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

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Date:   Sat, 9 Nov 2019 18:21:41 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "leoyang.li@....com" <leoyang.li@....com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] ARM: dts: ls1021a-tsn: Use interrupts for the SGMII PHYs

On Sat, Nov 09, 2019 at 05:16:48PM +0200, Vladimir Oltean wrote:
> On Saturday, 9 November 2019, Andrew Lunn <andrew@...n.ch> 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.

Hi Vladimir

                       t1                    t2

     ------------------\                     /----------------
                        \-------------------/

The interrupt output is active low. So it is high by default. At time
t1 something happens, say the link is established. The interrupt
becomes active, we have a failing edge. We want the interrupt
controller to fire. Lets say it does. The interrupt handler runs, and
clears the interrupt cause. This is at time t2. We then get a rising
edge and the PHY releases the interrupt, and the level returns to
high.

So how does this work if you have the interrupt controller triggering
on a rising edge? The edge won't rise until the interrupt handler
finishes its work.

	 Andrew