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

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Date:	Wed, 21 Jan 2015 22:32:23 +0200
From:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	atull <atull@...nsource.altera.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Pavel Machek <pavel@...x.de>, 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, 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,
	Andrew Morton <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

Hi Jason,

> On Jan 21, 2015, at 22:27 , Jason Gunthorpe <jgunthorpe@...idianresearch.com> wrote:
> 
> On Wed, Jan 21, 2015 at 06:33:12PM +0200, Pantelis Antoniou wrote:
>> Hi Alan,
>> 
>>> On Jan 21, 2015, at 18:01 , One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> wrote:
>>> 
>>> On Thu, 15 Jan 2015 22:54:46 +0200
>>> Pantelis Antoniou <pantelis.antoniou@...sulko.com> wrote:
>>> 
>>>> Hi Alan,
>>>> 
>>>>> On Jan 15, 2015, at 22:45 , One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> wrote:
>>>>> 
>>>>> On Thu, 15 Jan 2015 11:47:26 -0700
>>>>> Jason Gunthorpe <jgunthorpe@...idianresearch.com> wrote:
>>>>>> It is a novel idea, my concern would be that embedding the FPGA in the
>>>>>> DT makes it permanent unswappable kernel memory.
>>>>>> Not having the kernel hold the FPGA is best for many uses.
>>>>> 
>>>>> If you have a filesysytem before the FPGA is set up then it belongs in
>>>>> the file system. As you presumably loaded the kernel from somewhere there
>>>>> ought to be a file system (even an initrd).
>>>>> 
>>>> 
>>>> Request firmware does not imply keeping it around. You can always re-request
>>>> when reloading (although there’s a nasty big of caching that needs to be
>>>> resolved with the firmware loader).
>>> 
>>> Which comes down to the same thing. Unless you can prove that there is a
>>> path to recover the firmware file that does not have any dependancies
>>> upon the firmware executing (and those can be subtle and horrid at times)
>>> you need to keep it around for suspend/resume at least and potentially
>>> any unexpected error/reset.
>>> 
>> 
>> In that case the only safe place to put it is in the kernel image itself, which
>> is something the firmware loader already supports.
> 
> My point is that the current firmware layer is overly cautious and
> FPGAs are very big. My current project on small Xilinx device has a
> 10MB programming file. The biggest Xilinx device today has a max
> bitfile size around 122MB.
> 
> So keeping that much memory pinned in the kernel when I can prove it
> is uncessary for my system (either because there is no suspend/resume
> possibility, or because I know the CPU can always access the
> filesytem) is very undesirable.
> 
> Other systems might have to take the ram hit.
> 
> Since it seems like the kernel has no way to know, we probably have
> have a way to tell it not to cache the bitfile.
> 

The firmware loader was not originally meant to handle these cases, but
I’m sure it’s not an insurmountable obstacle.

> Jason

Regards

— Pantelis

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