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

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Date:	Wed, 28 Oct 2015 12:03:41 -0500
From:	atull <atull@...nsource.altera.com>
To:	Moritz Fischer <moritz.fischer@...us.com>
CC:	Josh Cartwright <joshc@...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, 28 Oct 2015, Moritz Fischer wrote:

> On Wed, Oct 28, 2015 at 9:18 AM, Josh Cartwright <joshc@...com> wrote:
> > 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.
> 
> Alright, I'll investigate that. Again, for the non-partial reconfig
> case I'm happy
> with the behavior as implemented, for the partial reconfig I just
> haven't run into
> issues with not dealing with the level shifters.

Are you suggesting pinctrl instead of introducing a FPGA Bridge Framework?
If it fits, that's great.  Steffen is urging us to include reconfiguring
width of the bridge so I'm trying to figure how and if that all fits in
here.

> 
> > Implementing the FPGA Bridge interface in the Zynq driver because it's
> > what the core expects is just backwards.  It's an abstraction inversion.
> 
> Yeah, you're probably right.

Sorry I wasn't clear earlier, but I am going to change simple FPGA bus
to be OK with the "fpga-bridges" property being left out.  The change is
to do that should just be for simple_fpga_bus_get_bridges() to return 0
if it doesn't find any bridges.  The FPGA can still get programmed and
the child devices get populated.  All the other FPGA bridge-related
function calls (simple_fpga_bus_bridge_enable/disable and
simple_fpga_bus_put_bridges) won't have any effect when
simple_fpga_bus_get_bridges leaves num_bridges==0 and bridges==NULL.

So if you don't need bridges, you can leave that out (once I've fixed
that).

Actually if you want to use simple fpga bus with FPGA bridges compiled out,
I could even have the fpga bridge APIs become empty functions.

Alan

> 
> Thanks,
> 
> Moritz
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