linux-kernel - Re: [PATCH v9 6/7] staging: add simple-fpga-bus

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Message-ID: <20150723221510.GA16971@obsidianresearch.com>
Date:	Thu, 23 Jul 2015 16:15:10 -0600
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Moritz Fischer <moritz.fischer@...us.com>
Cc:	Alan Tull <atull@...nsource.altera.com>,
	Greg KH <gregkh@...uxfoundation.org>, hpa@...or.com,
	Michal Simek <monstr@...str.eu>,
	Michal Simek <michal.simek@...inx.com>, rdunlap@...radead.org,
	mark.rutland@....com, linux-doc@...r.kernel.org, rubini@...dd.com,
	Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
	s.trumtrar@...gutronix.de, devel@...verdev.osuosl.org,
	sameo@...ux.intel.com, Nicolas Pitre <nico@...aro.org>,
	ijc+devicetree@...lion.org.uk, kyle.teske@...com,
	Grant Likely <grant.likely@...aro.org>,
	David Brown <davidb@...eaurora.org>,
	Linus Walleij <linus.walleij@...aro.org>, cesarb@...arb.net,
	devicetree@...r.kernel.org, jason@...edaemon.net,
	pawel.moll@....com, iws@...o.caltech.edu, broonie@...nel.org,
	Philip Balister <philip@...ister.org>,
	Petr Cvek <petr.cvek@....cz>, dinguyen@...nsource.altera.com,
	pavel@...x.de, yvanderv@...nsource.altera.com,
	linux-kernel@...r.kernel.org, balbi@...com,
	Alan Tull <delicious.quinoa@...il.com>, robh+dt@...nel.org,
	Rob Landley <rob@...dley.net>,
	Kumar Gala <galak@...eaurora.org>, akpm@...ux-foundation.org,
	davem@...emloft.net, m.chehab@...sung.com
Subject: Re: [PATCH v9 6/7] staging: add simple-fpga-bus

On Thu, Jul 23, 2015 at 02:55:52PM -0700, Moritz Fischer wrote:
> Hi Alan,
> 
> I saw that your socfpga driver doesn't support the partial reconfig
> use case (not a big deal).
> What I currently do for Zynq is if I'm doing a non-partial reconfig is
> that I disable input
> level shifters and assert *all* resets while reprogramming in my FPGA
> manager .write_init() and .write_complete().

I do this as well, but it is a bit more complex.. FPGA specific code
has to run around and ensure all DMA is shut off, then we need to make
sure no CPU issued AXI transactions can happen, then we can tear down
the FPGA side.

If the FPGA is torn down while an AXI op is inprogress things go
sideways, we have to work to prevent that :)

This happens almost for free, I use DT and the device model to
disconnect the drivers. The drivers are careful to synchronously fence
off in-progress DMA. Then drop the DT nodes associated with the
FPGA, finally the actual FPGA cells can be reset.

> In a partial reconfiguration situation, would I have separate
> simple-fpga buses for each of the parts that I swap out, each with
> it's own reset and bitfile attached?

I'd think of partial reconfiguration as another nested FPGA. The
resets and so forth could be attached to soft controllers in the
unswappable part of the FPGA.

DT nodes have to surround it in some way...

Jason
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