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Date:   Sun, 4 Jun 2017 15:55:46 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mason <slash.tmp@...e.fr>
cc:     Marc Zyngier <marc.zyngier@....com>,
        Jason Cooper <jason@...edaemon.net>,
        Mark Rutland <mark.rutland@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Design of interrupt controller driver

On Sat, 3 Jun 2017, Mason wrote:
> 1) The interrupt router has 128 inputs and 24 outputs.
> Therefore, several devices have to share an output line.
> I believe they *must* be of the same interrupt type?
> In the limit, we could use
> - 1 output line for level high
> - 1 output line for level low
> - 1 output line for edge rising
> - 1 output line for edge falling
> Is that correct?

No. You CANNOT share edge type interrupts, simply because it might be
impossible to figure out which device fired the interrupt.

> As for the DMA interrupt, the HW designers consider it
> a level interrupt. When the engine is busy processing
> a command, the interrupt signal is low; when the engine's
> command queue is empty, the interrupt signal is high.
> Thus, there is no risk of "missing a pulse". But maz
> has stated twice that the relevant *event* is the
> transition from "busy" to "idle", which makes this
> an edge (rising) interrupt. However, it doesn't have
> the problem described in 2)  The interrupt signal
> will remain high, so there is no risk of "missing"
> a pulse. So they can be grouped. Is that correct?

That depends on the DMA controller. If you can disable the interrupt line
at the DMA device level, then you can use that scheme. If not, you'll get
an interrupt storm as long as the DAM engine is idle.

Thanks,

	tglx

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