linux-kernel - Re: [PATCH 02/16] fpga: add FPGA device framework

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Date:   Fri, 31 Mar 2017 11:03:28 +0200
From:   Greg KH <greg@...ah.com>
To:     Wu Hao <hao.wu@...el.com>
Cc:     atull@...nel.org, moritz.fischer@...us.com,
        linux-fpga@...r.kernel.org, linux-kernel@...r.kernel.org,
        luwei.kang@...el.com, yi.z.zhang@...el.com,
        Tim Whisonant <tim.whisonant@...el.com>,
        Enno Luebbers <enno.luebbers@...el.com>,
        Shiva Rao <shiva.rao@...el.com>,
        Christopher Rauer <christopher.rauer@...el.com>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Subject: Re: [PATCH 02/16] fpga: add FPGA device framework

On Fri, Mar 31, 2017 at 03:48:42PM +0800, Wu Hao wrote:
> On Fri, Mar 31, 2017 at 08:09:09AM +0200, Greg KH wrote:
> > On Thu, Mar 30, 2017 at 08:08:02PM +0800, Wu Hao wrote:
> > > 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.
> > > 
> > > '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
> > > 
> > > This following APIs are provided by FPGA device framework:
> > > * fpga_dev_create
> > >   Create fpga device under the given parent device.
> > > * fpga_dev_destroy
> > >   Destroy fpga device
> > > 
> > > The following sysfs files are created:
> > > * /sys/class/fpga/<fpga.x>/name
> > >   Name of the fpga device.
> > 
> > How does this interact with the existing "fpga class" that is in the
> > kernel already?
> 
> The fpga-dev introduced by this patch, is only a container device, and
> drivers could register different functions under it. Per my understanding,
> the existing "fpga class", including fpga-region, fpga-bridge and 
> fpga-manager, is used to provide reconfiguration function for FPGA. So
> driver can create child node using this existing "fpga class" to provide
> FPGA reconfiguration function, and more nodes under this container for
> different functions for given FPGA device.
> 
> For Intel FPGA device, partial reconfiguration is only one function of 
> Intel FPGA Management Engine (FME). FME driver creates fpga_manager under
> below path for partial reconfiguration, and other interfaces for more
> functions, e.g power management, virtualization support and etc.
> 
> /sys/class/fpga/<fpga.x>/<intel-fpga-fme.x>/fpga_manager

So there is now two different levels of fpga class interfaces?

I'm not disagreeing with this, just that it seems a bit confusing, don't
you think?

greg k-h