[libhid-discuss] Performance of libhid

[Peter Stuge]

shivaprasad javali wrote:
> unplug the USB devices which have to be reflected in my application
> immediately( there is a visual indicator as to how many USB devices are
> plugged in). So I open new handles to the USB devices once every
> second.

I strongly recommend that you make your application event based
instead. (Never poll, get plug/unplug events from the system and
update your view accordingly.)


> My target platform is the OLPC

I also work on coreboot, too bad for us you guys recruited Richard. :)


> In my application a Motion sensor can be connected to the computer
> through the USB device and when it is connected I read from the
> sensor every 5ms so that I can detect any sort of movement across
> the motion sensor. The OLPC in itself is very slow and that coupled
> with my requirement on the USB driver has led the motion sensor to
> be very sluggish in its response.

The OLPC is a Geode LX running at 400-500 MHz and the USB is 2.0.
I don't consider that "very slow." There are admittedly many layers
of software running on the CPU, but the system does not have to be
sluggish.


How USB performs is extremely dependant on the device. It is very
easy for inexperienced USB device developers to create devices with
very bad performance. Knowing when and how to use standardized device
classes and the four endpoint types will make or break a device.

HID devices are very easy to access in Windows. Many device vendors
choose to abuse the HID specification for this reason, in spite of
their devices having nothing to do with a Human Interface.

But, as Charles pointed out, HID devices are never high performance.
They use control or interrupt transfers, which have particular
benefits and drawbacks (just like bulk and isochronous do).


As you are targeting a particular system and need neither portability
nor full featured HID report processing you should write your driver
using either one of the native Linux APIs for USB (usbfs) and HID
(hidraw) or possibly libusb-1.0.

If you go for usbfs, make sure to keep multiple transfers pending at
all times.

You must write more low level code to get better performance.

Your device will be the bottleneck. The device descriptors and the
device firmware is critical. Most device vendors are not very skilled
with USB. Geode LX+CS5536 can drive a lot of USB data effortlessly.


//Peter