Re: [PATCH 2/3] mac80211: reset TSF to 0 when joining a mesh

[Thomas Pedersen <twpedersen@gmail.com>]

On Mon, Dec 16, 2013 at 1:55 PM, Sergey Ryazanov <ryazanov.s.a@gmail.com> wrote:
> 2013/12/16 Thomas Pedersen <twpedersen@gmail.com>:
>> On Mon, Dec 16, 2013 at 4:33 AM, Sergey Ryazanov <ryazanov.s.a@gmail.com> wrote:
>>> Hello Thomas,
>>>
>>> 2013/12/16 Thomas Pedersen <twpedersen@gmail.com>:
>>>> diff --git a/net/mac80211/mesh.c b/net/mac80211/mesh.c
>>>> index 330d1f7..1174157 100644
>>>> --- a/net/mac80211/mesh.c
>>>> +++ b/net/mac80211/mesh.c
>>>> @@ -802,6 +802,8 @@ int ieee80211_start_mesh(struct ieee80211_sub_if_data *sdata)
>>>>                 return -ENOMEM;
>>>>         }
>>>>
>>>> +       /* next beacon will be DTIM-1, so TSF=0 was DTIM=0 */
>>>> +       drv_set_tsf(local, sdata, 0);
>>>>         ieee80211_bss_info_change_notify(sdata, changed);
>>>>
>>>>         netif_carrier_on(sdata->dev);
>>>
>>> What happen with AP interface on the same radio if we configure mesh
>>> portal? Clients could be confused by such TSF jump.
>>
>> Like Johannes said; either the driver supports per-vif TSF, or well,
>> it doesn't. Anyway wouldn't clients reset their own TSF when the new
>> beacon is received?
>>
> Yeah, IEEE 802.11 says that stations must blindly use AP time, but it
> says nothing about the situation when time is accidentally reset to
> zero. I can't predict reaction of each chip/firmware/driver to such
> situation.
>
> Another one unclear situation is the reaction of peers of mesh STA,
> which share the same radio (and same TSF). If we silently reset its
> time, what happens to the time synchronization with neighbors?

Peer mesh STAs will notice an overly large TSF difference since last
measurement, and reset Toffset to this new value.

> Why could not we calculate DTIM counter value on basis of known TSF
> and Beacon/DTIM interval, instead of primitive time reset?

Yeah this sounds like  a nicer solution. So drv_get_tsf() on mesh
join, then calculate the right dtim_count? I'll give this a try.

Thomas