Subject: Re: Ethernet over PCIe driver for Inter-Processor Communication

From: David Hawkins <[email-masked]>
Date: Thu, 22 Aug 2013 14:43:38 -0700

Hi S.Saravanan,

> I have a custom board  with four MPC8640 nodes connected over
> a transparent PCI express switch . In this configuration one node is
> configured as host(Root Complex) and others as agents(End Point). Thus
> the legacy PCI software works fine . However the mainline kernel lacks
> any standard support for Inter-processor communication over PCI. I am
> in the process of developing an Ethernet over  PCI driver for the same
> on the lines of rionet . However I am facing the following problems.
>
> a)   I can generate MSI interrupts from End Point to Root Complex over
> PCI . But the vice-versa is not possible . However i need a method to
> interrupt the End Point from the Root Complex to complete my driver.

Root complex's would normally interrupt a device via a PCIe write
to a register in a BAR on the end-point (or in extended configuration
space registers depending on the hardware implementation).

> Only previous references I can find are this post
> http://www.mail-archive.com/[email-masked]/msg25765.html
> However this uses doorbells and I think may not be possible in MPC8640.

PCIe drivers need some way to interrupt the processor, so there must
be an option somewhere ... for example, what are the message register
interrupts intended for? See p479

http://cache.freescale.com/files/32bit/doc/ref_manual/MPC8641DRM.pdf

(Ira and myself have not used the MPC8640 so are not familiar with
its user manual).

> Any pointers on this issue and guidance on this driver development would
> be helpful .

We use the Ethernet-over-PCI driver that Ira developed. Our next boards
will use an MPC8308, but we don't currently have any in a PCIe device
form-factor (just the MPC8038RDB), so he has not ported it to PCIe.

Feel free to discuss your ideas for your PCIe driver (eg., why start
with rionet rather than Ira's driver), either on-list, or email Ira
and myself directly.

Cheers,
Dave