Re: [PATCH v2 02/16] fpga: bridge: support getting bridge from device

[Alan Tull]

On Mon, May 8, 2017 at 3:44 AM, Wu, Hao <hao.wu@xxxxxxxxx> wrote:
>> On Wed, May 3, 2017 at 3:07 PM, Alan Tull <atull@xxxxxxxxxx> wrote:
>> > On Wed, May 3, 2017 at 6:58 AM, Wu Hao <hao.wu@xxxxxxxxx> 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.

> Thanks
> Hao