linux-kernel - Re: Portable DT Connectors with regard to FPGAs

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Date:   Tue, 25 Sep 2018 13:47:43 +1000
From:   David Gibson <david@...son.dropbear.id.au>
To:     Alan Tull <atull@...nel.org>
Cc:     Frank Rowand <frowand.list@...il.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Rob Herring <robh+dt@...nel.org>,
        Moritz Fischer <mdf@...nel.org>, linux-fpga@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Stephen Boyd <stephen.boyd@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Grant Likely <grant.likely@...retlab.ca>
Subject: Re: Portable DT Connectors with regard to FPGAs

On Mon, Sep 24, 2018 at 03:32:44PM -0500, Alan Tull wrote:
> My interest here was in having some discussion on whether connectors
> are a good match for handling FPGAs.
> 
> The relevant use model is where a user applies a DT overlay targeting
> an FPGA region after the kernel has booted.  That overlay initiates
> FPGA programming and then adds nodes for the new FPGA hardware. This
> is discussed more completely in the FPGA manager DT binding document
> [1].  The main deal here is that I'd like to be able to add nodes
> in/below a FPGA region node to support devices in the FPGA (and be
> able also to remove them if we are going to reconfigure the FPGA.)
> 
> Previous discussions about DT connectors focused on the types of
> things likely to be on a physical connector. GPIO and SPI got named as
> good examples for discussion while MMIO specifically was dismissed
> [2].  That's problematic for embedded FPGAs for example since the FPGA
> is on a mmio bus and hardware that is programmed into the FPGA lives
> on that mmio bus similar to any embedded peripherals.  So there's a
> question - are mmio busses intended to be left un-connectorizable?

I don't see any particular reason that a connector couldn't be used
for mmio devices.  I think you'd want to treat the connection point as
a bridge on the mmio bus - that can have a 'ranges' property mapping
the connected device into the parent bus's address space (as an
identity mapping or otherwise).

> 
> Alan
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/fpga/fpga-region.txt
> 
> [2] https://lkml.org/lkml/2016/7/20/560
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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