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

[delicious quinoa]

On Tue, Oct 8, 2013 at 4:44 PM, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Oct 08, 2013 at 12:00:14PM -0500, Alan Tull wrote:
>> 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?
>
> Because we really hate to add new ioctls to the kernel if at all
> possible.

I don't see any need for adding any ioctls.

> Using sysfs (and it's one-value-per-file rule), makes
> userspace tools easier, and managing the different devices in the system
> easier (you know _exactly_ which device you are talking to, you don't
> have to guess based on minor number).

That's cool.  The interface we could use is writing the raw fpga data
to /sys/class/fpga_manager/fpga0/fpga_config_data

Reading or setting the fpga state could be from
/sys/class/fpga_manager/fpga0/fpga_config_state

Or do I misunderstand?  Do you include sysfs attributes when you
are talking about ioctls?

Alan

>
> thanks,
>
> greg k-h
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