Date:   Mon, 8 May 2017 13:52:45 -0700
From:   Moritz Fischer <mdf@...nel.org>
To:     Alan Tull <atull@...nel.org>
Cc:     "Wu, Hao" <hao.wu@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "linux-fpga@...r.kernel.org" <linux-fpga@...r.kernel.org>,
        "matthew.gerlach@...ux.intel.com" <matthew.gerlach@...ux.intel.com>
Subject: Re: [PATCH v2 02/16] fpga: bridge: support getting bridge from device

On Mon, May 8, 2017 at 1:44 PM, Alan Tull <atull@...nel.org> wrote:
> On Mon, May 8, 2017 at 3:44 AM, Wu, Hao <hao.wu@...el.com> wrote:
>>> On Wed, May 3, 2017 at 3:07 PM, Alan Tull <atull@...nel.org> wrote:
>>> > On Wed, May 3, 2017 at 6:58 AM, Wu Hao <hao.wu@...el.com> wrote:
>>> >> On Thu, Apr 20, 2017 at 09:09:47AM -0500, Alan Tull wrote:
>>> >>> Add two functions for getting the FPGA bridge from the device
>>> >>> rather than device tree node.  This is to enable writing code
>>> >>> that will support using FPGA bridges without device tree.
>>> >>> Rename one old function to make it clear that it is device
>>> >>> tree-ish.  This leaves us with 3 functions for getting a bridge:
>>> >>>
>>> >>> * fpga_bridge_get
>>> >>>   Get the bridge given the device.
>>> >>>
>>> >>> * fpga_bridges_get_to_list
>>> >>>   Given the device, get the bridge and add it to a list.
>>> >>>
>>> >>> * of_fpga_bridges_get_to_list
>>> >>>   Renamed from priviously existing fpga_bridges_get_to_list.
>>> >>>   Given the device node, get the bridge and add it to a list.
>>> >>>
>>> >>
>>> >> Hi Alan
>>> >>
>>> >> Thanks a lot for providing this patch set for non device tree support. :)
>>> >> Actually I am reworking the Intel FPGA device drivers based on this patch
>>> >> set, and I find some problems with the existing APIs including fpga bridge
>>> >> and manager. My idea is to create all fpga bridges/regions/manager under
>>> >> the same platform device (FME), it allows FME driver to establish the
>>> >> relationship for the bridges/regions/managers it creates in an easy way.
>>> >> But I found current fpga class API doesn't support this very well.
>>> >> e.g fpga_bridge_get/get_to_list only accept parent device as the input
>>> >> parameter, but it doesn't work if we have multiple bridges (and
>>> >> regions/manager) under the same platform device. fpga_mgr has similar
>>> >> issue, but fpga_region APIs work better, as they accept fpga_region as
>>> >> parameter not the shared parent device.
>>> >
>>> > That's good feedback.  I can post a couple patches that apply on top
>>> > of that patchset to add the APIs you need.
>>> >
>>> > Probably what I'll do is add
>>> >
>>> > struct fpga_manager *fpga_mgr_get(struct fpga_manager *mgr);
>>> >
>>> > And rename fpga_bridge_get() to fpga_bridge_dev_get() and add the
>>> following:
>>> >
>>> > struct fpga_bridge *fpga_bridge_get(struct fpga_bridge *br,
>>> >                                 struct fpga_image_info *info);
>>> >
>>> > int of_fpga_bridge_get_to_list(struct fpga_bridge *br,
>>> >                                struct fpga_image_info *info,
>>> >                                struct list_head *bridge_list);
>>> >
>>> > Working on it now.
>>> >
>>> >>
>>> >> Do you think if having multiple fpga-* under one parent device is in the
>>> >> right direction?
>>> >
>>> > That should be fine as long as it's coded with an eye on making things
>>> > reusable and seeing beyond the current project.  Just thinking of the
>>> > future and of what can be of general usefulness for others.  And there
>>> > will be others interested in reusing this.
>>> >
>>> > Alan
>>>
>>> Actually,  I don't think you will need the additional APIs we were
>>> just discussing after all.  What you have is a multifunction device
>>> (single piece of hardware, multi functions such as in drivers/mfd).
>>> It will have child devices for the mgr, bridges, and regions.  When
>>> registering the mgr and bridges you will need to allocate child
>>> devices and use them to create the mgr and bridges.
>>>
>>> Alan
>>
>> Hi Alan
>>
>> I tried to create child devices as the parent device for the mgr and
>> bridges in fme platform driver module. If only creates the device without
>> driver, it doesn't work as try_module_get(dev->parent->driver->owner)
>> always failed in mgr_get and bridge_get functions.
>
> I tried it and it wasn't hard.
>
> Each mgr or bridge driver should be a separate file which registers
> its driver using 'module_platform_driver".  That way the drivers are
> registered with the kernel in a normal fashion.  The thing we want
> here is to not bypass the kernel driver model.
>
> You'll need to keep the platform_device pointers in private data somewhere.
>
> For each child platform device, do a platform_device_alloc and
> platform_device_add.
>
> Then to get the manager, you can do
>
> mgr = fpga_mgr_get(&priv->mgr_pdev->dev);
>
> If this is in your probe function, you can use -EPROBE_DEFER if
> platform_device_alloc or fpga_mgr_get fail.  Then you could destroy
> whatever you've created and return -EPROBE_DEFER to wait for the
> drivers you need to be registered and ready for devices to be added.
>
>>
>> If it creates platform devices as child devices, and introduce new platform
>> device drivers for bridge and mgr, then it will be difficult to establish the
>> relationship for region/mgr/bridges (e.g when should region->mgr be
>> configured and cleared, as mgr is created/destroyed when mgr parent
>> device platform driver module is loaded/unload), and it maybe not really
>> necessary to introduce more different driver modules here.
>
> It should be pretty easy to create/destroy child devices as shown
> above.  The kernel does this all the time.
>
>>
>> But if it allows multiple fpga-* created under one device in one device
>> driver, it will be much easier to avoid above problems. So I asked if it
>> is possible to create multiple fpga-* under one parent device,
>
> I think it's fine for your FME to create child platform devices.  It's
> similar to a mfd, but the mfd framework hides the platform devices
> from the module that creates them, unfortunately.
>
>> I feel
>> this will not impact to current fpga drivers a lot, but provide more
>> flexibility for drivers to use fpga-region/bridge/manager to create
>> the topology in a device specific way, especially for non device
>> tree case.
>>
>
> I would like to see most of this code as FME enumeration code + a mgr
> driver + a bridge driver + a region driver.  If the FME and the
> enumeration code can be separate files, so much the better for general
> usability.
>
> The enumeration code can build a set of regions by doing something like this:
> 1. figure out what type of mgr and bridges your hardware FME has.
> 2. do platform_device_alloc and platform_device_add to create the mgr
> device, save a pointer to its platform_device in your FME driver's
> private data.
> 2. For each port, create a region and a bridge device.  Save the
> region's platform device or struct in a list in your FME driver's
> priv.
> 3. then you can create the sub function devices.

The above sounds like a poster-child application for MFD. If you do it
in a clever
way (i.e. write your platform drivers in a reusable way) you might be able to
just reuse them on your next generation.

Cheers,

Moritz

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