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i.MX6 PCIE to XCHI USB3.0 Performance Issues

Question asked by ottawa-imx6 on Jul 23, 2013
Latest reply on Apr 10, 2017 by tom gao
Branched to a new discussion

Hello everyone,
I have an issue that I am trying to resolve which involves PCIE, XHCI, and USB 3.0 on the i.MX6Q systems.
The issue I am having is that my data transfer rates are too slow and as such this leads me to believe that PCIE2.0 and USB3.0 XHCI might not be configured optimally using the freescale kernel.


Attempted hardware:
Renesas USB3.0 Controller upd720201


up to and including 3.0.35-4.0.0 with PCIE, XHCI_HCD, and MSI enabled.




/dev/sda = external sata drive connected through USB3.0 and the PCIE controller.
/dev/sdb = sata drive connected directly to the i.MX6
/dev/sdc = second external sata drive


Through seperate tests on intel machines, I have verified that:
All drives are capable of (and proven on the same renesas and ti controllers) of well over 150MB/s write speed.
Both the TI and Rensesas controllers both are also capable of the bandwidth required.


My results on the i.MX6


CASE 1 : dd if=/dev/zero of=/dev/sda bs=1M write-speed ~60-70MB/s


CASE 2 : dd if=dev/sda of=/dev/sdb bs=1M write-speed ~35-40MB/s


CASE 3 : dd if=/dev/sda of=/dev/sdc bs=1M write-speed ~30-35MB/s


As I can tell there is a significant drop in the already limited maximum bandwidth (70MB/s while writing zeros from ram) down to 35MB/s when I try to transfer from one disk to another through the PCIE and usb3.0 interface.
Granted that I am using the interface for two drives simultaneously, it makes sense that it would be slower. I would imagine that if CASE 1 was performing at a higher rate then CASE 3 would be less relatively so.


The main issue seems to be with the baseline slow speed in CASE 1. 70MB/s is nowhere near the functional rates for PCIE2.0 + USB3.0 (I understand that there is overhead, but 70MB/s is still multiple times slower than it should be).


My questions to anyone who has experience with this is:
1. Has anyone seen any better performance than this?
2. Can anyone at freescale verify that this is the maximum performance possible through these interfaces? Are there any plans to update the kernel to improve this?