linux-kernel - Re: [PATCH v3 1/5] fpga: dfl: rename the bus type "dfl" to "fpga-dfl"

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Message-ID: <20200929041900.GA113620@archbook>
Date:   Mon, 28 Sep 2020 21:19:00 -0700
From:   Moritz Fischer <mdf@...nel.org>
To:     Xu Yilun <yilun.xu@...el.com>
Cc:     mdf@...nel.org, Greg KH <gregkh@...uxfoundation.org>,
        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"

Hi Xu,

On Tue, Sep 29, 2020 at 09:23:23AM +0800, Xu Yilun wrote:
> Hi moritz:
> 
> On Sun, Sep 27, 2020 at 04:36:47PM +0800, Xu Yilun wrote:
> > Hi Greg,
> > 
> > On Sun, Sep 27, 2020 at 09:54:01AM +0200, Greg KH wrote:
> > > On Sun, Sep 27, 2020 at 03:37:54PM +0800, Xu Yilun wrote:
> > > > Hi Greg,
> > > > 
> > > > On Sun, Sep 27, 2020 at 07:51:08AM +0200, Greg KH wrote:
> > > > > 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.
> > 
> > I thought we have consensus that "dfl" could be used out of fpga domain.
> > And we are all good that we keep the user ABIs & the bus name - "dfl", so "dfl"
> > is good as a global name from linux user's point of view, is it?
> > 
> > But why we reject the "dfl" in kernel code domain? I thought it is very
> > similar situation.
> > 
> > 
> > I think we have 2 options, to make the dfl self-consistent:
> > 
> > 1. "dfl-fpga" for everything - bus name, user ABIs, structures & APIs for
> >    other kernel subsystems. Then we lose the chance to support ASIC based DFL,
> >    it would be hard if we change user ABIs later.
> > 
> > 2. "dfl" for everything.
> > 
> > BTW, no ASIC based DFL devices in kernel today.
> > 
> > I fully understand the word "naming is hard" now, help me :)
> 
> Seems now we have different opinions on this:
> 
> - Hao thinks self-consistent is important to dfl framework.
Agreed. I mostly care about userspace facing ABI, though.

> - "dfl" for everything seems not preferable to Greg.
Maybe now that we re-explained, we can take another look at that?

> - From your previous mail, I assume you prefer to keep the bus name as "dfl"
> but change the stuff for other subsystem, is it?

I mostly think we should keep DFL generic where it touches userspace and
defines ABI, since we cannot change it afterwards.

I rest my point with the bus being independent of FPGAs despite the FPGA
being the (currently) only user.

> So I got a little stuck here.
> 
> Do you think "dfl-fpga" for everything would be an acceptable solution
> for you?

I just think it doesn't make a lot of sense to call it fpga-dfl or
dfl-fpga. But if everyone else disagrees ... naming is hard :-)

Cheers,
Moritz