[Question] DT-Bindings for run-time configurable bus?

[Arnd Bergmann]

On Tuesday 01 December 2015 13:30:25 Masahiro Yamada wrote:
> Hello experts,
> 
> I am tackling on a new bus driver, but I am worndering
> what the DT-binding specification should be.
> 
> Here is my hardware situation:
> 
> My SoC has an external bus (it is called UniPhier System Bus).
> This is a simple parallel bus with address, data,
> chip-selects, and some other control signals.
> It supports up to 8 chip-selects.
> 
> Each CS address space can be mapped onto the CPU view,
> and it must be configured run-time via the bus controller registers.
> 
> Let's assume this situation:
> 
>  - An ethernet device is connected at the offset address 0x01f00000 of CS1
>  - A UART device is connected at the offset address 0x00200000 of CS5
> 
> 
> A quick draft of device tree would be as follows:
> 
> 
> amba {
>      compatible = "simple-bus";
>      #address-cells = <1>;
>      #size-cells = <1>;
>      ranges;
> 
>      extbus {
>            compatible = "socionext,uniphier-system-bus";
>            reg = <0x58c00000 0x400>; /* registers of bus controller */
>            #address-cells = <2>;
>            #size-cells = <1>;
> 
>            ethernet at 1,01f00000 {
>                  compatible = "smsc,lan9115";
>                  reg = <1 0x01f00000 0x1000>;
>                  interrupts = <0 48 4>
>                  phy-mode = "mii";
>            };
> 
>            uart at 5,00200000 {
>                  compatible = "ns16550a";
>                  reg = <5 0x00200000 0x20>;
>                  interrupts = <0 49 4>
>                  clock-frequency = <12288000>;
>            };
>      };
> };
> 

That looks reasonable.

> Please note "ranges" property is missing from the extbus node.
> 
> As mentioned above, the address translation from the external bus
> to the parent bus is run-time configurable.
> 
> 
> It is possible to map
>   CS1 of extbus  to  0x40000000-0x41ffffff of CPU view
>   CS5 of extbus  to  0x42000000-0x43ffffff of CPU view
> 
> It is also possible to map
>   CS1 of extbus  to  0x46000000-0x47ffffff of CPU view
>   CS5 of extbus  to  0x44000000-0x45ffffff of CPU view
> 
> There is nothing preventing us from a particular
> address mapping.
> It is completely up to the software (driver)
> to choose one mapping from another.

Ok.

> And I notice a conflict between the followings.
> 
> [1] Device Tree is a hardware description language.
> It should not describe the software configuration.
> 
> So, ranges such as
>    ranges = <1 0 0x40000000 0x02000000
>              5 0 0x42000000 0x02000000>;
> 
>    or
> 
>    ranges = <1 0 0x46000000 0x02000000
>              5 0 0x44000000 0x02000000>;
> 
> are configuration information, which should not be
> included in the device tree.
> 
> Any address mapping is OK as long as no region overlap occurs.
> No point to specify "ranges" from the device tree.
> 
> 
> [2] of_translate_address() expects "ranges" in every bus node
> 
> When we need to translate the "reg" property into the CPU-viewed address,
> we call of_translate_address().  It translates addresses,
> parsing "ranges" property when crossing buses.
> It potentially means, "ranges" properties are statically defined
> in the device tree.
> 
> 
> I have not been able to find a good way
> to solve the conflict between [1] and [2].
> 
> 
> To sum up, what I want is:
> 
>  - Let the driver to configure the address translation on run-time
>  - Once the bus is configured, I want the sub nodes to be accessed
>    from the CPU, like the other statically instantiated devices.
> 
> 
> Any comment is welcome!
> 
> 
> BTW, perhaps Marvell Mbus has a similar situation (run-time configurable)?
> (Documentation/devicetree/bindings/bus/mvebu-mbus.txt)
> I am not familiar with such SoCs, though.

Yes, this is the example I was thinking of. For mbus, we decided that
doing the full dynamic reassignment of addresses is too bothersome
for the OS, so the DT contains a "reasonable" default that the OS can
use. This is also what we do for most PCI host controllers on embedded
systems. They tend to have programmable translations, and the DT contains
the settings that are known to work and that the driver uses to set up
the I/O windows even if a lot of other settings would work just as
well.

A more traditional setup that we use on server-class machines is that
the bootloader decides what the windows should be, sets them up and
documents them in DT. The OS can still change them if it wants to,
but it doesn't actually have to worry about the fact that those are
programmable, or what registers are used for programming them.

	Arnd