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

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Message-ID: <CAL_Jsq+ZfCajLXhLCi4MFbLRYGgs+BcDTsKg3r5rSBPv9yjGEQ@mail.gmail.com>
Date:	Mon, 12 Jan 2015 10:05:49 -0600
From:	Rob Herring <robherring2@...il.com>
To:	atull <atull@...nsource.altera.com>
Cc:	Pavel Machek <pavel@...x.de>,
	Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Simek <monstr@...str.eu>,
	Michal Simek <michal.simek@...inx.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Grant Likely <grant.likely@...aro.org>, iws@...o.caltech.edu,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Mark Brown <broonie@...nel.org>,
	Philip Balister <philip@...ister.org>,
	Alessandro Rubini <rubini@...dd.com>,
	Steffen Trumtrar <s.trumtrar@...gutronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Kyle Teske <kyle.teske@...com>,
	Nicolas Pitre <nico@...aro.org>, Felipe Balbi <balbi@...com>,
	Mauro Carvalho Chehab <m.chehab@...sung.com>,
	David Brown <davidb@...eaurora.org>,
	Rob Landley <rob@...dley.net>,
	David Miller <davem@...emloft.net>,
	Cesar Eduardo Barros <cesarb@...arb.net>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, devel@...verdev.osuosl.org,
	Alan Tull <delicious.quinoa@...il.com>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>,
	"yvanderv@...nsource.altera.com" <yvanderv@...nsource.altera.com>
Subject: Re: [PATCH v8 2/4] fpga manager: add sysfs interface document

On Sun, Jan 11, 2015 at 10:29 AM, atull <atull@...nsource.altera.com> 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.
>
> 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.
>
> 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.

That's not really true. Some WiFi devices have to load different
firmware for client and AP modes and switching is a common usecase in
phones.

There may be other instances of drivers wanting configurable firmware
names already. A common interface here would be nice. It could also be
useful to expose the firmware names such that you could generate a
list of all firmware files needed for a machine (that may already be
possible with uevents?).

Rob
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