > concern,
> > > > > > > > though. Let me know otherwise.
> > > > > > >
> > > > > > > No, I was talking about the fractional part, e.g with 256
> > > > > > > frames
> > > > > > > with
> > > > > > > 44.1kHz you have a period of 5804988.662131519274376 - so
> > > > > > > your
> > > > > > > math
> > > > > > > adds
> > > > > > > a truncation. same with 48khz, the fractional part is
> > > > > > > .333
> > > > > > >
> > > > > > > I burned a number of my remaining neurons chasing a <100
> > > > > > > ppb
> > > > > > > error
> > > > > > > which
> > > > > > > led to underruns after 10 hours, so careful now with
> > > > > > > truncation...
> > > > > >
> > > > > > Thanks for clarifying.
> > > > > >
> > > > > > Yes, we can end up having a fractional period which is
> > > > > > truncated.
> > > > > > Note
> > > > > > that both 'frames' and 'rate' are configured by the user.
> > > > > > The
> > > > > > user
> > > > > > should set 'frames' as multiple of 'rate' whenever possible
> > > > > > to
> > > > > > avoid
> > > > > > inaccuracy.
> > > > >
> > > > > It's unlikely to happen. it's classic in audio that people
> > > > > want
> > > > > powers
> > > > > of two for fast filtering, and don't really care that the
> > > > > periods
> > > > > are
> > > > > fractional. If you cannot guarantee long-term operation
> > > > > without
> > > > > timing
> > > > > issues, you should add constraints to the frames and rates so
> > > > > that
> > > > > there
> > > > > is no surprise.
> > > >
> > > > Fair enough. So for now I'll add a constraint on frames and
> > > > rates
> > > > to
> > > > unsure no surprises. Later we can revisit this and implement
> > > > the
> > > > compesation mechanism you described below.
> > >
> > > In my understanding, transmission timing of 'AVTP Audio format'
> > > in
> > > IEEE
> > > 1722:2016 is similar to 'blocking transmission' of IEC 61883-6.
> > > Packets
> > > have fixed size of data in its payload, thus include the same
> > > number
> > > of
> > > PCM frames. Talkers are expected to fill data till the size, then
> > > transmit the packet. Receivers are expected to perform buffering
> > > till
> > > presentation timestamp is elapsed, with one (or more) AVTPDUs.
> >
> > I'm not familiar with the 'blocking transmission' of IEC 61883-6
> > but
> > from the description above, yes, it looks similar indeed.
>
> Further investigation, I realized that transmission timing of AAF is
> not similar to IEC 61883-1/6 at all... I'm sorry to address to it
> into
> this topic.
>
> > > In clause 7.7 'AAF and SRP', I can see below sentence:
> > > '... A 44.1-kHz stream with 6 samples per AAF AVTPDU and an FQTSS
> > > observation interval of 125 us also has an SRP reservation of 1
> > > frame
> > > per observation interval even though there will periodically be
> > > an
> > > observation interval where no AAF AVTPDU will be transmitted
> > > since
> > > it has a transmission interval of 136.054 us as can be seen in
> > > the
> > > example given in Figure 36.' This means that packet transmission
> > > is
> > > not always periodically. There's a blank cycle per several
> > > cycles;
> > > like
> > > IEC 61883-1/6.
> >
> > My understanding of the periodicity of packet transmission is
> > different. I believe it is always periodic. Let me elaborate on
> > this.
> >
> > The first paragraph from Section 7.7 states that "AAF transmission
> > interval is defined by the clock rate of the media rather than the
> > FQTSS observation interval." Since the clock is periodic, the AAF
> > transmission interval is periodic too. For instance, if we take the
> > 44.1 kHz example from Figure 36, we can see the AAF transmission
> > interval is always ~136us. So, from AAF perspective (i.e. the
> > plugin
> > perspective), the packet transmission is always periodic.
