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

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Date:   Wed, 4 Jan 2017 10:05:42 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     valmiki <valmikibow@...il.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

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.