Date:   Wed, 7 Feb 2018 16:37:02 -0600
From:   Alan Tull <atull@...nel.org>
To:     Wu Hao <hao.wu@...el.com>
Cc:     "Luebbers, Enno" <enno.luebbers@...el.com>,
        Moritz Fischer <mdf@...nel.org>, linux-fpga@...r.kernel.org,
        linux-kernel <linux-kernel@...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>,
        Shiva Rao <shiva.rao@...el.com>,
        Christopher Rauer <christopher.rauer@...el.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Subject: Re: [PATCH v3 14/21] fpga: dfl: add fpga manager platform driver for FME

On Tue, Feb 6, 2018 at 10:52 PM, Wu Hao <hao.wu@...el.com> wrote:
> On Tue, Feb 06, 2018 at 12:53:44PM -0600, Alan Tull wrote:
>> On Tue, Feb 6, 2018 at 12:47 AM, Wu Hao <hao.wu@...el.com> wrote:
>> > On Mon, Feb 05, 2018 at 10:25:54PM -0600, Alan Tull wrote:
>> >> On Mon, Feb 5, 2018 at 7:47 PM, Wu Hao <hao.wu@...el.com> wrote:
>> >> > On Mon, Feb 05, 2018 at 10:36:45AM -0800, Luebbers, Enno wrote:
>> >> >> Hi Hao,
>> >> >>
>> >> >> On Sun, Feb 04, 2018 at 05:37:06PM +0800, Wu Hao wrote:
>> >> >> > On Fri, Feb 02, 2018 at 04:26:26PM -0800, Luebbers, Enno wrote:
>> >> >> > > Hi Hao, Alan,
>> >> >> > >
>> >> >> > > On Fri, Feb 02, 2018 at 05:42:13PM +0800, Wu Hao wrote:
>> >> >> > > > On Thu, Feb 01, 2018 at 04:00:36PM -0600, Alan Tull wrote:
>> >> >> > > > > On Mon, Nov 27, 2017 at 12:42 AM, Wu Hao <hao.wu@...el.com> wrote:
>> >> >> > > > >
>> >> >> > > > > Hi Hao,
>> >> >> > > > >
>> >> >> > > > > A few comments below.   Besides that, looks good.
>> >> >> > > > >
>> >> >> > > > > > This patch adds fpga manager driver for FPGA Management Engine (FME). It
>> >> >> > > > > > implements fpga_manager_ops for FPGA Partial Reconfiguration function.
>> >> >> > > > > >
>> >> >> > > > > > Signed-off-by: Tim Whisonant <tim.whisonant@...el.com>
>> >> >> > > > > > Signed-off-by: Enno Luebbers <enno.luebbers@...el.com>
>> >> >> > > > > > Signed-off-by: Shiva Rao <shiva.rao@...el.com>
>> >> >> > > > > > Signed-off-by: Christopher Rauer <christopher.rauer@...el.com>
>> >> >> > > > > > Signed-off-by: Kang Luwei <luwei.kang@...el.com>
>> >> >> > > > > > Signed-off-by: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
>> >> >> > > > > > Signed-off-by: Wu Hao <hao.wu@...el.com>
>> >> >> > > > > > ----
>> >> >> > > > > > v3: rename driver to dfl-fpga-fme-mgr
>> >> >> > > > > >     implemented status callback for fpga manager
>> >> >> > > > > >     rebased due to fpga api changes
>> >> >> > > > > > ---
>> >> >> > > > > >  .../ABI/testing/sysfs-platform-fpga-dfl-fme-mgr    |   8 +
>> >> >> > > > > >  drivers/fpga/Kconfig                               |   6 +
>> >> >> > > > > >  drivers/fpga/Makefile                              |   1 +
>> >> >> > > > > >  drivers/fpga/fpga-dfl-fme-mgr.c                    | 318 +++++++++++++++++++++
>> >> >> > > > > >  drivers/fpga/fpga-dfl.h                            |  39 ++-
>> >> >> > > > > >  5 files changed, 371 insertions(+), 1 deletion(-)
>> >> >> > > > > >  create mode 100644 Documentation/ABI/testing/sysfs-platform-fpga-dfl-fme-mgr
>> >> >> > > > > >  create mode 100644 drivers/fpga/fpga-dfl-fme-mgr.c
>> >> >> > > > > >
>> >> >> > > > > > diff --git a/Documentation/ABI/testing/sysfs-platform-fpga-dfl-fme-mgr b/Documentation/ABI/testing/sysfs-platform-fpga-dfl-fme-mgr
>> >> >> > > > > > new file mode 100644
>> >> >> > > > > > index 0000000..2d4f917
>> >> >> > > > > > --- /dev/null
>> >> >> > > > > > +++ b/Documentation/ABI/testing/sysfs-platform-fpga-dfl-fme-mgr
>> >> >> > > > > > @@ -0,0 +1,8 @@
>> >> >> > > > > > +What:          /sys/bus/platform/devices/fpga-dfl-fme-mgr.0/interface_id
>> >> >> > > > > > +Date:          November 2017
