Re: rtlwifi/rtl8192cu AP mode broken with PS STA

[Maciej S. Szmigiero]

On 06.04.2021 12:00, Kalle Valo wrote:
> "Maciej S. Szmigiero" <mail@xxxxxxxxxxxxxxxxxxxxx> writes:
>
> > On 29.03.2021 00:54, Maciej S. Szmigiero wrote:
> > > Hi,
> > >
> > > It looks like rtlwifi/rtl8192cu AP mode is broken when a STA is using PS,
> > > since the driver does not update its beacon to account for TIM changes,
> > > so a station that is sleeping will never learn that it has packets
> > > buffered at the AP.
> > >
> > > Looking at the code, the rtl8192cu driver implements neither the set_tim()
> > > callback, nor does it explicitly update beacon data periodically, so it
> > > has no way to learn that it had changed.
> > >
> > > This results in the AP mode being virtually unusable with STAs that do
> > > PS and don't allow for it to be disabled (IoT devices, mobile phones,
> > > etc.).
> > >
> > > I think the easiest fix here would be to implement set_tim() for example
> > > the way rt2x00 driver does: queue a work or schedule a tasklet to update
> > > the beacon data on the device.
> >
> > Are there any plans to fix this?
> > The driver is listed as maintained by Ping-Ke.
>
> Yeah, power save is hard and I'm not surprised that there are drivers
> with broken power save mode support. If there's no fix available we
> should stop supporting AP mode in the driver.

https://wireless.wiki.kernel.org/en/developers/documentation/mac80211/api
clearly documents that "For AP mode, it must (...) react to the set_tim()
callback or fetch each beacon from mac80211".

The driver isn't doing either so no wonder the beacon it is sending
isn't getting updated.

As I have said above, it seems to me that all that needs to be done here
is to queue a work in a set_tim() callback, then call
send_beacon_frame() from rtlwifi/core.c from this work.

But I don't know the exact device semantics, maybe it needs some other
notification that the beacon has changed, too, or even tries to
manage the TIM bitmap by itself.

It would be a shame to lose the AP mode for such minor thing, though.

I would play with this myself, but unfortunately I don't have time
to work on this right now.

That's where my question to Realtek comes: are there plans to actually
fix this?

Maciej