[PATCH v8 2/4] fpga manager: add sysfs interface document

[Pantelis Antoniou]

Hi Jason,

> On Jan 21, 2015, at 22:27 , Jason Gunthorpe <jgunthorpe at obsidianresearch.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 at lxorguk.ukuu.org.uk> wrote:
>>> 
>>> On Thu, 15 Jan 2015 22:54:46 +0200
>>> Pantelis Antoniou <pantelis.antoniou at konsulko.com> wrote:
>>> 
>>>> Hi Alan,
>>>> 
>>>>> On Jan 15, 2015, at 22:45 , One Thousand Gnomes <gnomes at lxorguk.ukuu.org.uk> wrote:
>>>>> 
>>>>> On Thu, 15 Jan 2015 11:47:26 -0700
>>>>> Jason Gunthorpe <jgunthorpe at obsidianresearch.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