Re: [PATCH v1 00/12] add FPGA hotplug manager driver

From: Greg KH <gregkh@linuxfoundation.org>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Tianfei Zhang <tianfei.zhang@intel.com>,
	bhelgaas@google.com, linux-pci@vger.kernel.org,
	linux-fpga@vger.kernel.org, lukas@wunner.de, kabel@kernel.org,
	mani@kernel.org, pali@kernel.org, mdf@kernel.org,
	hao.wu@intel.com, yilun.xu@intel.com, trix@redhat.com,
	jgg@ziepe.ca, ira.weiny@intel.com,
	andriy.shevchenko@linux.intel.com, dan.j.williams@intel.com,
	keescook@chromium.org, russell.h.weight@intel.com,
	corbet@lwn.net, linux-doc@vger.kernel.org,
	ilpo.jarvinen@linux.intel.com, lee@kernel.org,
	matthew.gerlach@linux.intel.com
Subject: Re: [PATCH v1 00/12] add FPGA hotplug manager driver
Date: Thu, 19 Jan 2023 16:33:05 +0100	[thread overview]
Message-ID: <Y8lisT7xmkd1/g+J@kroah.com> (raw)
In-Reply-To: <CAJZ5v0jW28k5+CN_F4dLo=OzVVJEL+U=gW4bzeSPjU=j53BJKg@mail.gmail.com>

On Thu, Jan 19, 2023 at 02:43:21PM +0100, Rafael J. Wysocki wrote:
> On Thu, Jan 19, 2023 at 2:34 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Wed, Jan 18, 2023 at 08:35:50PM -0500, Tianfei Zhang wrote:
> > > This patchset introduces the FPGA hotplug manager (fpgahp) driver which
> > > has been verified on the Intel N3000 card.
> > >
> > > When a PCIe-based FPGA card is reprogrammed, it temporarily disappears
> > > from the PCIe bus. This needs to be managed to avoid PCIe errors and to
> > > reprobe the device after reprogramming.
> > >
> > > To change the FPGA image, the kernel burns a new image into the flash on
> > > the card, and then triggers the card BMC to load the new image into FPGA.
> > > A new FPGA hotplug manager driver is introduced that leverages the PCIe
> > > hotplug framework to trigger and manage the update of the FPGA image,
> > > including the disappearance and reappearance of the card on the PCIe bus.
> > > The fpgahp driver uses APIs from the pciehp driver. Two new operation
> > > callbacks are defined in hotplug_slot_ops:
> > >
> > >   - available_images: Optional: available FPGA images
> > >   - image_load: Optional: trigger the FPGA to load a new image
> > >
> > >
> > > The process of reprogramming an FPGA card begins by removing all devices
> > > associated with the card that are not required for the reprogramming of
> > > the card. This includes PCIe devices (PFs and VFs) associated with the
> > > card as well as any other types of devices (platform, etc.) defined within
> > > the FPGA. The remaining devices are referred to here as "reserved" devices.
> > > After triggering the update of the FPGA card, the reserved devices are also
> > > removed.
> > >
> > > The complete process for reprogramming the FPGA are:
> > >     1. remove all PFs and VFs except for PF0 (reserved).
> > >     2. remove all non-reserved devices of PF0.
> > >     3. trigger FPGA card to do the image update.
> > >     4. disable the link of the hotplug bridge.
> > >     5. remove all reserved devices under hotplug bridge.
> > >     6. wait for image reload done via BMC, e.g. 10s.
> > >     7. re-enable the link of hotplug bridge
> > >     8. enumerate PCI devices below the hotplug bridge
> > >
> > > usage example:
> > > [root@localhost]# cd /sys/bus/pci/slot/X-X/
> > >
> > > Get the available images.
> > > [root@localhost 2-1]# cat available_images
> > > bmc_factory bmc_user retimer_fw
> > >
> > > Load the request images for FPGA Card, for example load the BMC user image:
> > > [root@localhost 2-1]# echo bmc_user > image_load
> >
> > Why is all of this tied into the pci hotplug code? Shouldn't it be
> > specific to this one driver instead?  pci hotplug is for removing/adding
> > PCI devices to the system, not messing with FPGA images.
> >
> > This feels like an abuse of the pci hotplug bus to me as this is NOT
> > really a PCI hotplug bus at all, right?
> >
> > Or is it?  If so, then the slots should show up under the PCI device
> > itself, not in /sys/bus/pci/slot/.  That location is there for old old
> > stuff, we probably should move it one of these days as there's lots of
> > special-cases in the driver core just because of that :(
> 
> I'm not sure if I can agree with this statement.
> 
> The slot here is what is registered via pci_hp_register(), isn't it?

Yes, but is it really a "slot" like a normal PCI slot?

> There are multiple users of this in the tree, including ACPI-based PCI
> hotplug, which is not really that old.

It's really old, I think I worked on that in the 2.4/2.5 days?  Anyway,
it's been around a long time.

> Are you saying that this should not be used?

I'm saying that PCI is the only subsystem/bus that has something like
this and we have a number of functions exported in the driver core only
for the pci hotplug slot list.  Which kind of implies that maybe it
should be moved to something else?  I don't have any specific ideas what
it should be, just that it feels really odd as-is still.

thanks,

greg k-h