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

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Date:	Wed, 28 Oct 2015 12:59:16 -0500
From:	Josh Cartwright <joshc@...com>
To:	atull <atull@...nsource.altera.com>
Cc:	Moritz Fischer <moritz.fischer@...us.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 12:03:41PM -0500, atull wrote:
> 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?

I'm suggesting that for the set of operations/configuration states that
need to be managed _for the Zynq[1]_ during reprogramming, I think
pinctrl might be a good fit.

But the pinctrl state activation would happen in the context of the zynq
fpga_mgr_ops write

I do not think it's a good fit for the socfpga, or for the lower level
fpga drivers _in general_.  Nor do I think that the FPGA Bridge
framework, as written, is a good fit for fpgas in general.

  Josh

[1]: Speaking only of the Zynq 7000-series, I don't know anything about
     the fancy new Zynq MPSoC :)
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