Re: dvb usb issues since kernel 4.9

On Mon, 8 Jan 2018, Mauro Carvalho Chehab wrote:

Em Sun, 7 Jan 2018 10:41:37 -0500 (EST)
Alan Stern [off-list ref] escreveu:

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On Sun, 7 Jan 2018, Mauro Carvalho Chehab wrote:

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It seems that the original patch were designed to solve some IRQ issues
with network cards with causes data losses on high traffic. However,
it is also causing bad effects on sustained high bandwidth demands
required by DVB cards, at least on some USB host drivers.

Alan/Greg/Eric/David:

Any ideas about how to fix it without causing regressions to
network?    

It would be good to know what hardware was involved on the x86 system
and to have some timing data.  Can we see the output from lsusb and
usbmon, running on a vanilla kernel that gets plenty of video glitches?  

From Josef's report, and from the BZ, the affected hardware seems
to be based on Montage Technology M88DS3103/M88TS2022 chipset.  

What type of USB host controller does the x86_64 system use?  EHCI or
xHCI?

I'll let Josef answer this.

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The driver it uses is at drivers/media/usb/dvb-usb-v2/dvbsky.c,
with shares a USB implementation that is used by a lot more drivers.
The URB handling code is at:

	drivers/media/usb/dvb-usb-v2/usb_urb.c

This particular driver allocates 8 buffers with 4096 bytes each
for bulk transfers, using transfer_flags = URB_NO_TRANSFER_DMA_MAP.

This become a popular USB hardware nowadays. I have one S960c
myself, so I can send you the lsusb from it. You should notice, however,
that a DVB-C/DVB-S2 channel can easily provide very high sustained bit
rates. Here, on my DVB-S2 provider, a typical transponder produces 58 Mpps
of payload after removing URB headers.  

You mentioned earlier that the driver uses bulk transfers.  In USB-2.0,
the maximum possible payload data transfer rate using bulk transfers is
53248 bytes/ms, which is 53.248 MB/s (i.e., lower than 58 MB/s).  And
even this is possible only if almost nothing else is using the bus at
the same time.

No, I said 58 Mbits/s (not bytes).

Well, what you actually _wrote_ was "58 Mpps of payload" (see above),
and I couldn't tell how to interpret that.  :-)

58 Mb/s is obviously almost 8 times less than the full USB bus 
bandwidth.

On DVB-C and DVB-S2 specs, AFAIKT, there's no hard limit for the maximum
payload data rate, although industry seems to limit it to be around
60 Mbits/s. On those standards, the maximal bit rate is defined by the
modulation type and by the channel symbol rate.

To give you a practical example, my DVB-S2 provider modulates each
transponder with 8/PSK (3 bits/symbol), and define channels with a
symbol rate of 30 Mbauds/s. So, it could, theoretically, transport
a MPEG-TS stream up to 90 Mbits/s (minus headers and guard intervals).
In practice, the streams there are transmitted with 58,026.5 Kbits/s.

Okay.  This is 58 Kb/ms or 7.25 KB/ms.  So your scheme of eight 4-KB 
buffers gives a latency of 0.57 ms with a total capacity of 4.5 ms, 
which is a lot better than what I was thinking.

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In any case, you might be able to attack the problem simply by using
more than 8 buffers.  With just eight 4096-byte buffers, the total
pipeline capacity is only about 0.62 ms (at the maximum possible
transfer rate).  Increasing the number of buffers to 65 would give a
capacity of 5 ms, which is probably a lot better suited for situations
where completions are handled by the ksoftirqd thread.

Increasing it to 65 shouldn't be hard. Not sure, however, if the hardware
will actually fill the 65 buffers, but it is worth to try.

Given the new information, 65 would be overkill.  But going from 8 to 
16 might help.

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Perhaps media drivers could pass some quirk similar to URB_ISO_ASAP,
in order to revert the kernel logic to prioritize latency instead of
throughput.  

It can't be done without pervasive changes to the USB subsystem, which
I would greatly prefer to avoid.  Besides, this wouldn't really solve
the problem.  Decreasing the latency for one device will cause it to be
increased for others.

If there is a TV streaming traffic at a USB bus, it means that the
user wants to either watch and/or record a TV program. On such
usecase scenario, a low latency is highly desired for the TV capture
(and display, if the GPU is USB), even it means a higher latency for
other traffic.

Not if the other traffic is also a TV capture.  :-)

It might make sense to classify softirq sources as "high priority" or 
"low priority", and only defer the "low priority" work to ksoftirqd.

Alan Stern