Date:   Thu, 23 Apr 2020 08:23:31 +0200
From:   Sascha Hauer <s.hauer@...gutronix.de>
To:     Moritz Fischer <mdf@...nel.org>
Cc:     linux-fpga@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: How to upload fpga firmware

Hi Moritz,

On Wed, Apr 22, 2020 at 06:36:48PM -0700, Moritz Fischer wrote:
> Hi Sascha,
> 
> On Wed, Apr 22, 2020 at 01:44:32PM +0200, Sascha Hauer wrote:
> > Hi,
> > 
> > I wonder what can be done with the mainline state of drivers/fpga/. The
> > entry to the framework seems to be fpga_mgr_load(). The only user of
> > this function is fpga_region_program_fpga(). This in turn is only called
> > in response of applying a device tree overlay. A device tree overlay is
> > applied with of_overlay_fdt_apply() which has no users in the Kernel.
> 
> Yes. It is waiting for dt_overlays one way or another. I personally
> don't currently have the bandwidth to work actively on this.
> 
> > My current task is to load a firmware to a FPGA. The code all seems to
> > be there in the Kernel, it only lacks a way to trigger it. I am not very
> > interested in device tree overlays since the FPGA appears as a PCI
> > device (although applying a dtbo could enable the PCIe controller device
> > tree node). Is there some mainline way to upload FPGA firmware? At the
> > moment we are using the attached patch to trigger loading the firmware
> > from userspace. Would something like this be acceptable for mainline?
> 
> We've looked into this sort of patches over the years and never came to
> a general interface that really works.
> 
> The OPAE folks (and other users I know of) usually use FPGA Manager with
> a higher layer on top of it that moves the bitstream into the kernel via
> an ioctl().
> 
> One concept I had toyed with mentally, but haven't really gotten around
> to implement is a 'discoverable' region, that would deal with the
> necessary re-enumeration via a callback and have a sysfs interface
> similar to what the patch below has.
> This would essentially cover use-cases where you have a discoverable
> device implemented in FPGA logic, such as say an FPGA hanging off of
> PCIe bus that can get loaded over USB, a CPLD or some other side-band
> mechanism. After loading the image you'd have to rescan the PCIe bus -
> which - imho is the kernel's job.
> 
> What I really wanna avoid is creating another /dev/fpga0 / /dev/xdevcfg
> that completely leaves the kernel in the dark about the fact that it
> reconfigures a bit of hardware hanging off the bus.

Yes, makes sense. While this would suffice my needs at the moment it
really sounds like a dead end.

> 
> In my ideal world you'd create a pci driver that binds to your device,
> and creates mfd style subdevices for whatever you'd want your design to
> do. One of these devices would be an FPGA and a FPGA region attached to
> that FPGA manager. Your top level driver would co-ordinate the fact that
> you are re-programming parts of the FPGA and create / destroy devices as
> needed for the hardware contained in the bitstream.

In my case there is no pci device visible before loading the firmware,
so creating a pci driver is not an option. Maybe pci host controllers
could register themselves as fpga-bridges. With this we could put the
pci host controller (or the pci device, AFAIK there is a PCI device tree
binding) where the fpga is connected into the fpga-bridges phandles list
of the fpga-region.

Regards,
  Sascha

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