Re: [Ksummit-discuss] [TECH TOPIC] Driver model/resources, ACPI, DT, etc (sigh)

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Tuesday, May 13, 2014 02:14:42 PM Olof Johansson wrote:
> On Tue, May 06, 2014 at 03:35:41PM +0200, Arnd Bergmann wrote:
> > On Tuesday 06 May 2014 14:18:56 Rafael J. Wysocki wrote:
> > > On Monday, May 05, 2014 09:31:04 PM Benjamin Herrenschmidt wrote:
> > > > On Mon, 2014-05-05 at 13:32 +0200, Rafael J. Wysocki wrote:
> > > > > The interpretation - yes, the translation - not necessarily.  By putting
> > > > > the translation part into drivers we'll duplicate a lot of code used for
> > > > > that.  In my opinion there needs to be a layer doing the translation
> > > > > automatically.
> > > > 
> > > > I'm afraid I don't understand. How can you do the translation
> > > > automatically if you have fundamentally different representation of
> > > > device-specific concepts ?
> > > 
> > > I'm not entirely sure how fundamentally different it is to be honest, but in
> > > any case I'm talking about the situation in which the hardware is the same
> > > and the driver is the same, only the firmware interface is different.  In
> > > that case either the firmware supplies the information the driver needs, or
> > > it doesn't (and as long as the information is supplied, the format doesn't
> > > really matter, because it only is a matter of translation).
> > 
> > One example for something that is fundamentally different between an
> > embedded system using DT and any ACPI system is how a PCI(e) host bridge
> > gets treated, we are currently having long discussions about this elsewhere.
> > 
> > With ACPI (or a server running OF for that matter), you assume that the
> > PCI host is operational and all buses are fully probed and then you
> > have methods for doing hotplugging, you just look at the device that
> > are already initialized by the firmware.
> > 
> > With embedded systems, you (in general, some steps may get skipped
> > in practice) go through a lot more steps:
> > - detect the presence of the host bridge device
> > - enable clocks, voltages, phys, resets, etc.
> > - set up inbound and outbound MMIO and PIO address translation windows
> > - perform a full bus scan and assign bus and device resources
> > 
> > We probably woulnd't go through that hassle on ACPI systems. I think
> > we shouldn't attempt any translation here, and keep the code completely
> > separate, with the common parts being handled by the PCI core as we
> > do today.
> > 
> > An example about a subsystem where translate complex information
> > from both DT and ACPI into a common format is the dmaengine: We can
> > have references to dma channels in completely incompatible ways
> > but still use the same dma_request_slave_channel() interface for
> > both. The ACPI path currently is lacking a bit (e.g. there is no
> > way to specify the channel name in ACPI, so the code makes
> > assumptions that don't hold true in general), but that's fixable.
> > 
> > I assume we will see more of the second kind in the future, and this
> > would be limited to embedded systems on both ACPI and DT, since you
> > shouldn't see dma slave devices on general purpose systems with
> > either.
> > 
> > > Bottom line: I'd like to avoid modifying drivers that use the of_ interface
> > > today if possible.  Instead, I'd prefer to modify the firmware layer so that
> > > those drivers can get what they ask for regardless of what firmware interface
> > > the platform is using.
> > 
> > Looking at the most trivial example:
> > 
> > bool of_property_read_bool(const struct device_node *np,                                       				    const char *propname);
> > 
> > We should definitely be able to have an interface like this for
> > ACPI, but you don't have a 'struct device_node' pointer then.
> > We can add a new
> > 
> > bool dev_property_read_bool(const struct device *dev,
> >                             const char *propname);
> 
> > 
> > that handles both, but then you have to change every driver
> > that is already using of_property_read_bool() and that can
> > get used with ACPI.
> 
> It's actually not that easy in practice. There's not a one-to-one
> correspondence between device tree nodes and struct devices in the
> kernel. There are many bindings that use subnodes for sections of their
> information, and so on.
> 
> > An advantage of the dev_property_read_bool()
> > interface however is that it would also work for platform
> > devices that are created in platform code using
> > platform_device_register_full() or similar if we add a way
> > to define simple properties in C code. That would also simplify
> > a lot of drivers that use both (simple) platform_data and DT
> > today.
> 
> I'm not a fan of this "grand unified abstraction layer". Driver writes are
> already complaining that things are too complicated with the move to device
> tree, and we're making things even more abstracted and complicated here. People
> will need to peel back even more layers to figure out what's actually going on.
> 
> 
> For the truly trivial stuff, there are some quite simple ways to handle this:
> 
> 1. Make sure that anything that can be a named IORESOURCE_* is so. Which could
>    mean moving over GPIO and possibly clocks to that, and use those for lookup.
> 
> 2. If Rafael's arguments are to be believed, that the translation should be
>    trivial and easy to do, then we might as well reuse the structures and APIs
>    that are already in the kernel. In other words, allocate a struct
>    device_node, populate it with the properties from the ACPI structure, and
>    attach it to the struct device. Then there's no need for any churn in the
>    drivers (until they need ACPI-specific tweaks, which they will all need).

The problem with that is that we have either device_node or acpi_node.companion
under struct device and they are mutually exclusive.  So this isn't going to fly.
[And it doesn't make sense to duplicate information already available from
acpi_node.companion this way IMHO.]

Also device references are problematic here, because both interfaces use
different ways to do that (phandles vs object paths in the ACPI namespace).

So actually, what Arnd proposed seems to be the simplest approach.

> The real problem is the non-trivial stuff. There, I think there's no real
> alternative than to teach the drivers about both firmware interfaces.

I'm not sure about that.  I won't say "this isn't the case" until I actually
try things out, but it *may* be avoidable AFAICS today.

Rafael