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am335x - which kernel has a working USB host driver with DMA support?

Andrzej Szombierski
Intellectual 260 points

I'm trying to use an rtl8188eu-based wifi stick with a BeagleBone Black board. Is there any linux kernel with solid USB host support (including DMA)?

My observations:

The 3.2.0 kernel (from am335x-evm-sdk-src-06) is the best I have found, it doesn't trigger any errors with DMA enabled, and supports about 1.2MB/s throughput with about 10% CPU load. This is really poor performance, also using an ancient kernel is not an option for me.

All other versions I've checked (beaglebone 3.8, ti-linux-kernel 3.12, Linus' 3.13) work properly only with PIO mode. In this mode I can transfer more than 5MB/s, but at the price of 90% CPU load (=unusable).

Attempting to use DMA results in an unstable connection, some kernels also spew dma-related error mesages in dmesg.

For reference, I've checked the same wifi stick on a rockchip-based board (converted tablet). With 7% CPU usage it supports about 3.6MB/s throughput.

  • 0
    TI__Guru**** 168291 points
    Andrzej,

    Have you tried TI 3.14 kenel?

    git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git
    tag ti2014.10.01.
  • 0 in reply to Bin Liu
    Intellectual 260 points
    Thank you for the pointer. I'm testing this kernel right now. While DMA mode seems to be working, the performance is less than ideal.

    With the same WiFi setup, I get about 3.3MB/s with 50% (!) CPU usage.

    In another test, reading data from a USB drive (dd if=/dev/sda of=/dev/null bs=1M) fully saturates the CPU while maintaining about 14MB/s transfer rate. It is a rather small improvement from the PIO mode (9MB/s with saturated CPU).

    Is the hardware capable of fast USB transfers with low CPU usage at all?
  • 0 in reply to Andrzej Szombierski
    TI__Guru**** 168291 points

    Andrzej,

    If you used omap2plus_defconfig, please then manually disable CONFIG_SMP. It is known SMP enabled in 3.12+ kernel on single ARM core affects USB, Ethernet (maybe other peripherals as while) perofrmance.

    With USB drives, you should get at least 17+ MB/s for read, 23+ MB/s for write, with 20~25% CPU load, if the drive is not the bottle neck.

  • 0 in reply to Bin Liu
    Intellectual 260 points
    I didn't have CONFIG_SMP enabled. But I've noticed that I've measured the CPU utilization incorrectly.

    When doing the transfer, 'top' shows about 20-30% cpu idle. On the other hand, when I run some cpu-heavy operation in the background, it can get about 60-70% cpu time, and the network transfer is not affected, indicating that these 60-70% of cpu time was in fact available.

    Thank you for your support.