Re: [RFC 00/12] On-demand device registration

[Alexander Holler]

Am 29.04.2015 um 08:58 schrieb Tomeu Vizoso:
> On 28 April 2015 at 20:17, Alexander Holler <holler@xxxxxxxxxxxxx> wrote:
> > Am 28.04.2015 um 14:49 schrieb Tomeu Vizoso:
> > > On 25 April 2015 at 01:15, Alexander Holler <holler@xxxxxxxxxxxxx> wrote:
> > > > Am 24.04.2015 um 16:47 schrieb Tomeu Vizoso:
> > > > > Hi,
> > > > >
> > > > > while reading the thread [0] that Alexander Holler started with his
> > > > > series to make probing order deterministic, it occurred to me that it should
> > > > > be possible to achieve the same by probing devices as they are referenced by
> > > > > other devices.
> > > > >
> > > > > This basically reuses the information that is already embedded in the
> > > > > probe() implementations, saving us from refactoring existing drivers or
> > > > > adding information to DTBs.
> > > > >
> > > > > The main issue I see is that the registration code path in some
> > > > > subsystems may not be reentrant, so some refactoring of the locking will be
> > > > > needed. In my testing I have found this problem with regulators, as the
> > > > > supply of a regulator might end up being registered during the registration
> > > > > of the first one.
> > > > >
> > > > > Something I'm not completely happy with is that I have had to move the
> > > > > population of the device tree after all platform drivers have been
> > > > > registered. Otherwise I don't see how I could register drivers on demand as
> > > > > we don't have yet each driver's compatible strings.
> > > > >
> > > > > I have done my testing on a Tegra124-based Chromebook, and these patches
> > > > > were enough to eliminate all the deferred probes.
> > > >
> > > >
> > > > First you have to solve a problem which is totally unrelated to DT or
> > > > ACPI or x86 or ARM:
> > > >
> > > > I think as long as drivers don't register themself whithout any side
> > > > effect, this problem isn't solvable. In order to make an ordered list of
> > > > drivers to start, you need to know which drivers are actually available.
> > >
> > >
> > > Yeah, I kind of side-stepped that issue by waiting until all drivers
> > > have been registered before registering devices. I think someone
> > > suggested doing so in your thread (maybe Grant?).
> >
> >
> > That doesn't work. As said above, several drivers doing a lot more than just
> > registering in their initcall. They might even crash if some prerequisits
> > aren't given. And several of these prerequisits (init orders) are hardcoded
> > by various means.
>
> But aren't those dependencies being taken care currently by the
> initcall level the driver is placed in? That remains the same in this
> approach.

In short, no. There are various very ugly things done in several drivers 
to enforce some order.

Regards,

Alexander Holler
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/