Re: DTS parsing and kernel bus support query

[Josh Boyer <jwboyer@linux.vnet.ibm.com>]

On Tue, Dec 08, 2009 at 01:53:41PM +0000, vinayak.kale@gmail.com wrote:
> Hi
>
> I have following queries about DTS parsing and kernel bus support. I 
> would really appreciate any help.
>
> I have a PowerPC 440 core based board. We are porting Linux kernel – 2.6.31.
>
> 1) For other PowerPC 4xx based boards, what we found is devices like  
> UART/I2C/GPIO are connected through PLB->OPB buses. Accordingly we 
> specify the node structure inside a dts file. Does kernel uses the actual 
> path to locate a node eg /plb/opb/i2c? or it uses the compatible property 
> to locate a node?

It calls "of_platform_bus_probe" very early with a list of buses to walk to
create the of_platform devices.  Then the device drivers will use the
compatible properties to bind to the right of_platform devices.  If the bus
isn't walked, then those devices won't get created in the kernel.  See the
beginning portions of arch/powerpc/platforms/44x/ppc44x_simple.c.

> 2)In case of our board, instead of OPB and EBC we have buses like AI and  
> APB. So our DTS node struct for UART looks like /plb/ai/apb/serial. For  
> this do we need to add any extra support in kernel in order to recognize  
> the devices on AI and APB buses? Similar question for Flash device. It's 
> on the path /plb/ai/flash.

You would need to clearly have the AI and APB buses in the DTS file, and then
have them walked via of_platform_bus_probe.  If they are simple busses without
odd translations, it shouldn't be too hard to do.

> I need to access UART/I2C/GPIO/Flash devices over AI and APB bus. PLB is  
> connected to AI bus. AI is further connected to APB.
>
> 3) Do we need to add extra support for AI/APB bus in kernel?

Not sure.  I don't know how those busses operate.

josh