>> >> >> > > > > > +KernelVersion:  4.15
>> >> >> > > > > > +Contact:       Wu Hao <hao.wu@...el.com>
>> >> >> > > > > > +Description:   Read-only. It returns interface id of partial reconfiguration
>> >> >> > > > > > +               hardware. Userspace could use this information to check if
>> >> >> > > > > > +               current hardware is compatible with given image before FPGA
>> >> >> > > > > > +               programming.
>> >> >> > > > >
>> >> >> > > > > I'm a little confused by this.  I can understand that the PR bitstream
>> >> >> > > > > has a dependency on the FPGA's static image, but I don't understand
>> >> >> > > > > the dependency of the bistream on the hardware that is used to program
>> >> >> > > > > the bitstream to the FPGA.
>> >> >> > > >
>> >> >> > > > Sorry for the confusion, the interface_id is used to indicate the version of
>> >> >> > > > the hardware for partial reconfiguration (it's part of the static image of
>> >> >> > > > the FPGA device). Will improve the description on this.
>> >> >> > > >
>> >> >> > >
>> >> >> > > The interface_id expresses the compatibility of the static region with PR
>> >> >> > > bitstreams generated for it. It changes every time a new static region is
>> >> >> > > generated.
>> >> >> > >
>> >> >> > > Would it make more sense to have the interface_id exposed as part of the FME
>> >> >> > > device (which represents the static region)? I'm not sure - it kind of also
>> >> >> > > makes sense here, where you would have all the information in one place (if the
>> >> >> > > interface_id matches, I can use this component to program a bitstream).
>> >> >> >
>> >> >> > Hi Enno
>> >> >> >
>> >> >> > Yes, this interface is under fpga-dfl-fme-mgr.0, and fpga-dfl-fme-mgr.0 is
>> >> >> > under fpga-dfl-fme.0. It's part of the FME device for sure. From another
>> >> >> > point of view, it means if anyone wants to do PR on this Intel FPGA device,
>> >> >> > he needs to find the FME device firstly, and then check if any fpga manager
>> >> >> > created under this FME device, if yes, check the interface_id before PR via
>> >> >> > the FME device node ioctl.
>> >> >>
>> >> >> That sounds good, thank you!
>> >> >>
>> >> >> >
>> >> >> > >
>> >> >> > > Sorry for my limited understanding of the infrastructure - would this same
>> >> >> > > "fpga-dfl-fme-mgr.0" be used for PR if we had multiple PR regions? In that case
>> >> >> > > it would need to expose multiple interface_ids (or we'd have to track both
>> >> >> > > interface IDs and an identifier for the target PR region).
>> >> >> >
>> >> >> > Yes, the fpga manager could be shared with different PR regions.
>> >> >> >
>> >> >> > Sorry, I'm not sure where we need to expose multiple interface_ids and why.
>> >> >>
>> >> >> It's basically a question of how to determine bitstream compatibility - either,
>> >> >> there's a separate interface_id per reconfigurable region, or there is a single
>> >> >> interface_id for the entire device. Both make sense from a certain perspective.
>> >> >>
>> >> >> If there are multiple interface_ids per device (one per region), the driver
>> >> >> would need to expose all of them. If there's only a single one, the driver only
>> >> >> exposes that one ID - compatibility would be determined by looking at both that
>> >> >> single interface_id _and_ the identifier/number of the targeted region.
>> >> >>
>> >> >> I would prefer a separate interface_id per region - it seems more generic and
>> >> >> flexible.
>> >>
>> >> Hi Enno,
>> >>
>> >> I agree with this.
>> >>
>> >> >
>> >> > It's possible to have per region interface_id (or even both per dev interface_id
