linux-kernel - Re: [PATCH v8 2/4] fpga manager: add sysfs interface document

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Date:	Mon, 12 Jan 2015 09:45:30 +0100
From:	Pavel Machek <pavel@...x.de>
To:	atull <atull@...nsource.altera.com>
Cc:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	jgunthorpe@...idianresearch.com, hpa@...or.com,
	Michal Simek <monstr@...str.eu>,
	Michal Simek <michal.simek@...inx.com>, rdunlap@...radead.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devicetree@...r.kernel.org, robh+dt@...nel.org,
	Grant Likely <grant.likely@...aro.org>, iws@...o.caltech.edu,
	linux-doc@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	philip@...ister.org, rubini@...dd.com,
	Steffen Trumtrar <s.trumtrar@...gutronix.de>,
	jason@...edaemon.net, kyle.teske@...com, nico@...aro.org,
	Felipe Balbi <balbi@...com>, m.chehab@...sung.com,
	davidb@...eaurora.org, Rob Landley <rob@...dley.net>,
	davem@...emloft.net, cesarb@...arb.net, sameo@...ux.intel.com,
	akpm@...ux-foundation.org,
	Linus Walleij <linus.walleij@...aro.org>, pawel.moll@....com,
	mark.rutland@....com, ijc+devicetree@...lion.org.uk,
	galak@...eaurora.org, devel@...verdev.osuosl.org,
	Alan Tull <delicious.quinoa@...il.com>,
	dinguyen@...nsource.altera.com, yvanderv@...nsource.altera.com
Subject: Re: [PATCH v8 2/4] fpga manager: add sysfs interface document

On Sun 2015-01-11 10:29:00, atull wrote:
> On Sat, 10 Jan 2015, Pavel Machek wrote:
> 
> > On Sat 2015-01-10 10:10:51, Pantelis Antoniou wrote:
> > > Hi Pavel,
> > > 
> > > > On Jan 9, 2015, at 22:56 , Pavel Machek <pavel@...x.de> wrote:
> > > > 
> > > > On Fri 2015-01-09 13:14:24, atull wrote:
> > > >> On Wed, 7 Jan 2015, Pavel Machek wrote:
> > > >> 
> > > >>> On Tue 2015-01-06 14:13:37, atull@...nsource.altera.com wrote:
> > > >>>> +
> > > >>>> +What:		/sys/class/fpga_manager/<fpga>/firmware
> > > >>>> +Date:		October 2014
> > > >>>> +KernelVersion:	3.18
> > > >>>> +Contact:	Alan Tull <atull@...nsource.altera.com>
> > > >>>> +Description:	Name of the FPGA image file to load using firmware
> > > >>>> class.
> > > >>> 
> > > >>> This one is ugly: it unneccessarily passes firmware name through the
> > > >>> kernel. Just make interface and code simpler by always passing
> > > >>> "socfpga-fpga-image" or something like that.
> > > >>> 
> > > >>> Thanks,
> > > >>> 									Pavel
> > > >> 
> > > >> Hi Pavel,
> > > >> 
> > > >> It might be ugly.  It's not unnecessary.  Some uses of FPGAs involve
> > > >> switching out the FPGA images dynamically under control of the userspace.
> > > > 
> > > > Then configure udev to load right firmware for you, or ln -s
> > > > image-i-want-now socfpga-fpga-image to select the one to read…?
> > > > 
> > > 
> > > Who said that udev is going to be available in the kind of system this is going to end in?
> > 
> > Noone.
> > 
> > > Doing ln -s tricks to load a different image? Really?
> > 
> > But if you don't have udev, you can do ln -s. It is better than
> > open-coding symlink functionality into
> > /sys/class/fpga_manager/<fpga>/firmware ... cause that's what this is.
> > 
> > Actually, clean implementation of "firmware" would be symlink in
> > sysfs; but I'd say that would be overdoing it.
> > 
> > > I say the interface is fine as it is.
> > 
> > I say it is not.
> > 									Pavel
> 
> Hi Pavel,
> 
> I see that we could do it that way and it would eliminate one of the
> sysfs files (../fpga_manager/<fpga>/firmware).  Either way we are assuming
> that there are fpga images in the filesystem in the firmware search path,
> so I don't see why adding a piece of indirection (symlink) makes things
> better.

Well.. you are basically re-implementing symlink with
../fpga_manager/<fpga>/firmware "file" in sysfs, and doing it
badly. It has no advantages over real symlink.

> We want to be able to switch out the FPGA image
> under control of userspace.  So we don't want an interface that makes
> it more cumbersome or tries to pretend that there's only one image.

What is cumbersome about symlink? Why is "fake" symlink in sysfs better?

> Previous uses of the firmware layer has been to use it to load once after
> bootup; this is different since some use cases will want to switch out
> the FPGA image.  If someone wants there to be only one FPGA image on
> the FGPA forever, they will probably not be using this framework; their
> FPGA will probably be loaded before Linux boots up.

Why? I have just one image on the fpga, and would prefer to load it
from Linux.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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