linux-kernel - Re: KOBJ

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Date:   Mon, 5 Nov 2018 14:44:21 +0100
From:   Jonas Bonn <jonas@...rbonn.se>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, rafael@...nel.org
Subject: Re: KOBJ_BIND uevent

Hi,

On 05/11/2018 14:21, Greg KH wrote:
> On Mon, Nov 05, 2018 at 11:35:57AM +0100, Jonas Bonn wrote:
>> Hi,
>>
>> I have a question about the ordering of uevents, specifically concerning
>> complex USB devices that present multiple interfaces/functions.
>>
>> Before KOBJ_BIND, a USB device would typically present itself as:
>>
>> add usb_device
>> add usb_interface-1
>> add subsystem-device-1.0
>> add subsystem-device-1.1
>> add usb_interface-2
>> add subsystem-device-2.0
>>
>> I have noted that the recently added "bind" actions, however, present in the
>> reverse order.
>>
>> bind subsystem-device-1.0
>> bind subsystem-device-1.1
>> bind usb-interface-1
>> bind subsystem-device-2.0
>> bind usb_interface-2
>> bind usb_device
>>
>> This secondary ordering could be useful in the sense that the final "bind"
>> action on the usb_device is an indication that the kernel has finished
>> enumeration of all endpoints and has bound all drivers that it could to the
>> available interfaces... i.e. no further events for this device are expected.
> 
> Maybe.  Maybe not, as userspace might still be in the process of loading
> new kernel drivers based on the add uevents that got sent out.  Then
> binding would happen later after the usb_device was "bound".
> 
>> The question, then, is:  is the above ordering of "bind" events stable, or
>> is it just a consequence of the current implementation and may change in the
>> future?
> 
> Not stable at all, sorry, you can not depend on it.
> 
> Nor should you even try to, what problem are you wanting to solve here?

Specifically, I'm dealing with USB modems that fall into two categories:

i)  present a bunch of USB interfaces with one ACM (or similar) class 
device on each interface (one TTY for data, control, GPS, etc.)

ii)  present one interface with multiple endpoints that end up as 
multiple Linux devices (typically usbmisc device + network interface)

In both cases, the modem really isn't useful until all the 
interfaces/endpoints are ready.  It would be nice to be able to have one 
uevent that indicates that the kernel has done all it can with the 
device so we can evaluate whether we have everything we need.

Currently, we set a 1 second delay from the "add usb_device" before 
proceeding to evaluate the available modem interfaces; this delay is 
normally sufficient for everything to be enumerated and drivers bound, 
but it's still just an arbitrary delay.

Thanks,
Jonas

> 
> thanks,
> 
> greg k-h
>