linux-kernel - Re: [net-next 01/18] ieee802154: hwsim: Ensure proper channel selection at probe time

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Date:   Tue, 4 Jan 2022 18:08:46 -0500
From:   Alexander Aring <alex.aring@...il.com>
To:     Miquel Raynal <miquel.raynal@...tlin.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
        Stefan Schmidt <stefan@...enfreihafen.org>,
        linux-wpan - ML <linux-wpan@...r.kernel.org>,
        David Girault <david.girault@...vo.com>,
        Romuald Despres <romuald.despres@...vo.com>,
        Frederic Blain <frederic.blain@...vo.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [net-next 01/18] ieee802154: hwsim: Ensure proper channel
 selection at probe time

Hi,

On Tue, 4 Jan 2022 at 10:44, Miquel Raynal <miquel.raynal@...tlin.com> wrote:
>
> Hi Alexander,
>
> alex.aring@...il.com wrote on Tue, 28 Dec 2021 16:05:43 -0500:
>
> > Hi,
> >
> > On Wed, 22 Dec 2021 at 10:57, Miquel Raynal <miquel.raynal@...tlin.com> wrote:
> > >
> > > A default channel is selected by default (13), let's clarify that this
> > > is page 0 channel 13. Call the right helper to ensure the necessary
> > > configuration for this channel has been applied.
> > >
> > > So far there is very little configuration done in this helper but we
> > > will soon add more information (like the symbol duration which is
> > > missing) and having this helper called at probe time will prevent us to
> > > this type of initialization at two different locations.
> > >
> >
> > I see why this patch is necessary because in later patches the symbol
> > duration is set at ".set_channel()" callback like the at86rf230 driver
> > is doing it.
> > However there is an old TODO [0]. I think we should combine it and
> > implement it in ieee802154_set_channel() of "net/mac802154/cfg.c".
> > Also do the symbol duration setting according to the channel/page when
> > we call ieee802154_register_hw(), so we have it for the default
> > settings.
>
> While I totally agree on the background idea, I don't really see how
> this is possible. Every driver internally knows what it supports but
> AFAIU the core itself has no easy and standard access to it?
>

I am a little bit confused here, because a lot of timing related
things in the phy information rate points to "x times symbols". If
this value depends on the transceiver, how are they compatible then?

> Another question that I have: is the protocol and center frequency
> enough to always derive the symbol rate? I am not sure this is correct,
> but I thought not all symbol rates could be derived, like for example
> certain UWB PHY protocols which can use different PRF on a single
> channel which has an effect on the symbol duration?

Regarding UWB PHY I see that for values like LIFS/SIFS they reference
a "preambleSymbols" value which is defined.

I need to do more research regarding this.

- Alex