linux-kernel - Re: [PATCH 2/2] fpga: add FPGA manager debugfs

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Date:   Thu, 16 Aug 2018 15:00:34 -0700
From:   Moritz Fischer <mdf@...nel.org>
To:     Federico Vaga <federico.vaga@...n.ch>
Cc:     Alan Tull <atull@...nel.org>, Moritz Fischer <mdf@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Appana Durga Kedareswara Rao <appanad@...inx.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fpga@...r.kernel.org,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Alan Tull <atull@...nsource.altera.com>,
        Matthew Gerlach <matthew.gerlach@...ux.intel.com>
Subject: Re: [PATCH 2/2] fpga: add FPGA manager debugfs

Hi Federico,

On Thu, Aug 16, 2018 at 11:21:32PM +0200, Federico Vaga wrote:
> Hi,
> 
> On Thursday, August 16, 2018 10:04:51 PM CEST Alan Tull wrote:
> > On Thu, Aug 16, 2018 at 1:59 PM, Moritz Fischer <mdf@...nel.org> wrote:
> > > Hi Alan,
> > 
> > Hi Moritz,
> > 
> > > comments inline. While I see how this is useful, I have the
> > > suspicion that from the moment this gets merged vendor kernels
> > > will just default to use this ...
> > 
> > Yeah, I have that suspicion as well.  That's probably why I sat on
> > this and didn't upstream it for 2 years.  But on the other hand, I
> > keep hearing of lots of cases of people implementing this
> > independently anyway.  At least if it is debugfs, it makes it clear
> > that it's not intended for production use.
> 
> I'm one of those guys who implemented this independently.

We all have in one way or another ;) Most people on ARM run an out of tree
patch using devicetree overlays these days I hope rather than /dev/mem
and UIO ... or other vender solutions...
> 
> @Mortiz
> I do not see how this can be a bad thing (from what you wrote I guess you 
> prefer another interface). Which interface to use depends on the use case.
> If you have this suspicion it's, I guess, because such interface it is 
> extremely easy to use.

What happens to a kernel driver doing MMIO with devices while you reload
the entire FPGA from userland?
> 
> @Alan
> DebugFS can be a first step, but I would go for a normal device in /dev at 
> some point. I do not see why this should not be used in production

I'm not against having a userland interface to reprogram the FPGA, the 
Intel DFL code is a good example of a sensible one, doing so in a safe
manner.

Ideally we'll get around to have a more generic interface, as we get
time to work on it.

- Moritz