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

[greg@kroah.com (Greg KH)]

On Thu, Aug 01, 2013 at 07:35:31PM +0100, Will Deacon wrote:
> Hello,
> 
> Whilst Linux implements a bunch of different bus types (many of which
> are in fact virtual), devices sitting on non-probable, memory mapped
> buses inside SoCs typically live on either the platform_bus or the
> amba_bus. So far, this has worked out alright; the buses haven't needed
> to be visible to software and no additional software control is really
> required from the OS. However, as I/O coherency and hardware
> virtualisation capabilities start to creep into ARM-based SoCs, Linux
> needs to know the topology of the system on which it is running.
> 
> 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" :)

>   - 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.

> 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?

thanks,

greg k-h