Re: [very-RFC 0/8] TSN driver for the kernel

John Fastabend
2016-06-13T15:57:10Z

On 16-06-13 04:47 AM, Richard Cochran wrote:
> Henrik,
>
> On Sun, Jun 12, 2016 at 01:01:28AM +0200, Henrik Austad wrote:
>> There are at least one AVB-driver (the AV-part of TSN) in the kernel
>> already,
>
> Which driver is that?
>
>> however this driver aims to solve a wider scope as TSN can do
>> much more than just audio. A very basic ALSA-driver is added to the end
>> that allows you to play music between 2 machines using aplay in one end
>> and arecord | aplay on the other (some fiddling required) We have plans
>> for doing the same for v4l2 eventually (but there are other fishes to
>> fry first). The same goes for a TSN_SOCK type approach as well.
>
> Please, no new socket type for this.
>
>> What remains
>> - tie to (g)PTP properly, currently using ktime_get() for presentation
>> time
>> - get time from shim into TSN and vice versa
>
> ... and a whole lot more, see below.
>
>> - let shim create/manage buffer
>
> (BTW, shim is a terrible name for that.)
>
> [sigh]
>
> People have been asking me about TSN and Linux, and we've made some
> thoughts about it. The interest is there, and so I am glad to see
> discussion on this topic.
>
> Having said that, your series does not even begin to address the real
> issues. I did not review the patches too carefully (because the
> important stuff is missing), but surely configfs is the wrong
> interface for this. In the end, we will be able to support TSN using
> the existing networking and audio interfaces, adding appropriate
> extensions.
>
> Your patch features a buffer shared by networking and audio. This
> isn't strictly necessary for TSN, and it may be harmful. The
> Listeners are supposed to calculate the delay from frame reception to
> the DA conversion. They can easily include the time needed for a user
> space program to parse the frames, copy (and combine/convert) the
> data, and re-start the audio transfer. A flexible TSN implementation
> will leave all of the format and encoding task to the userland. After
> all, TSN will some include more that just AV data, as you know.
>
> Lets take a look at the big picture. One aspect of TSN is already
> fully supported, namely the gPTP. Using the linuxptp user stack and a
> modern kernel, you have a complete 802.1AS-2011 solution.
>
> Here is what is missing to support audio TSN:
>
> * User Space
>
> 1. A proper userland stack for AVDECC, MAAP, FQTSS, and so on. The
> OpenAVB project does not offer much beyond simple examples.
>
> 2. A user space audio application that puts it all together, making
> use of the services in #1, the linuxptp gPTP service, the ALSA
> services, and the network connections. This program will have all
> the knowledge about packet formats, AV encodings, and the local HW
> capabilities. This program cannot yet be written, as we still need
> some kernel work in the audio and networking subsystems.
>
> * Kernel Space
>
> 1. Providing frames with a future transmit time. For normal sockets,
> this can be in the CMESG data. For mmap'ed buffers, we will need a
> new format. (I think Arnd is working on a new layout.)
>
> 2. Time based qdisc for transmitted frames. For MACs that support
> this (like the i210), we only have to place the frame into the
> correct queue. For normal HW, we want to be able to reserve a time
> window in which non-TSN frames are blocked. This is some work, but
> in the end it should be a generic solution that not only works
> "perfectly" with TSN HW but also provides best effort service using
> any NIC.
>

When I looked at this awhile ago I convinced myself that it could fit
fairly well into the DCB stack (DCB is also part of 802.1Q). A lot of
the traffic class to queue mappings and priories could be handled here.
It might be worth taking a look at ./net/sched/mqprio.c and ./net/dcb/.

Unfortunately I didn't get too far along but we probably don't want
another mechanism to map hw queues/tcs/etc if the existing interfaces
work or can be extended to support this.

> 3. ALSA support for tunable AD/DA clocks. The rate of the Listener's
> DA clock must match that of the Talker and the other Listeners.
> Either you adjust it in HW using a VCO or similar, or you do
> adaptive sample rate conversion in the application. (And that is
> another reason for *not* having a shared kernel buffer.) For the
> Talker, either you adjust the AD clock to match the PTP time, or
> you measure the frequency offset.
>
> 4. ALSA support for time triggered playback. The patch series
> completely ignore the critical issue of media clock recovery. The
> Listener must buffer the stream in order to play it exactly at a
> specified time. It cannot simply send the stream ASAP to the audio
> HW, because some other Listener might need longer. AFAICT, there
> is nothing in ALSA that allows you to say, sample X should be
> played at time Y.
>
> These are some ideas about implementing TSN. Maybe some of it is
> wrong (especially about ALSA), but we definitely need a proper design
> to get the kernel parts right. There is plenty of work to do, but we
> really don't need some hacky, in-kernel buffer with hard coded audio
> formats.
>
> Thanks,
> Richard
>