Re: [PATCH 2/8] driver-core: add asynchronous probing support for drivers

[Dan Williams]

On Mon, Jul 6, 2015 at 4:40 PM, Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> wrote:
> On Tue, Jul 07, 2015 at 01:23:15AM +0200, Luis R. Rodriguez wrote:
>> On Sat, Jul 04, 2015 at 07:09:19AM -0700, Dan Williams wrote:
>> > On Fri, Jul 3, 2015 at 11:30 AM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
>> > > On Sat, Jun 27, 2015 at 04:45:25PM -0700, Dan Williams wrote:
>> > >> On Mon, Mar 30, 2015 at 4:20 PM, Dmitry Torokhov
>> > >> <dmitry.torokhov@xxxxxxxxx> wrote:
>> > >> > Some devices take a long time when initializing, and not all drivers are
>> > >> > suited to initialize their devices when they are open. For example,
>> > >> > input drivers need to interrogate their devices in order to publish
>> > >> > device's capabilities before userspace will open them. When such drivers
>> > >> > are compiled into kernel they may stall entire kernel initialization.
>> > >> >
>> > >> > This change allows drivers request for their probe functions to be
>> > >> > called asynchronously during driver and device registration (manual
>> > >> > binding is still synchronous). Because async_schedule is used to perform
>> > >> > asynchronous calls module loading will still wait for the probing to
>> > >> > complete.
>> > >> >
>> > >> > Note that the end goal is to make the probing asynchronous by default,
>> > >> > so annotating drivers with PROBE_PREFER_ASYNCHRONOUS is a temporary
>> > >> > measure that allows us to speed up boot process while we validating and
>> > >> > fixing the rest of the drivers and preparing userspace.
>> > >> >
>> > >> > This change is based on earlier patch by "Luis R. Rodriguez"
>> > >> > <mcgrof@xxxxxxxx>
>> > >> >
>> > >> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
>> > >> > ---
>> > >> >  drivers/base/base.h    |   1 +
>> > >> >  drivers/base/bus.c     |  31 +++++++---
>> > >> >  drivers/base/dd.c      | 149 ++++++++++++++++++++++++++++++++++++++++++-------
>> > >> >  include/linux/device.h |  28 ++++++++++
>> > >> >  4 files changed, 182 insertions(+), 27 deletions(-)
>> > >>
>> > >> Just noticed this patch.  It caught my eye because I had a hard time
>> > >> getting an open coded implementation of asynchronous probing to work
>> > >> in the new libnvdimm subsystem.  Especially the messy races of tearing
>> > >> things down while probing is still in flight.  I ended up implementing
>> > >> asynchronous device registration which eliminated a lot of complexity
>> > >> and of course the bugs.  In general I tend to think that async
>> > >> registration is less risky than async probe since it keeps wider
>> > >> portions of the traditional device model synchronous
>> > >
>> > > but its not see -DEFER_PROBE even before async probe.
>> >
>> > Except in that case you know probe has been seen by the driver at
>> > least once.  So I see that as less of a surprise, but point taken.
>> >
>> > >> and leverages the
>> > >> fact that the device model is already well prepared for asynchronous
>> > >> arrival of devices due to hotplug.
>> > >
>> > > I think this sounds reasonable, do you have your code upstream or posted?
>> >
>> > Yes, see nd_device_register() in drivers/nvdimm/bus.c
>>
>> It should be I think rather easy for Dmitry to see if he can convert this input
>> driver (not yet upstream) to this API and see if the same issues are fixed.
>
> No, I would rather not as it means we lose error handling on device
> registration.
>

I think this is a red herring as I don't see how async probing is any
better at handling device registration errors.  The error is logged
and "handled" by the fact that a device fails to appear, what other
action would you take?  In fact libnvdimm does detect registration
failures and reports that in a parent device attribute (at least for a
region device and their namespace child devices).
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