Re: [PATCH v2 2/3] fpga manager: framework core

[atull <atull@opensource.altera.com>]

On Mon, 8 Dec 2014, One Thousand Gnomes wrote:

> On Sat, 6 Dec 2014 13:00:17 +0000
> Grant Likely <grant.likely@linaro.org> wrote:
> 
> > On Fri, Oct 24, 2014 at 11:52 AM, Pavel Machek <pavel@denx.de> wrote:
> > > Hi!
> > >
> > >> * /sys/class/fpga_manager/<fpga>/firmware
> > >>   Name of FPGA image file to load using firmware class.
> > >>   $ echo image.rbf > /sys/class/fpga_manager/<fpga>/firmware
> > >
> > > I .. still don't think this is good idea. What about namespaces?
> > > The path corresponds to path in which namespace?
> > 
> > I don't understand your concern here. This allows userspace to name
> > the FPGA bitstream that the kernel will use during request_firmware(),
> > and it will show up as the $FIRMWARE value in the uevent file, but it
> > is still the responsibility of userspace to choose what to load, and
> > it can freely ignore the setting of $FIRMWARE if it needs to.
> 
> I think the entire model here is basically pedicated on a bogus
> assumption that an FPGA is a one shot device. It's not. It's a fast
> reloadable reusable device. A lot of work being done with FPGAs in
> operating systems already involves basically task switching and
> scheduling FPGAs as a shared resource pool. Trying to nail something
> together with request_firmware is several years behind the curve.
> 
> From userspace it needs to be a open, load, use, close type model, not a
> static or semi-static pile of mappings.
> 
> Alan
> 

Hi Alan,

I agree with the view that a FPGA is something that can get reprogrammed a lot.
That's a flexibility we want to use.  I don't see a problem with using firmware
to do the programming as long as we have a lightweight interface where we can
load an image, use it, then later reset the FGPA and load a different image
instead.

This assumes that the system will have a pile of FPGA images sitting on
the filesystem for us to switch between.

My intent is to also support loading using device tree overlays.  This is a lot
more linux-like and less of something just bolted on.  The flow here is:

* load a DT overlay
* this causes the fpga to get programmed
* appropriate bridges get enabled
* appropriate drivers get probed

When the DT overlay is removed, all these get undone in the reverse order.

Alan Tull