Date:   Wed, 2 Aug 2017 16:01:14 -0500
From:   Alan Tull <atull@...nel.org>
To:     Wu Hao <hao.wu@...el.com>
Cc:     Rob Herring <robh+dt@...nel.org>, Moritz Fischer <mdf@...nel.org>,
        linux-fpga@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "Kang, Luwei" <luwei.kang@...el.com>,
        "Zhang, Yi Z" <yi.z.zhang@...el.com>,
        Tim Whisonant <tim.whisonant@...el.com>,
        Enno Luebbers <enno.luebbers@...el.com>,
        Shiva Rao <shiva.rao@...el.com>,
        Christopher Rauer <christopher.rauer@...el.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 02/22] fpga: add FPGA device framework

On Wed, Aug 2, 2017 at 9:07 AM, Wu Hao <hao.wu@...el.com> wrote:
> On Tue, Aug 01, 2017 at 04:04:44PM -0500, Alan Tull wrote:
>> On Tue, Aug 1, 2017 at 3:43 AM, Wu Hao <hao.wu@...el.com> wrote:
>> > On Mon, Jul 31, 2017 at 04:40:16PM -0500, Alan Tull wrote:
>> >> On Thu, Jul 27, 2017 at 2:10 PM, Rob Herring <robh+dt@...nel.org> wrote:
>> >> > On Thu, Jul 27, 2017 at 11:35 AM, Alan Tull <atull@...nel.org> wrote:
>> >> >> On Sun, Jun 25, 2017 at 8:51 PM, Wu Hao <hao.wu@...el.com> wrote:
>> >> >>
>> >> >> Hi Rob,
>> >> >>
>> >> >> I was hoping to pick your brain a bit on a DT question.
>> >> >>
>> >> >>> During FPGA device (e.g PCI-based) discovery, platform devices are
>> >> >>> registered for different FPGA function units. But the device node path
>> >> >>> isn't quite friendly to applications.
>> >> >>>
>> >> >>> Consider this case, applications want to access child device's sysfs file
>> >> >>> for some information.
>> >> >>>
>> >> >>> 1) Access using bus-based path (e.g PCI)
>> >> >>>
>> >> >>>   /sys/bus/pci/devices/xxxxx/fpga_func_a.0/sysfs_file
>> >> >>>
>> >> >>>   From the path, it's clear which PCI device is the parent, but not perfect
>> >> >>>   solution for applications. PCI device BDF is not fixed, application may
>> >> >>>   need to search all PCI device to find the actual FPGA Device.
>> >> >>>
>> >> >>> 2) Or access using platform device path
>> >> >>>
>> >> >>>   /sys/bus/platform/devices/fpga_func_a.0/sysfs_file
>> >> >>>
>> >> >>>   Applications find the actual function by name easily, but no information
>> >> >>>   about which fpga device it belongs to. It's quite confusing if multiple
>> >> >>>   FPGA devices are in one system.
>> >> >>
>> >> >> There's a proposal for adding sysfs nodes that correspond to each FPGA
>> >> >> device., with the devices located on each FPGA under them.  It makes
>> >> >> it easier to see which device is on which FPGA.
>> >> >
>> >> > Makes sense.
>> >> >
>> >> >>> 'FPGA Device' class is introduced to resolve this problem. Each node under
>> >> >>> this class represents a fpga device, which may have one or more child
>> >> >>> devices. Applications only need to search under this FPGA Device class
>> >> >>> folder to find the child device node it needs.
>> >> >>>
>> >> >>> For example, for the platform has 2 fpga devices, each fpga device has
>> >> >>> 3 child devices, the hierarchy looks like this.
>> >> >>>
>> >> >>> Two nodes are under /sys/class/fpga/:
>> >> >>> /sys/class/fpga/fpga.0
>> >> >>> /sys/class/fpga/fpga.1
>> >> >>>
>> >> >>> Each node has 1 function A device and 2 function B devices:
>> >> >>> /sys/class/fpga/fpga.0/func_a.0
>> >> >>> /sys/class/fpga/fpga.0/func_b.0
>> >> >>> /sys/class/fpga/fpga.0/func_b.1
>> >> >>>
>> >> >>> /sys/class/fpga/fpga.1/func_a.1
>> >> >>> /sys/class/fpga/fpga.1/func_b.2
>> >> >>> /sys/class/fpga/fpga.1/func_b.3
>> >> >
>> >> > A class is generally what is the function of the device, not how it is
>> >> > attached. Seems like what you want here is a new bus type if the
>> >> > existing PCI and platform bus types don't work.
>> >> >
>> >> >>
>> >> >> I can see the value of having sysfs nodes that correspond to fpga
>> >> >> devices and being able to find devices under them.  I'm thinking what
>> >> >> that would mean for Device Tree when fpga-dev is used on DT enabled
>> >> >> systems.  In Device Tree, what is a fpga-dev?
>> >> >
>> >> > Just properly setting the parent struct device on the functions should
>> >> > be enough to figure out which function is in which fpga. I don't see
>> >> > why a new class is needed.
>> >> >
>> >> >> Currently the DT would have a FPGA bridge corresponding to each FPGA's
>> >> >> hardware bridge and a heirarchy of bridges, regions and devices under
>> >> >> it.  On systems that don't support partial reconfiguration under the
>> >> >> OS (so not main bridge that was controlled by the OS), there would be
>> >> >> a FPGA region, then its child regions, bridges, and devices.
>> >> >
>> >> > The FPGA bridges could instantiate fpga bus type devices instead of
>> >> > platform devices.
>> >>
>> >> Yes
>> >>
>> >> Some FPGA use cases already have a base bridge per FPGA that could
>> >> serve as this bus.  But this use case has a static FPGA image +
>> >> reprogrammable child fpga regions.  There's no base bridge under Linux
>> >> since the FPGA was programmed and the bridge enabled before Linux
>> >> boots.   An added base bridge that doesn't touch hardware will be
>> >> required for this type of use.
>> >
>> > Hi Alan
>> >
>> > Does 'base bridge' mentioned above mean a hardware bridge just like
>> > PCIe or USB?
>>
>> Whatever connects each FPGA to the CPU.  One base bridge per FPGA
>> device to create the fpga bus type devices.  Each PR region's bridge
>> would also be a bus.

