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 > -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