>> >> > and per region interface_id at the same time), but per FME PR sub feature
>> >> > implementation, it supports multiple PR regions, but only provide one interface
>> >> > id, so at least in this case, it's not per-region information per my
>> >> > understanding. We can consider it later when hardware really supports it. : )
>> >>
>> >> Hi Hao,
>> >>
>> >> I understand that in the case of this PR hardware, the region to
>> >> program is selected when the region_id to program is written to a PR
>> >> hardware control register.  For another example, Arria10 has a hard PR
>> >> hardware and the PR bitstream lands in the area of the FPGA for which
>> >> it was compiled.  In that case, for the PR bitstream to be compatible
>> >> with a PR region, the layout of the edge connections also needs to be
>> >> compatible, so compatibility is per-region in that case instead of
>> >> per-PR hardware.
>> >
>> > Hi Alan,
>> >
>> > Thanks a lot for the explanation. :)
>> >
>> > I fully understand the consideration of adding per-region interface_id.
>> >
>> >> And besides, as I said yesterday, the hard PR
>> >> hardware would not know what the static region ID is when this
>> >> framework is used with such a device.
>> >
>> > Yes, is it possible that hard PR hardware with different versions, requires
>> > different images or different methods for compatibility checking?
>>
>> Because it is really hardware and not something in the FPGA fabric,
>> the hard PR hardware isn't going to change versions very often.  It
>> has to be designed to be flexible and not add any constraints on the
>> PR regions.  If some feature is added or a bug is fixed, that's just a
>> driver issue at most and should not affect PR region compatibility.
>> PR region compatibility would only be dependent on the static FPGA
>> image and the regions that are created in it.  It could be exported in
>> terms of a single static region ID or per-region ID.
>>
>> >
>> >>
>> >> That's why I think making the id per-region may be more future proof,
>> >> even if it may see unnecessary in the case of the original blue bits
>> >> this was written for.
>> >
>> > I feel that per-PR hardware interface id is useful in some cases, and maybe
>> > in some cases, both per-PR hardware and per-region interface ids are needed
>> > for its compatibility checking, so shall we leave developers to decide to
>> > implement per-PR hardware or per-region or both interface ids based on their
>> > own hardware implementations? How do you think? :)
>>
>> That gives us 3 sets of id's.  Seems overly complicated and the
>> userspace would have to figure out which set of id's to use.  I want
>> to see an interface that isn't more complicated than it needs to be
>> but still can be expected to be ok for the future (as far as we can
>> anticipate).
>>
>> Would per-region id's cause any problems that you can see?  I
>> understand that the region id's would all be the same value for a
>> given PR hardware in your use case, but that doesn't seem like it
>> would be hard to implement or that it opens up some possible failure.
>
> Hi Alan
>
> No, I don't have any concern on per-region id at all. I will remove this
> sysfs interface_id interface from this driver, and another patch to add
> a common sysfs interface under the fpga-region. I feel maybe we can use
> "compat_id" instead of "id" here to avoid confusion for that sysfs
> interface under fpga-region, as we added a region_id to fpga_image_info,
> and they have totally different purposes. How do you think?

Hi Hao,

Sounds good.  Yes, that that would keep things clear.

Alan

>
> Thanks
> Hao

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