device.h [1] says it better than me:

 * A bus is a channel between the processor and one or more devices.
For the
 * purposes of the device model, all devices are connected via a bus,
even if
 * it is an internal, virtual, "platform" bus. Buses can plug into
each other.
 * A USB controller is usually a PCI device, for example. The device
model
 * represents the actual connections between buses and the devices
they control.

A fpga-bridge could be the bus even if the FPGA is on PCIe.  The
advantage of doing it that way is that this bus will be usable for
both embedded and PCIe FPGAs.

Alan

[1] https://github.com/torvalds/linux/blob/master/include/linux/device.h

>>
>> >
>> > I tried to use fpga bus type device instead of fpga-dev class today,
>> > it works for me, e.g Intel FPGA device PCIe driver could create a
>> > fpga bus type dev as a child of PCIe device and its sysfs path will be
>> > changed to /sys/bus/fpga/devices/fpga.x/ from /sys/class/fpga/fpga.x/.
>> > For now, this fpga bus type device is only used as container device,
>> > so no driver needed for it.
>>
>> That's great!  I'd like to see the code to try it out with device
>> tree.  Is it part of fpga-bridge or something separate for now?
>>
>
> Hi Alan
>
> I just sent the patch I did as a RFC Patch[1] to the mailing list. Please
> take a look. I only replaced the original fpga-dev class with new 'fpga'
> bus type, and keep the original interface not changed.
>
> [1] http://marc.info/?l=linux-fpga&m=150167682312708&w=2
>
>> >
>> > Do you have any concern on this? I see fpga bus type works fine, but
>> > I didn't see other advantages for this case, as we only use it as a
>> > container device to represent a FPGA device in sysfs hierarchy. :)
>>
>> I could not see a way to make the fpga-dev class compatible with the
>> FPGA Device Tree bindings.  This was a red flag. That's why I asked
>> Rob's opinion.  Sysfs classes collect devices of a specific type
>> together; busses describe topology.  I think the goal of fpga-dev was
>> to describe topology.  It's more correct to define this as a bus, not
>> a class.  If it's done right, it can work for device tree also.
>
> Got it. Thanks. :)
>
> Hao
>
>>
>> Alan
>>
>> >
>> > Thanks
>> > Hao
>> >
>> >>
>> >> > That's really up to Linux and outside the scope of
>> >> > the bindings.
>> >>
>> >> Thanks for the feedback.
>> >>
>> >> Alan Tull
>> >>
>> >> >
>> >> > Rob

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