Re: [RFC PATCH v2 0/1] FPGA subsystem core

[Alan Tull <atull@altera.com>]

On Fri, 2013-10-04 at 16:33 -0700, Greg Kroah-Hartman wrote:
> On Fri, Oct 04, 2013 at 11:12:13AM -0700, H. Peter Anvin wrote:
> > On 10/04/2013 10:44 AM, Michal Simek wrote:
> > > 
> > > If you look at it in general I believe that there is wide range of 
> > > applications which just contain one bitstream per fpga and the 
> > > bitstream is replaced by newer version in upgrade. For them 
> > > firmware interface should be pretty useful. Just setup firmware 
> > > name with bitstream and it will be automatically loaded in startup 
> > > phase.
> > > 
> > > Then there is another set of applications especially in connection 
> > > to partial reconfiguration where this can be done statically by 
> > > pregenerated partial bitstreams or automatically generated on 
> > > target cpu. For doing everything on the target firmware interface 
> > > is not the best because everything can be handled by user 
> > > application and it is easier just to push this bitstream to do 
> > > device and not to save it to the fs.
> > > 
> > > I think the question here is if this subsystem could have several 
> > > interfaces. For example Alan is asking for adding char support. 
> > > Does it even make sense to have more interfaces with the same 
> > > backend driver? When this is answered then we can talk which one 
> > > make sense to have. In v2 is sysfs and firmware one. Adding char
> > > is also easy to do.
> > > 
> > 
> > Greg, what do you think?
> > 
> > I agree that the firmware interface makes sense when the use of the
> > FPGA is an implementation detail in a fixed hardware configuration,
> > but that is a fairly restricted use case all things considered.
> 
> Ideally I thought this would be just like "firmware", you dump the file
> to the FPGA, it validates it and away you go with a new image running in
> the chip.
> 
> But, it sounds like this is much more complicated, so much so that
> configfs might be the correct interface for it, as you can do lots of
> things there, and it is very flexible (some say too flexible...)
> 
> A char device, with a zillion different custom ioctls is also a way to
> do it, but one that I really want to avoid as that gets messy really
> quickly.

Hi Greg,

We are discussing a char device that has very few interfaces:
 - a way of writing the image to fpga
 - a way of getting fpga manager status
 - a way of setting fpga manager state

This all looks like standard char driver interface to me.  Writing the
image could be writing to the devnode (cat image.bin > /dev/fpga0). The
status stuff would be sysfs attributes.  All normal stuff any char
driver in the kernel would do.  Why not just go with that?

> 
> thanks,
> 
> greg k-h
>