linux-kernel - Re: Need clarity on PCIe MSI interrupt in device tree

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Message-ID: <1365da01-e990-5ad1-3b26-978d86bca5f8@gmail.com>
Date:   Wed, 4 Jan 2017 22:59:48 +0530
From:   valmiki <valmikibow@...il.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        helgaas@...nel.org, marc.zyngier@....com, arnd@...db.de
Subject: Re: Need clarity on PCIe MSI interrupt in device tree

Thanks Mark

On 1/4/2017 3:35 PM, Mark Rutland wrote:
> On Wed, Jan 04, 2017 at 08:47:43AM +0530, valmiki wrote:
>> Hi,
>
> Hi,
>
>> I have confusion on MSI interrupt flags in PCIe documetation.
>>
>> MSI interrupts are edge triggered, but i see some controllers use
>> Ex:tegra <0 99 0x4>, here interrupt flags show 0x4 which means level
>> sensitive as per include/dt-bindings/interrupt-controller/irq.h.
>
> As Marc says, this isn't actually a description of an MSI, but rather
> the interrupt generated by the interrupt controller, which is often (but
> not always) built into PCIe host controllers.
>
> This looks something like:
>
> +--------+        +-----------------+            +----------+
> |        |        +----------+      |~~msi~irq~~>|          |
> | Device |==MSI==>| MSI ctrl | PCIe |            | IRQ ctrl |~~~> CPU
> |        |        +----------+      |~~intx~irq~>|          |
> +--------+        +-----------------+            +----------+
>
> Each PCIe device sends MSIs to the MSI controller. In response to this,
> the MSI controller raises a wired interrupt, which is named "msi" in the
> DT binding because it is the interrupt generated by the MSI controller,
> and the PCIe controller may raise several distinct interrupts which all
> need names.
>
>> May i know why is it like this, why MSI depicted as level sensitive
>> in device tree.
>
> As above, the interrupt from the MSI controller to the interrupt
> controller is level sensitive, but this is not the case for the actual
> MSIs.
>
> Thanks,
> Mark.
>