Re: [GIT PULL] On-demand device probing

["Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx>]

On Monday, October 19, 2015 05:58:40 PM Rob Herring wrote:
> On Mon, Oct 19, 2015 at 4:40 PM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> > On Monday, October 19, 2015 10:58:25 AM Rob Herring wrote:
> >> On Mon, Oct 19, 2015 at 10:29 AM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> >> > On Mon, 2015-10-19 at 15:50 +0100, Mark Brown wrote:
> >> >> > But the point I'm making is that we are working towards *fixing* that,
> >> >> > and *not* using DT-specific code in places where we should be using the
> >> >> > generic APIs.
> >> >>
> >> >> What is the plan for fixing things here?  It's not obvious (at least to
> >> >> me) that we don't want to have the subsystems having knowledge of how
> >> >> they are bound to a specific firmware which is what you seem to imply
> >> >> here.
> >> >
> >> > I don't know that there *is* a coherent plan here to address it all.
> >> >
> >> > Certainly, we *will* need subsystems to have firmware-specific
> >> > knowledge in some cases. Take GPIO as an example; ACPI *has* a way to
> >> > describe GPIO, and properties which reference GPIO pins are intended to
> >> > work through that — while in DT, properties which reference GPIO pins
> >> > will have different contents. They'll be compatible at the driver
> >> > level, in the sense that there's a call to get a given GPIO given the
> >> > property name, but the subsystems *will* be doing different things
> >> > behind the scenes.
> >> >
> >> > My plan, such as it is, is to go through the leaf-node drivers which
> >> > almost definitely *should* be firmware-agnostic, and convert those. And
> >> > then take stock of what we have left, and work out what, if anything,
> >> > still needs to be done.
> >>
> >> Many cases are already agnostic in the drivers in terms of the *_get()
> >> functions. Some are DT specific, but probably because those subsystems
> >> are new and DT only. In any case, I don't think these 1 line changes
> >> do anything to make doing conversions here harder.
> >>
> >> >> It seems like we're going to have to refactor these bits of code when
> >> >> they get generalised anyway so I'm not sure that the additional cost
> >> >> here is that big.
> >> >
> >> > That's an acceptable answer — "we're adding legacy code here but we
> >> > know it's going to be refactored anyway". If that's true, all it takes
> >> > is a note in the commit comment to that effect. That's different from
> >> > having not thought about it :)
> >>
> >> Considering at one point we did create a fwnode based API, we did
> >> think about it. Plus there was little input from ACPI folks as to
> >> whether the change was even useful for ACPI case.
> >
> > Well, sorry, but who was asking whom, specifically?
> 
> You and linux-acpi have been copied on v2 and later of the entire
> series I think.

Yes, but it wasn't like a direct request, say "We need your input, so can you
please have a look and BTW we want this in 4.4, so please do it ASAP".  In
which case I'd prioritize that before other things I needed to take care of.

> > The underlying problem is present in ACPI too and we don't really have a good
> > solution for it.  We might benefit from a common one if it existed.
> 
> The problem for DT is we don't generically know what are the
> dependencies at a core level. We could know some or most dependencies
> if phandles (links to other nodes) were typed, but they are not. If
> the core had this information, we could simply control the device
> creation to order probing. Instead, this information is encoded into
> the bindings and binding parsing resides in the subsystems. That
> parsing happens during probe of the client side and is done by the
> subsystems (for common bindings). Since we already do the parsing at
> this point, it is a convenient place to trigger the probe of the
> dependency. Is ACPI going to be similar in this regard?

It is similar in some ways.  For example, if a device's functionality depends
on an I2C resource (connection), the core doesn't know that at the device
creation time at least in some cases.  Same for GPIO, SPI, DMA engines etc.

There is a _DEP object in ACPI that can be used by firmware to tell the OS
about those dependencies, but there's no way in the driver core to use that
information anyway today.

> Fundamentally, it is a question of probe devices when their
> dependencies are present or drivers ensure their dependencies are
> ready. IIRC, init systems went thru a similar debate for service
> dependencies.

The probe ordering is not the entire picture, though.

Even if you get the probe ordering right, the problem is going to show up in
multiple other places: system suspend/resume, runtime PM, system shutdown,
unbinding of drivers.  In all of those cases it is necessary to handle things
in a specific order if there is a dependency.

> >> In any case, we're talking about adding 1 line.
> >
> > But also about making the driver core slighly OF-centric.
> 
> How so? The one line is in DT binding parsing code in subsystems, not
> driver core. The driver core change is we add every device (that
> happened to be created by DT) to the deferred probe list, so they
> don't probe right away.

The "that happened to be created by DT" part is of concern here.  What is there
that makes DT special in that respect?  Why shouldn't that be applicable to
devices created by the ACPI core, for example, or by a board file or something
else?

> > Sure, we need OF-specific code and ACPI-specific code wherever different
> > handling is required, but doing that at the driver core level seems to be
> > a bit of a stretch to me.
> >
> > Please note that we don't really have ACPI-specific calls in the driver core,
> > although we might have added them long ago even before the OF stuff appeared
> > in the kernel for the first time.  We didn't do that, (among other things)
> > because we didn't want that particular firmware interface to appear special
> > in any way and I'm not really sure why it is now OK to make OF look special
> > instead.
> 
> I don't think DT is special and we avoid DT specific core changes as
> much as possible. I think the difference is DT uses platform_device
> and ACPI does not.

ACPI uses platform devices too.  In fact, ACPI device objects are enumerated as
platform devices by default now.

Or do you means something else here?

> It used to be separate, but got merged together primarily to support the
> plethora of existing drivers. Anyway, that is all outside of anything in this
> series.

It explains the context of the series, so it is useful to talk about IMO.

> 
> > If it is trivial to avoid that (and you seem to be arguing that it is), why
> > do we have to do it?
> 
> Sorry, I don't follow what "that" or "it" is.

OK, so maybe I misunderstood you, sorry about that.

Thanks,
Rafael

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