netdev - Re: [PATCH v3 07/41] net: ieee802154: mcr20a: Fix lifs/sifs periods

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Message-ID: <20220118192013.46c42f82@xps13>
Date:   Tue, 18 Jan 2022 19:20:13 +0100
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Alexander Aring <alex.aring@...il.com>
Cc:     Stefan Schmidt <stefan@...enfreihafen.org>,
        linux-wpan - ML <linux-wpan@...r.kernel.org>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
        "linux-wireless@...r.kernel.org Wireless" 
        <linux-wireless@...r.kernel.org>,
        David Girault <david.girault@...vo.com>,
        Romuald Despres <romuald.despres@...vo.com>,
        Frederic Blain <frederic.blain@...vo.com>,
        Nicolas Schodet <nico@...fr.eu.org>,
        Michael Hennerich <michael.hennerich@...log.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Varka Bhadram <varkabhadram@...il.com>,
        Xue Liu <liuxuenetmail@...il.com>, Alan Ott <alan@...nal11.us>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v3 07/41] net: ieee802154: mcr20a: Fix lifs/sifs periods

Hi Alexander,

alex.aring@...il.com wrote on Mon, 17 Jan 2022 17:52:10 -0500:

> Hi,
> 
> On Mon, 17 Jan 2022 at 06:54, Miquel Raynal <miquel.raynal@...tlin.com> wrote:
> >
> > These periods are expressed in time units (microseconds) while 40 and 12
> > are the number of symbol durations these periods will last. We need to
> > multiply them both with phy->symbol_duration in order to get these
> > values in microseconds.
> >
> > Fixes: 8c6ad9cc5157 ("ieee802154: Add NXP MCR20A IEEE 802.15.4 transceiver driver")
> > Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>
> > ---
> >  drivers/net/ieee802154/mcr20a.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/ieee802154/mcr20a.c b/drivers/net/ieee802154/mcr20a.c
> > index f0eb2d3b1c4e..e2c249aef430 100644
> > --- a/drivers/net/ieee802154/mcr20a.c
> > +++ b/drivers/net/ieee802154/mcr20a.c
> > @@ -976,8 +976,8 @@ static void mcr20a_hw_setup(struct mcr20a_local *lp)
> >         dev_dbg(printdev(lp), "%s\n", __func__);
> >
> >         phy->symbol_duration = 16;
> > -       phy->lifs_period = 40;
> > -       phy->sifs_period = 12;
> > +       phy->lifs_period = 40 * phy->symbol_duration;
> > +       phy->sifs_period = 12 * phy->symbol_duration;  
> 
> I thought we do that now in register_hw(). Why does this patch exist?

The lifs and sifs period are wrong.

Fixing this silently by generalizing the calculation is simply wrong. I
feel we need to do this in order:
1- Fix the period because it is wrong.
2- Now that the period is set to a valid value and the core is able to
   do the same operation and set the variables to an identical content,
   we can drop these lines from the driver. 

#2 being a mechanical change, doing it without #1 means that something
that appears harmless actually changes the behavior of the driver. We
generally try to avoid that, no?

Thanks,
Miquèl