[Linuxptp-devel] Issues with mutex locking and ptp_find_pin

[Andrew S. <and...@gm...>]

Hi there

I just modified the Linux TI CPTS driver to support external hardware time
stamping. In my driver's .enable function callback on the *ptp_clock_info*
struct I made use of the *ptp_find_pin* to map a (function,channel) to a
(pin). Using the testptp.c app I am able to successfully switch to func=1
(EXTTS) mode and time stamping works. However, when I try and switch back
to func=0 (NONE) the testptp application hangs, and I get a kernel panic
(slow mutex).

To repeat:

root@arm:~# testptp -l
name HW1_TS_PUSH index 0 func 0 chan 0
name HW2_TS_PUSH index 1 func 0 chan 1
name HW3_TS_PUSH index 2 func 0 chan 2
name HW4_TS_PUSH index 3 func 0 chan 3
root@arm:~# testptp -i 3 -L 3,1
set pin function okay
root@arm:~# testptp -i 3 -e 1
external time stamp request okay
event index 3 at 946685182.188502679
root@arm:~#
root@arm:~# testptp -i 3 -L 3,0
...
(testptp application hangs)

I believe that the underlying reason for this is that *ptp_find_pin* function
attempts to perform a mutex lock on &ptp->pincfg_mux, which is already
locked by the calling function (see the mutex lock
<https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/drivers/ptp/ptp_chardev.c?id=refs/tags/v4.4#n244>
in the *ptp_ioctl* function in /drivers/ptp/ptp_chardev.c).

Perhaps it isn't a good practice to call *ptp_find_pin *from within the
enable function callback in driver code, as done in
/drivers/net/ethernet/intel/igb/igb_ptp.c.

Andrew Symington