devctl(8): A device control utility

[John Baldwin]

On 1/12/15 12:01 PM, Warner Losh wrote:
> 
>> On Jan 12, 2015, at 9:16 AM, John Baldwin <jhb at FreeBSD.org> wrote:
>>
>> On 1/5/15 4:18 PM, John Baldwin wrote:
>>> On Monday, January 05, 2015 09:58:19 PM Hans Petter Selasky wrote:
>>>> On 01/05/15 21:37, John Baldwin wrote:
>>>>> On 1/5/15 3:13 PM, Hans Petter Selasky wrote:
>>>>>> On 01/05/15 21:01, John Baldwin wrote:
>>>>>>> The devctl(8) utility is then a thin wrapper around libdevctl (and
>>>>>>> does not
>>>>>>> yet have a manpage).
>>>>>>>
>>>>>>> Do folks have any feedback?
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> In the USB area attach and detach must be synchronized to the USB stack
>>>>>> and "usbconfig -d X.Y set_config Z" or "usbconfig -d X.Y reset" should
>>>>>> be used to avoid races attaching and detaching drivers!
>>>>>
>>>>> I think this points to one or more missing bus methods so that the bus
>>>>> can hook into device_probe_and_attach() to reset a device as needed.
>>>>> (e.g. if you had bus_probe_started / bus_probe_finished and possibly
>>>>> similar methods for attach.  PCI could use a bus_attach_finished()
>>>>> callback so that it could clean up any dangling resources and possibly
>>>>> power down on a failed attach the way it does in bus_child_detached as
>>>>> well).
>>>>
>>>> Hi,
>>>>
>>>> USB has its own threads to allocate/free devices. Another problem is how
>>>> to atomically get a reference count across multiple layers like PCI and
>>>> USB. It doesn't allow probe/attach when called from outside these threads.
>>>
>>> That just means you need to use some locks. :)  Cardbus also uses an event
>>> thread to handle auto-attach of devices when it detected a card change event,
>>> but that doesn't prevent it from servicing an ioctl request.
>>
>> Another option btw would be to add bus methods that wrap probe and
>> attach (rather than pre and post event hooks).  I wish bus_add_child()
>> were done this way such that device_add_child_ordered() were renamed to
>> bus_generic_add_child() (and was the default add_child method) and that
>> device_add_child_ordered() called 'BUS_ADD_CHILD()' so that
>> 'device_add_child()' was the proper public API (instead of exposing
>> BUS_ADD_CHILD()).  Similarly, I think that 'device_attach()' and
>> 'device_probe_and_attach()' should be the public API and that one way or
>> another we should add hooks to allow bus drivers to modify their
>> behavior if needed.  However, they should be fine for devctl ioctls to
>> invoke as well as other kernel bits.
> 
> When I was doing CardBus and PC Card I wished for similar things.  Then
> I realized I didn’t need them because as the bus author, I know when these
> events happened and could take appropriate actions for the bus. I didn’t have
> that atomic access issues though, since as the bus author I also controlled how
> and when mutexes were taken out and when I allowed access to the bus. I
> only used mutexes in CardBus and PC Card because most of the sleeps were
> short, but other ways to do locking are quite possible...

I think the problem here is that devctl introduces events that happen
without the bus's knowledge.

-- 
John Baldwin