Re: [PATCH] ARM: dts: ls1021a-tsn: Use interrupts for the SGMII PHYs

[Vladimir Oltean]

On Sat, 9 Nov 2019 at 23:37, Vladimir Oltean <olteanv@xxxxxxxxx> wrote:
>
> On Sat, 9 Nov 2019 at 23:05, Andrew Lunn <andrew@xxxxxxx> wrote:
> >
> > On Sat, Nov 09, 2019 at 08:52:54PM +0100, Alexander Stein wrote:
> > >  On Saturday, November 9, 2019, 4:21:51 PM CET Vladimir Oltean wrote:
> > > > On 09/11/2019, Andrew Lunn <andrew@xxxxxxx> wrote:
> > > > > On Sat, Nov 09, 2019 at 12:56:42PM +0200, Vladimir Oltean wrote:
> > > > >> On the LS1021A-TSN board, the 2 Atheros AR8031 PHYs for eth0 and eth1
> > > > >> have interrupt lines connected to the shared IRQ2_B LS1021A pin.
> > > > >>
> > > > >> The interrupts are active low, but the GICv2 controller does not support
> > > > >> active-low and falling-edge interrupts, so the only mode it can be
> > > > >> configured in is rising-edge.
> > > > >
> > > > > Hi Vladimir
> > > > >
> > > > > So how does this work? The rising edge would occur after the interrupt
> > > > > handler has completed? What triggers the interrupt handler?
> > > > >
> > > > >   Andrew
> > > > >
> > > >
> > > > Hi Andrew,
> > > >
> > > > I hope I am not terribly confused about this. I thought I am telling
> > > > the interrupt controller to raise an IRQ as a result of the
> > > > low-to-high transition of the electrical signal. Experimentation sure
> > > > seems to agree with me. So the IRQ is generated immediately _after_
> > > > the PHY has left the line in open drain and it got pulled up to Vdd.
> > >
> >
> > > It is correct GIC only supports raising edge and active-high. The
> > > IRQ[0:5] on ls1021a are a bit special though.  They not directly
> > > connected to GIC, but there is an optional inverter, enabled by
> > > default.
> >
> > Ah, O.K. So configuring for a rising edge is actually giving a falling
> > edge. Which is why it works.
> >
> > Actually supporting this correctly is going a cause some pain. I
> > wonder how many DT files currently say rising/active high, when in
> > fact falling/active low is actually being used? And when the IRQ
> > controller really does support active low and falling, things brake?
> >
> > Vladimir, since this is a shared interrupt, you really should use
> > active low here. Maybe the first step is to get control of the
> > inverter, and define a DT binding which is not going to break
> > backwards compatibility. And then wire up this interrupt.
> >
> >           Andrew
>
> Oh, ok, this is what you mean, thanks Alexander for the clarification.
> This sure escalated quickly and is going to keep me busy for a while.
>
> -Vladimir

Sorry, I'm still a bit in shock, since this hit me in the face from
nowhere, so I hadn't followed the entire history when I sent the above
email.
It looks after all that Kurt and Rasmus have picked this up again and
that the latest patch set is from 2 days ago, I'll take a look at
that...
https://lwn.net/Articles/804103/

Thanks,
-Vladimir