Re: [PATCH v8 2/4] fpga manager: add sysfs interface document

Pantelis Antoniou
2015-01-14T19:01:44Z

Hi Jason,

> On Jan 14, 2015, at 20:12 , Jason Gunthorpe <jgunthorpe@obsidianresearch.com> wrote:
>
> On Wed, Jan 14, 2015 at 04:06:17PM +0000, One Thousand Gnomes wrote:
>
>> and I think you effectively have the user usage covered here for such
>> things. It much like GPIO pins - we can describe them but we can also
>> declare they are not visible to the user.
>
> A missing element in mainline is a kind of VFIO scheme to let user
> space access the FPGA registers designated for user space use.
>
> We have been using these patches since 2.6.16 to allow user space to
> access the FPGA register resources via a PCI like /sys/../resource0
> mmapable:
>
> https://github.com/jgunthorpe/linux/commit/59d5d13ddeffa8980ccc6248ebb5f1678ccb23f4
> https://github.com/jgunthorpe/linux/commit/7c29c4344627be8a3906d64d32db533bc131df86
> https://github.com/jgunthorpe/linux/commit/e41bb4a197368a9d505d66b627aee82f2d2b8895
>
> We deliberately split up the FPGA registers and the assign user space
> permissions to the resource0 files in a way that makes sense for our
> app. Typically there are 10-20 FPGA register regions. User space does
> not access register regions that control DMA.
>

Interesting.

>> The swappable case mostly comes out of the /dev node. Once you have the
>> dev node it becomes a detail of the OS not the FPGA driver as to who may
>> open it, and how it is handed about. It might be an FPU manager daemon it
>> might not.
>
> Right, but the thing that scares me about the swappable case is the
> kernel side support.. The FPGA has to connect to the CPU in some way,
> it must have some address assigned, etc. Swapping needs to take care
> of all those details in some way.
>
> A fixed bus interface block and dynamic reconfiguration for the
> remainder is probably the way to manage that. But, that implies that
> even a family of swappable FPGAs will have a DT overlay associated
> with it.
>
> Ideally, I could see wanting to have a file format that combines the
> overlay and FPGA bitfile. A loader tool would use the /dev/ interface
> to setup both elements. That would be much more robust than the
> current scheme I see (eg Xilinx) using where the bitfile and DT bit
> live in completely different places and have to be perfectly matched.
>

A single DT property can be 4 gigs of binary data (cheating a bit, the whole blob can be
4 gigs). You can conceivably include the whole bitfile in the DT blob.

Whether this is a perversion or not it’s left to the reader. The bitfile does
describe the hardware though.

> Jason

Regards

— Pantelis