[asterisk-dev] DAHDI_CHECK_HOOKSTATE and setting rxisoffhook

[Tzafrir Cohen]

The following has been a subject of a bug report or two, but sadly I
still fail to understand the rationale of them.

The following is code from available() in chan_dahdi.c . This function
returns 1 if a channel can be used for an outgoing call or 0 otherwise.

Code from 1.4.21 (chan_zap.c):

                        if (!p->sig || (p->sig == SIG_FXSLS))
                                return 1;
                        /* Check hook state */
                        if (p->subs[SUB_REAL].zfd > -1)
                                res = ioctl(p->subs[SUB_REAL].zfd, ZT_GET_PARAMS
                        else {
                                /* Assume not off hook on CVRS */
                                res = 0;
                                par.rxisoffhook = 0;
                        }
                        if (res) {
                                ast_log(LOG_WARNING, "Unable to check hook state
                        } else if ((p->sig == SIG_FXSKS) || (p->sig == SIG_FXSGS
                                /* When "onhook" that means no battery on the li
                                  it is out of service..., if it's on a TDM card
                                  bank, there is no telling... */
                                if (par.rxbits > -1)
                                        return 1;
                                if (par.rxisoffhook)
                                        return 1;
                                else
#ifdef ZAP_CHECK_HOOKSTATE
                                        return 0;
#else
                                        return 1;
#endif
                        } else if (par.rxisoffhook) {
                                ast_log(LOG_DEBUG, "Channel %d off hook, can't u
                                /* Not available when the other end is off hook
                                return 0;
                        }
                }
                return 1;

As you can see, '0' may only be returned here if ZAP_CHECK_HOOKSTATE is
defined or if the signalling is not fxs_ls, fxs_ks or fxs_gs (that is: 
not an FXO with LS, KS or GS)

Code in recent 1.4 is very similar.


The code in 1.6.0, however, is slightly different:

                        if (!p->sig || (p->sig == SIG_FXSLS))
                                return 1;
                        /* Check hook state */
                        if (p->subs[SUB_REAL].dfd > -1) {
                                memset(&par, 0, sizeof(par));
                                res = ioctl(p->subs[SUB_REAL].dfd, DAHDI_GET_PAR
                        } else {
                                /* Assume not off hook on CVRS */
                                res = 0;
                                par.rxisoffhook = 0;
                        }
                        if (res) {
                                ast_log(LOG_WARNING, "Unable to check hook state
                        } else if ((p->sig == SIG_FXSKS) || (p->sig == SIG_FXSGS
                                /* When "onhook" that means no battery on the li
                                  it is out of service..., if it's on a TDM card
                                  bank, there is no telling... */
                                if (par.rxbits > -1)
                                        return 1;
                                if (par.rxisoffhook)
                                        return 1;
                                else
                                        return 0;
                        } else if (par.rxisoffhook) {
                                ast_debug(1, "Channel %d off hook, can't use\n",
                                /* Not available when the other end is off hook
#ifdef DAHDI_CHECK_HOOKSTATE
                                return 0;
#else
                                return 1;
#endif
                        }
                }
                return 1;

As you can see, this code may also return 0 (fail) if signalling was 
fxs_ks or fxs_gs, if the rxisoffhook flag is not set. This flag gets
set by asking the driver.

So far I have failed to get any explanation as to why this test is
needed. It is said to be some issue with channel bank, but when I asked
how it can be reproduced, I failed to get more information.

The impact of this issue is that FXO interfaces fail to work with
Asterisk >= 1.6.0 unless set to use LS signalling or unless there is an
incoming call before.

There is a related issue that rxisoffhook is not properly reported for
various drivers. It gets reset to an "uninitialized" value after
dahdi_cfg but the low-level driver has no decent way of setting it, as
it is called right after the optional span startup hook.

Sean Ruffel fixed the Digium analog drivers to properly set it. However
as he could not use any hook, he had to check for an unintialized state
at the code run at interrupt time. I don't think this is such a good
idea to add extra code at this place, which is the most timing critical
part of the driver. I would prefer to just avoid testing it in Asterisk
if I don't have to.

-- 
               Tzafrir Cohen
icq#16849755              jabber:tzafrir.cohen at xorcom.com
+972-50-7952406           mailto:tzafrir.cohen at xorcom.com
http://www.xorcom.com  iax:guest at local.xorcom.com/tzafrir