Re: [PATCH v2 0/7] mt76x02: Beacon support for USB

[Kalle Valo <kvalo@codeaurora.org>]

Stanislaw Gruszka <sgruszka@redhat.com> writes:

> On Tue, Jan 29, 2019 at 01:10:08PM +0100, Felix Fietkau wrote:
>> On 2019-01-29 13:07, Kalle Valo wrote:
>> > Felix Fietkau <nbd@nbd.name> writes:
>> > 
>> >> On 2019-01-29 12:47, Kalle Valo wrote:
>> >>> Stanislaw Gruszka <sgruszka@redhat.com> writes:
>> >>> 
>> >>>> We can configure beaconing, but without TBTT interrupt we
>> >>>> can not support PS buffering. This can be added later using
>> >>>> kernel hrtimer, if we can keep it in sycn with device timer.
>> >>>>
>> >>>> I tested AP and IBSS modes.
>> >>> 
>> >>> So how does this work reliably so that there's no packet loss with
>> >>> clients using power save?
>> >>
>> >> There will be multicast packet loss for clients using power save.
>> > 
>> > Isn't that a problem? At least as a normal user I would very frustrated
>> > if sometimes my connection work and sometimes not, for example if I'm
>> > trying discover devices from my network. Hopefully nobody won't use USB
>> > devices for any real AP stuff, but still enabling something which we
>> > know doesn't work realiably is concerning.
>> I agree. Maybe we should leave out the flag for AP mode in this patch
>> until we have PS buffering and leave the rest of the code intact.
>
> But how serious problem of dropping multicast frames for PS stations
> is?

It's _both_ broadcast and multicast frames. Felix already mentioned ARP
but there are also other protocols which rely on broadcast/multicast
frames.

> I don't think from user perspective this is "sometimes my connection
> work and sometimes not", but something much less annoying.

So basically you are saying that we should just depend on lock and hope
that users don't notice the packet loss. I don't think that's really
good design philosophy.

> Moreover in the tree we have already bunch of drivers that do 
> advertise AP mode support without HOST_BROADCAST_PS_BUFFERING 
> like iwlwifi or brcm80211 .

After seeing how much those drivers are tested I would be very surprised
to see that their broadcast and multicast packet handling is broken.

> So I don't think we should drop AP flag, AP mode works with
> this patch set quite well.

"Works" is a very broad statement and depends on what you have tested.
Sure, if you tested tested with ping and iperf from a client to AP
without using power save mode that will work. But if you try to connect
from another device on the network to another client using aggressive
power saving it's a totally different situation.

Back in the Nokia N800 days I wasted a lot of time debugging all sort of
buggy APs which didn't work correctly with power save, it was really
annoying and very frustrating for the users. Let's not do the same
mistakes, we are better than that.

-- 
Kalle Valo