linux-kernel - RE: [PATCH v3 1/2] gpio: mlxbf2: Introduce IRQ support

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Date:   Tue, 28 Sep 2021 15:02:02 +0000
From:   Asmaa Mnebhi <asmaa@...dia.com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "David S. Miller" <davem@...emloft.net>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        David Thompson <davthompson@...dia.com>
Subject: RE: [PATCH v3 1/2] gpio: mlxbf2: Introduce IRQ support

> So the PHY is level based. The PHY is combing multiple interrupt sources 
> into one external interrupt. If any of those internal interrupt sources are
> active, the external interrupt is active. If there are > multiple active sources
> at once, the interrupt stays low, until they are all cleared. This means
> there is not an edge per interrupt. There is one edge when the first internal
> source occurs, and no more edges, > even if there are more internal interrupts.

> The general flow in the PHY interrupt handler is to read the interrupt status
> register, which tells you which internal interrupts have fired. You then
> address these internal interrupts one by one.

In KSZ9031, Register MII_KSZPHY_INTCS=0x1B reports all interrupt events and
clear on read. So if there are 4 different interrupts, once it is read once, all 4 clear at once.
The micrel.c driver has defined ack_interrupt to read the above reg and is called every time the
interrupt handler phy_interrupt is called. So in this case, we should be good.
The code flow in our case would look like this:
- 2 interrupt sources (for example, link down followed by link up) set in MII_KSZPHY_INTCS
- interrupt handler (phy_interrupt) reads MII_KSZPHY_INT which automatically clears both
interrupts
- another internal source triggers and sets the register.
- The second edge will be caught accordingly by the GPIO.

> This can take some time, MDIO is a slow bus etc. While handling these interrupt sources,
> it could be another internal interrupt source triggers. This new internal interrupt source
> keeps the external interrupt active. But there has not been an edge, since the interrupt 
> handler is still clearing the sources which caused the first interrupt. With level interrupts,
> this is not an issue. When the interrupt handler exits, the interrupt is re-enabled. Since it
> is still active, due to the unhandled internal interrupt sources, the level interrupt
> immediately fires again. the handler then sees this new interrupt and handles it.
> At that point the level interrupt goes inactive.