On 3/9/22 00:59, Farhan Khan wrote:
Hi all,
I am trying to understand how FreeBSD's USB subsystem works and appear to be
running into a kernel panic due to a failed assertion that I do not understand.
the assertion in question is /usr/src/sys/dev/usb/usb_transfer.c:1954.
My understanding is that you first configure usbd_transfer_setup(9). To start a
bulk transfer, you populate the appropriate list (typically an STAIL_* list),
then initiate a transfer by calling usbd_transfer_start(9) with the correct
transfer. In my case, my callback is a simple printf().
Upon the usbd_transfer_start(9) call, I am hitting the assertion error. I have
looked at other device drivers and have not identified what they are doing
differently.
Hi,
When you call usbd_transfer_setup() you need to specify a mutex which
protects the internal data structures of USB transfers. This mutex must
be locked when you call:
usbd_transfer_start(9)
usbd_transfer_stop(9)
And this mutex is also automagically locked in your USB callback
function. This is to allow atomic stop of multiple USB transfers at the
same time.
It is similar to how the mutex enabled callout's work.
--HPS
