Hello,
I have implemented a USB device function using Linux functionfs and now there
is a problem being reported.
I need to ask this group for advice.
The problem is this:
1) device boots
2) some usb transfers happen, all are OK
3) a device app runs to completion (USB quiescent during this time, no USB
transfers required)
4) the controlling PC starts a 4 KByte USB transfer to the device, but this
transfer does not finish. Only 3 Kbytes are ACK'd by the device.
(A USB analyzer shows the host trying to send more, but the device
persistently NAK's)
If step (3) is omitted, everything works fine. It is reliable - 15/15 times it
is OK.
The USB device function is implemented with functionfs and aio. Most of the
implementation is in user space.
An off-the-shelf low level Linux driver is being used.
Regression tests show no problems with various sized USB transfers for over 24
hours.
A colleague has investigated and has asserted user space is not the right way
to do things.
He says:
"It appeared that running the <device app> was enough to swap the usb code out
that it wasn't able to swap back in quick enough to respond to the USB traffic
in a timely fashion" .... "This is the major drawback to user space drivers as
opposed to kernel drivers. Kernel drivers pages are locked into memory while
user space can be swapped out. There were numerous articles about this, but
the best one I found was:
http://www.makelinux.net/ldd3/chp-2-sect-9 "
Linux Device Drivers, 3rd Edition, By Jonathan Corbet, Greg Kroah-Hartman,
Alessandro Rubini : February 2005
"There pertinent part is:
o Response time is slower, because a context switch is required to transfer
information or actions between the client and the hardware.
o Worse yet, if the driver has been swapped to disk, response time is
unacceptably long. Using the mlock system call might help, but usually you'll
need to lock many memory pages, because a user-space program depends on a lot
of library code. mlock, too, is limited to privileged users.
Some articles I read stated that the swap could take seconds."
QUESTIONS:
- Did I make a mistake using user space and functionfs?
(I thought state-of-the-art way to do usb function drivers was to use
functionfs...)
- Should I add calls to mlock() to try to fix?
Any advice is appreciated.
AP
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