Hi all,
I have an EG20T-based board and have some issues with performance on the
USB device interface.
I made a libusb test program (using the async interface)[0] to read data
from the EG20T's USB device port which has the gadget zero source/sink
function bound. In theory, one would hope this would give the fastest
real-world results for the hardware connected.
The test program submits 32 IN transfers and re-submits on transfer
completion, counting received packets.
From running my test program for a few minutes I get the following:
elapsed: 548.468416 seconds
packets: 21503334
packets/sec: 39206.148199
bytes/sec: 20073547.877732
MBit/sec: 160.588383
160MBit/sec isn't terrible, but I hoped for better. A USB analyzer shows
7 transactions happening quickly (with about 14us separating them), but
every 8th transaction, the EG20T will NAK between 20-80 times[1], losing
50-100us[2].
This delay happens every 8th transaction without fail[3].
I've looked at the following:
1. The f_sourcesink.c function it queues up 8 responses at the
beginning. Changing this number up or down had no effect.
2. Analysis of pch_udc.c doesn't show anything which would obviously
cause a delay every 8th packet.
3. f_eem seems to have roughly the same performance with ping -f -s
64000 (160Mbit/sec).
The CPU load of the gadget-side Atom PC sits very close to zero.
System Details:
Linux 3.13.0-rc7 (With a defconfig from Yocto for Intel Crownbay)
Intel Atom E680 with EG20T
I seem to have eliminated everything on the host side, since the host is
asking for data, and the device is saying it doesn't have any for up to
100us at a time.
What am I missing?
Alan.
[0] http://www.signal11.us/~alan/recv.c
[1] The number of NAKs not important and doesn't correlate to the amount
of time spent NAK-ing.
[2] Analyzer screenshots:
http://www.signal11.us/~alan/nak.png
http://www.signal11.us/~alan/nak_open.png
[3] Often it looks like it's aligned to a SOF packet, but that's only
because when you delay 100us, the odds are good you will be NAK-ing when
a SOF packet arrives (every 125us). Sometimes the NAK-ing will start
before the SOF and sometimes after.
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