> >
> > The AAF transmission interval isn't necessarily equal to the FQTSS
> > observation interval. Again, if we take a look at the 44.1 kHz
> > example,
> > we can see the AAF transmission interval is ~136us while the FQTSS
> > observation interval is 125us. This, however, isn't an issue since
> > the
> > plugin is not expected to operate in terms of FQTSS observation
> > interval, but in terms of AAF transmission interval as stated in
> > the
> > first paragraph from Section 7.7.
>
> Indeed, thanks for your correction against my misunderstanding.
>
> > > In my opinion, it's better calculate proper interval of timerfd
> > > to
> > > create the black interval, without truncate the fraction. Then,
> > > give
> > > proper constrains to SND_PCM_IOPLUG_HW_PERIOD_BYTES to prevent
> > > applications from underrun.
> >
> > If the above understanding is correct, I'm not sure this approach
> > would
> > work. Let me know otherwise.
>
> I have another concern of buffering in a perspectives of delay of
> task
> scheduling.
>
> The interval of task scheduling for this plugin is decided mainly by
> the value of 'frames_per_pkt', given by users. In your documentation,
> the value is 6[1]. Of cource this is an example but in this case the
> interval is calculated as 125us at 48.0kHz. In my opinion, task
> scheduling in Linux kernel brings deadline misses for the interval,
> in most cases such as major Linux distribution on usual personal
> computers. When considering about the fact that recent motherboards
> implements Intel I210/220 series, it's better to care for the low-
> level
> realtime systems, in my opinion.
Agreed.
To mitigate this scheduling issue, a follow-up patchset will extend the
plugin to leverage the ETF qdisc [1] which will be available on next
kernel release. The idea is: instead of sending one AVTP packet at
every interval, the plugin will send several AVTP packets at once,
configuring their Tx time accordingly, so it can "sleep" for longer
periods of time. The goal is to ease on task scheduling by
offloading packet transmissions to the NIC.
> > > Furthermore, in your proposal, the number of PCM frames in one
> > > AVTPDU
> > > is
> > > decided according to plugin parameter. However, if compliant to
> > > specification, it's better to decide the number according to
> > > 'FQTSS
> > > observation interval'. I can see recommendations in two cases;
> > > FQTSS = 125 us and 256 us, in Table 17 and 18 of IEEE 1212:2016.
> >
> > It was designed that way on purpose. Let me share the rationale.
> >
> > The values in Table 17 and 18 are just recommendations. From the
> > plugin
> > perspective, we should enable users to configure the number of
> > frames
> > according to their needs. This way, users are free to configure the
> > values recommended by the spec or other values optimized to their
> > AVTP
> > application.
> >
> > Besides that, as stated in the NOTE right below Table 18, 125us and
> > 250us are not the only possible FQTSS observation intervals.
>
> I addressed to the reccomendation itself. If specification describes
> recommendations, there will be a reason to consider about it, just
> not
> refer to values on the table.
The point I was trying to make is that it's up to the users to
configure the plugin with the spec recommended values (to ensure
interoperabilty) or their own custom/application-specific values. For
example, for automotive use-cases, they have defined intervals equals
to 1333.33us and 1451.25us (see Table 15 from [2]) that are not covered
in Table 17 and 18.
Regards,
Andre
[1] https://marc.info/?l=linux-netdev&m=153065819412748
[2] https://avnu.org/wp-content/uploads/2014/05/Automotive-Ethernet-AVB
-Func-Interop-Spec-v1.5-Public.pdf
>
> I can see a sentence in section 7.7; 'in order to maintain
> interoperability between devices, the transmission intervals listed
> in
> Table 17 and Table 18 should be used.' I understand that 'if a talker
> end station transfers an AVTP packet with largely different number of
> sample frames from expectations on listener end station, there's a
> case
> that the listener cannot handles the sample frames due to processor
> loading or buffer overflow on listener side. To avoid this case,
> FQTSS
> observation interval is loosely used to decide intervals of AVTPDU
> transmission'. But it's within my imagination.
>
> [1]
> http://mailman.alsa-project.org/pipermail/alsa-devel/2018-August/1394
> 95.html
>
>
> Thanks
>
> Takashi Sakamoto
> _______________________________________________
> Alsa-devel mailing list
> Alsa-devel at alsa-project.org
> http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
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