Re: [PATCH v2 1/2] Documentation: fpga: dfl: add PCI Identification documentation

[matthew . gerlach]

On Fri, 4 Mar 2022, Russ Weight wrote:

> On 3/3/22 14:04, Tom Rix wrote:
> > On 3/2/22 4:35 PM, matthew.gerlach@xxxxxxxxxxxxxxx wrote:
> > > From: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>
> > >
> > > Add documentation on identifying FPGA based PCI cards prompted
> > > by discussion on the linux-fpga@xxxxxxxxxxxxxxx mailing list.
> > >
> > > Signed-off-by: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>
> > > ---
> > > v2: Introduced in v2.
> > > ---
> > >   Documentation/fpga/dfl.rst | 20 ++++++++++++++++++++
> > >   1 file changed, 20 insertions(+)
> > >
> > > diff --git a/Documentation/fpga/dfl.rst b/Documentation/fpga/dfl.rst
> > > index ef9eec71f6f3..5fb2ca8e76d7 100644
> > > --- a/Documentation/fpga/dfl.rst
> > > +++ b/Documentation/fpga/dfl.rst
> > > @@ -502,6 +502,26 @@ Developer only needs to provide a sub feature driver with matched feature id.
> > >   FME Partial Reconfiguration Sub Feature driver (see drivers/fpga/dfl-fme-pr.c)
> > >   could be a reference.
> > >   +PCI Device Identification
> > > +================================
> > > +Since FPGA based PCI cards can be reconfigured to a perform a completely
> > > +new function at runtime, properly identifying such cards and binding the
> > > +correct driver can be challenging. In many use cases, deployed FPGA based
> > > +PCI cards are essentially static and the PCI Product ID and Vendor ID pair
> > > +is sufficient to identify the card.  The DFL framework helps with the
> > > +dynamic case of deployed FPGA cards changing at run time by providing
> > > +more detailed information about card discoverable at runtime.
> > > +
> > > +At one level, the DFL on a PCI card describes the function of the card.
> > > +However, the same DFL could be instantiated on different physical cards.
> > > +Conversely, different DFLs could be instantiated on the same physical card.
> > > +Practical management of a cloud containing a heterogeneous set of such cards
> > > +requires a PCI level of card identification. While the PCI Product ID and
> > > +Vendor ID may be sufficient to bind the dfl-pci driver, it is expected
> > > +that FPGA PCI cards would advertise suitable Subsystem ID and Subsystem
> > > +Vendor ID values. PCI Vital Product Data (VPD) can also be used for
> > > +more granular information about the board.
> >
> > This describes a bit more of the problem, it should describe it wrt ofs dev id. The introduction of the ofs dev should be explicitly called out as a generic pci id.

The problem I'm describing exists for all FPGA based PCI cards; so I am 
purposely trying to be abstract as much as possible.

> > Why couldn't one of the old pci id's be reused ?

Yes, old pci id's could be reused, and people have done just that.  We 
thought a new PCI ID would minimize confusion with cards that have already 
been deployed.

> > How will the subvendor/subid be enforced ?

Subvendor and Subid are managed just like any other PCI card with or 
without a FPGA.

> > Is the current security manager patchset smart enough to save the board from being bricked when a user doesn't look beyond the pci id ?
>
> Yes - the security manager is invoked based of DFL feature ID and revision, and the functionality is differentiated based on the same information.
>
> > What happens if a board uses this device id but doesn't have a max10 to do the update ?

If a board doesn't have a max10, then there will be no DFH for a max10 in 
the board's DFLs.  Presumeably, the board would need some update process, 
and an approprate DFH would be in that board's DFL.

> > Tom
> >
> > > +
> > >   Location of DFLs on a PCI Device
> > >   ================================
> > >   The original method for finding a DFL on a PCI device assumed the start of the