[Ksummit-2013-discuss] [ARM ATTEND] Describing complex, non-probable system topologies

[Greg KH]

On Fri, Aug 02, 2013 at 12:53:34PM +0100, Will Deacon wrote:
> Hi Greg,
> 
> On Thu, Aug 01, 2013 at 08:27:30PM +0100, Greg KH wrote:
> > On Thu, Aug 01, 2013 at 07:35:31PM +0100, Will Deacon wrote:
> > > Naturally, this would need to be described as a device-tree binding and
> > > communicate:
> > > 
> > >   - Buses which can be configured as coherent, including which devices
> > >     on those buses can be made coherent.
> > > 
> > >   - How IOMMUs sit on the bus and interact with masters on that bus (the
> > >     current one-IOMMU-driver-per-bus may not work well for the
> > >     platform_bus).
> > 
> > I've been waiting for people to finally run into this one, and realize
> > that they shouldn't be using "platform_bus" :)
> 
> But, as pointed out later in this thread, people have been doing the exact
> opposite!

I guess they were wrong :(

I know I mentioned it a few times over the years, but I've been ignoring
ARM for a long time for a variety of reasons.

> We can change the mindset by yelling, but if you're writing a new
> driver for a peripheral on an ARM SoC, platform_bus is mighty tempting
> because you get a bunch of device-tree parsing code for free (see
> drivers/of/platform.c).

I know :(

So, who do I "yell at", and what do I do to make things easier for you
from the driver-core perspective?

> What's worse is that this nice-and-easy auto-probing doesn't work for nested
> device-nodes (i.e. a bunch of device-nodes under a common parent, something
> which you might think is pretty common in a `tree') so people shy away from
> nesting as a means to group devices too.

That's sad.

> > >   - QoS and PM constraints. This isn't really in my area, but we do have
> > >     buses that have these features and expect software to control them.
> > > 
> > >   - The system topology and linkages between buses and devices.
> > 
> > The driver core handles this really well, you just have to create new
> > busses, and don't rely on the "catch-all" platform_bus.
> 
> Agreed, it's time that we started to describe these non-probable buses as
> separate bus_types, with controller logic for configuring the bus itself
> (there are weird-and-wonderful ring-based designs on the horizon which can
> require a fair amount of setup).

I've heard rumors of those for a while now, I'll believe it when I see
them :)

> > > The last point is increasingly important as various blocks of ARM system
> > > IP start to require knowledge of masters and how things like memory
> > > traffic, DVM messages, interrupts (think MSI) etc are routed between
> > > them in order to configure the system correctly. For example, interfacing
> > > a PCIe device with an SMMU requires knowledge of both the requester id
> > > associated with the device and how that maps to incoming stream ids
> > > (based off the AXI bus id) on the SMMU. Even worse, this mapping is
> > > likely generated dynamically by the host controller, which would need to
> > > know about downstream buses and their SMMUs.
> > 
> > Hm, sounds like an ACPI tree is what you need to be using :)
> > 
> > Seriously, why not use ACPI for stuff like this?  You already are
> > starting to do that for ARM-based systems, why not just make it the
> > standard?
> 
> So, like a good proportion of the ARM community, ACPI isn't something I'm
> well-versed in. Yes, it's coming, but at the same time it's not going to be
> everywhere and we need to continue to support new SoCs using device-tree.
> Whilst it might even become a de-factor standard for servers, mobile devices
> will likely continue with the bootloaders they currently have. Furthermore,
> the mobile space is really the wild-west when it comes to system topology --
> exynos SoCs tend to have one IOMMU per device, for example:
> 
> http://lists.infradead.org/pipermail/linux-arm-kernel/2013-July/181922.html
> 
> On the back of that, how does ACPI describe these relationships? It would
> certainly be a good idea to see what's already being done so we don't
> reinvent everything again for device-tree.

I don't recall the specifics of how it does this, but the spec is open
(and bad to read, sorry), and the linux-acpi mailing list is very
welcoming, so I suggest you start there if you have questions.

thanks,

greg k-h