linux-kernel - Re: [PATCH v12 3/6] fpga: add simple-fpga-bus

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Message-ID: <20151028161859.GJ8245@jcartwri.amer.corp.natinst.com>
Date:	Wed, 28 Oct 2015 11:18:59 -0500
From:	Josh Cartwright <joshc@...com>
To:	Moritz Fischer <moritz.fischer@...us.com>
Cc:	Alan Tull <atull@...nsource.altera.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Michal Simek <monstr@...str.eu>,
	Michal Simek <michal.simek@...inx.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
	Alan Tull <delicious.quinoa@...il.com>,
	"dinguyen@...nsource.altera.com" <dinguyen@...nsource.altera.com>
Subject: Re: [PATCH v12 3/6] fpga: add simple-fpga-bus

On Wed, Oct 28, 2015 at 08:37:51AM -0700, Moritz Fischer wrote:
> On Wed, Oct 28, 2015 at 3:07 AM, Josh Cartwright <joshc@...com> wrote:
> > On Tue, Oct 27, 2015 at 05:09:12PM -0500, atull@...nsource.altera.com wrote:
> >> From: Alan Tull <atull@...nsource.altera.com>
> >>
> >> The Simple FPGA bus uses the FPGA Manager Framework and the
> >> FPGA Bridge Framework to provide a manufactorer-agnostic
> >> interface for reprogramming FPGAs that is Device Tree
> >> Overlays-based.
> >
> > Do you intend the "simple-fpga-bus" to be used on Zynq as well?  The
> > whole concept of the socfpga's "FPGA Bridge" doesn't map to the Zynq at
> > all, from what I can tell.
> 
> For Zynq the zynq-fpga driver takes care of the level shifters on full
> reconfiguration,
> and doesn't for partial reconfiguration. Now depending on which parts
> of the fabric
> are partial reconfigured (say AXI masters), one might run into issues
> with a setup like that.
> 
> My first plan was to counter that by using zynq-reset to hold the
> reset high during
> reconfiguration of that part of the FPGA.
> 
> I'm happy to rethink that part and maybe redo the level shifters and
> resets together in a bridge
> driver under devicetree control gives finer grained control.

There is already a framework which is used to describe and manipulate
level shifting/other IO properties, and that is pinctrl, and if we
wanted to use an appropriate abstraction, I think pinctrl would be the
best bet.

Implementing the FPGA Bridge interface in the Zynq driver because it's
what the core expects is just backwards.  It's an abstraction inversion.

  Josh
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