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

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Date:   Fri, 31 Mar 2017 12:01:13 -0700 (PDT)
From:   matthew.gerlach@...ux.intel.com
To:     Wu Hao <hao.wu@...el.com>
cc:     Greg KH <greg@...ah.com>, 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, 31 Mar 2017, 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

I completely understand the need for a container device.  The fpga-region 
is also primarily a container, and in some cases the fpga-region may 
represent the entire fpga.  Over time this code may become redundant.

> 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
>
> Thanks
> Hao
>
>>
>> thanks,
>>
>> greg k-h
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