Date:   Sun, 27 Sep 2020 07:51:08 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Moritz Fischer <mdf@...nel.org>
Cc:     Xu Yilun <yilun.xu@...el.com>, linux-fpga@...r.kernel.org,
        linux-kernel@...r.kernel.org, trix@...hat.com, lgoncalv@...hat.com,
        hao.wu@...el.com
Subject: Re: [PATCH v3 1/5] fpga: dfl: rename the bus type "dfl" to "fpga-dfl"

On Sat, Sep 26, 2020 at 12:22:19PM -0700, Moritz Fischer wrote:
> Hi Greg,
> 
> On Sat, Sep 26, 2020 at 08:09:13AM +0200, Greg KH wrote:
> > On Sat, Sep 26, 2020 at 10:23:46AM +0800, Xu Yilun wrote:
> > > Hi greg,
> > > 
> > > About the bus naming, I summarized some questions we've discussed to check
> > > with you. See inline.
> > > 
> > > On Thu, Sep 24, 2020 at 10:27:00AM -0700, Moritz Fischer wrote:
> > > > Hi Xu,
> > > > 
> > > > On Fri, Sep 25, 2020 at 12:59:57AM +0800, Xu Yilun wrote:
> > > > > Now the DFL device drivers could be made as independent modules and put
> > > > > in different subsystems according to their functionalities. So the name
> > > > > should be descriptive and unique in the whole kernel.
> > > > > 
> > > > > The patch changes the naming of dfl bus related structures, functions,
> > > > > APIs and documentations.
> > > > > 
> > > > > Signed-off-by: Xu Yilun <yilun.xu@...el.com>
> > > > > ---
> > > > >  Documentation/ABI/testing/sysfs-bus-dfl      |  15 --
> > > > >  Documentation/ABI/testing/sysfs-bus-fpga-dfl |  15 ++
> > > > >  MAINTAINERS                                  |   2 +-
> > > > >  drivers/fpga/dfl.c                           | 254 ++++++++++++++-------------
> > > > >  drivers/fpga/dfl.h                           |  77 ++++----
> > > > >  5 files changed, 184 insertions(+), 179 deletions(-)
> > > > >  delete mode 100644 Documentation/ABI/testing/sysfs-bus-dfl
> > > > >  create mode 100644 Documentation/ABI/testing/sysfs-bus-fpga-dfl
> > > > > 
> > > > > diff --git a/Documentation/ABI/testing/sysfs-bus-dfl b/Documentation/ABI/testing/sysfs-bus-dfl
> > > > > deleted file mode 100644
> > > > > index 23543be..0000000
> > > > > --- a/Documentation/ABI/testing/sysfs-bus-dfl
> > > > > +++ /dev/null
> > > > > @@ -1,15 +0,0 @@
> > > > > -What:		/sys/bus/dfl/devices/dfl_dev.X/type
> > > > > -Date:		Aug 2020
> > > > > -KernelVersion:	5.10
> > > > > -Contact:	Xu Yilun <yilun.xu@...el.com>
> > > > > -Description:	Read-only. It returns type of DFL FIU of the device. Now DFL
> > > > > -		supports 2 FIU types, 0 for FME, 1 for PORT.
> > > > > -		Format: 0x%x
> > > > > -
> > > > > -What:		/sys/bus/dfl/devices/dfl_dev.X/feature_id
> > > > > -Date:		Aug 2020
> > > > > -KernelVersion:	5.10
> > > > > -Contact:	Xu Yilun <yilun.xu@...el.com>
> > > > > -Description:	Read-only. It returns feature identifier local to its DFL FIU
> > > > > -		type.
> > > > > -		Format: 0x%x
> > > > 
> > > > You're changing userland facing ABI. I think that's something to avoid,
> > > > please check with Greg on the rules since this hasn't been in a release yet.
> > > > 
> > > 
> > > I'm going to change the name of bus stuff for other subsystems, to be
> > > aligned, I also consider change the bus_type.name and dfl dev_name. But
> > > it will cause the changing of user ABIs. No user case for these user ABI
> > > now cause they are just queued, is it good I change them?
> > 
> > Why change the user name here?  No need for that, right?  Unless you
> > really want to, and think that no one will notice.  If so, fine, change
> > them :)
> 
> Let's leave it as is -- An FPGA is one possible implementation and as for
> other buses, you wouldn't call it fpga-usb or usb-fpga just because the
> USB bus is implemented in an FPGA if it behaves like a normal USB bus.
> Having an ASIC based DFL bus show up under dfl-fpga / fpga-dfl in sysfs
> would be super confusing.
> 
> > > It is mentioned that although Device Feature List is introduced in FPGA,
> > > but it doesn't limit the usage in FPGA only. It's just a method to
> > > discover features from a device, for sure it can be extended and used
> > > in other devices too. So it can be bigger namespace than FPGA. Like in
> > > our existing code, we picked dfl_fpga (DFL based FPGA) for uapi (ioctl)
> > > and internal functions. This is suggested by Alan (The previous FPGA
> > > maintainer). It's possible to have "DFL based XXX" in the future, even
> > > currently only FPGA uses DFL. This is the reason we thought just "dfl"
> > > in the whole kernel space is OK.
> > > So, is there a chance we keep the "dfl" naming in the whole kernel?
> > 
> > No one knows what "DFL" is, and odds are, if a different subsystem wants
> > to use it, they will have their own variant, right?
> > 
> > And why didn't you all use device tree?  How did this sneak in past
> > everyone?
> 
> DFL is a pretty efficient implementation in terms of resource
> utilization on the FPGA end (a couple of registers / memories) vs
> several kilobytes of memory for a device-tree blob.
> 
> The hardware using DFL to describe its internal structure exists in the
> form of deployed accelerator cards and telling all its users to go and
> change their hardware design would be feasible -- If you think about an
> FPGA as a (albeit reconfigurable) ASIC you wouldn't go and tell people
> to redesign their ASIC to use Device-Tree? :)
> 
> I'm not sure where the 'sneaking in' anything comes from. It's been
> reviewed on the list (and by yourself back then). If you feel any of
> this wasn't kosher, let's talk about it, to make sure it doesn't happen
> again.

I can't remember reviewing it, sorry, too many patches, I probably was
only worrying about functionality issues, not what the code actually did
:)

So it's fine, just a surprise to me, no big deal.

thanks,

greg k-h

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