On Mon, Jul 31, 2017 at 04:40:16PM -0500, Alan Tull wrote: > On Thu, Jul 27, 2017 at 2:10 PM, Rob Herring <robh+dt@xxxxxxxxxx> wrote: > > On Thu, Jul 27, 2017 at 11:35 AM, Alan Tull <atull@xxxxxxxxxx> wrote: > >> On Sun, Jun 25, 2017 at 8:51 PM, Wu Hao <hao.wu@xxxxxxxxx> wrote: > >> > >> Hi Rob, > >> > >> I was hoping to pick your brain a bit on a DT question. > >> > >>> During FPGA device (e.g PCI-based) discovery, platform devices are > >>> registered for different FPGA function units. But the device node path > >>> isn't quite friendly to applications. > >>> > >>> Consider this case, applications want to access child device's sysfs file > >>> for some information. > >>> > >>> 1) Access using bus-based path (e.g PCI) > >>> > >>> /sys/bus/pci/devices/xxxxx/fpga_func_a.0/sysfs_file > >>> > >>> From the path, it's clear which PCI device is the parent, but not perfect > >>> solution for applications. PCI device BDF is not fixed, application may > >>> need to search all PCI device to find the actual FPGA Device. > >>> > >>> 2) Or access using platform device path > >>> > >>> /sys/bus/platform/devices/fpga_func_a.0/sysfs_file > >>> > >>> Applications find the actual function by name easily, but no information > >>> about which fpga device it belongs to. It's quite confusing if multiple > >>> FPGA devices are in one system. > >> > >> There's a proposal for adding sysfs nodes that correspond to each FPGA > >> device., with the devices located on each FPGA under them. It makes > >> it easier to see which device is on which FPGA. > > > > Makes sense. > > > >>> 'FPGA Device' class is introduced to resolve this problem. Each node under > >>> this class represents a fpga device, which may have one or more child > >>> devices. Applications only need to search under this FPGA Device class > >>> folder to find the child device node it needs. > >>> > >>> For example, for the platform has 2 fpga devices, each fpga device has > >>> 3 child devices, the hierarchy looks like this. > >>> > >>> Two nodes are under /sys/class/fpga/: > >>> /sys/class/fpga/fpga.0 > >>> /sys/class/fpga/fpga.1 > >>> > >>> Each node has 1 function A device and 2 function B devices: > >>> /sys/class/fpga/fpga.0/func_a.0 > >>> /sys/class/fpga/fpga.0/func_b.0 > >>> /sys/class/fpga/fpga.0/func_b.1 > >>> > >>> /sys/class/fpga/fpga.1/func_a.1 > >>> /sys/class/fpga/fpga.1/func_b.2 > >>> /sys/class/fpga/fpga.1/func_b.3 > > > > A class is generally what is the function of the device, not how it is > > attached. Seems like what you want here is a new bus type if the > > existing PCI and platform bus types don't work. > > > >> > >> I can see the value of having sysfs nodes that correspond to fpga > >> devices and being able to find devices under them. I'm thinking what > >> that would mean for Device Tree when fpga-dev is used on DT enabled > >> systems. In Device Tree, what is a fpga-dev? > > > > Just properly setting the parent struct device on the functions should > > be enough to figure out which function is in which fpga. I don't see > > why a new class is needed. > > > >> Currently the DT would have a FPGA bridge corresponding to each FPGA's > >> hardware bridge and a heirarchy of bridges, regions and devices under > >> it. On systems that don't support partial reconfiguration under the > >> OS (so not main bridge that was controlled by the OS), there would be > >> a FPGA region, then its child regions, bridges, and devices. > > > > The FPGA bridges could instantiate fpga bus type devices instead of > > platform devices. > > Yes > > Some FPGA use cases already have a base bridge per FPGA that could > serve as this bus. But this use case has a static FPGA image + > reprogrammable child fpga regions. There's no base bridge under Linux > since the FPGA was programmed and the bridge enabled before Linux > boots. An added base bridge that doesn't touch hardware will be > required for this type of use. Hi Alan Does 'base bridge' mentioned above mean a hardware bridge just like PCIe or USB? I tried to use fpga bus type device instead of fpga-dev class today, it works for me, e.g Intel FPGA device PCIe driver could create a fpga bus type dev as a child of PCIe device and its sysfs path will be changed to /sys/bus/fpga/devices/fpga.x/ from /sys/class/fpga/fpga.x/. For now, this fpga bus type device is only used as container device, so no driver needed for it. Do you have any concern on this? I see fpga bus type works fine, but I didn't see other advantages for this case, as we only use it as a container device to represent a FPGA device in sysfs hierarchy. :) Thanks Hao > > > That's really up to Linux and outside the scope of > > the bindings. > > Thanks for the feedback. > > Alan Tull > > > > > Rob -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
