linux-kernel - Re: Design of interrupt controller driver

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Date:   Mon, 5 Jun 2017 01:40:49 +0200
From:   Mason <slash.tmp@...e.fr>
To:     Thomas Gleixner <tglx@...utronix.de>
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 04/06/2017 22:13, Thomas Gleixner wrote:
> On Sun, 4 Jun 2017, Mason wrote:
>> On 04/06/2017 15:55, Thomas Gleixner wrote:
>>>> 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 DMA engine is idle.
>>
>> I don't follow.
> 
> Sorry, hit send too early. Let me rephrase:
> 
> When you configure the interrupt as edge then you cannot share it. No
> matter whether it stays high or not.

Could you explain why? (I must be missing something.)

If the signal is low when the controller is busy,
and high when the controller is idle, then the
transition from busy to idle is a rising edge,
which will trigger an interrupt. The intc driver
will look up the status of all devices, see that
the DMA engine interrupt request line is high,
and thus call the DMA engine's ISR.

I have that setup right now, sharing TX and RX
interrupts, and the device using the DMA engine
(a NAND Flash controller) works well.

Is there a problem if several shared edge interrupts
fire at the same time (or very close to one another).
I don't see the problem, as long as the line stays
high until software does something with the device.

> The only way to share it is, to configure it as level interrupt. But that
> requires that you can disable the interrupt at the DMA device level once it
> triggered. Otherwise you get an interrupt storm.

I'm not sure what you mean with "disable the interrupt at the
DMA device level". The interrupt can be masked at the system
interrupt controller (i.e. before sharing the interrupt
signal). The DMA engine just outputs 0 when busy, 1 when idle.

Regards.