How to describe FPGA-based devices in the device tree ?

How to describe FPGA-based devices in the device tree ?

[Laurent Pinchart]

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Hi everybody,

I'm (at last) trying to move from ARCH=ppc to ARCH=powerpc. After reading 
Documentation/powerpc/booting-without-of.txt and various device trees in 
arch/powerpc/boot/dts, I still don't know how to express some devices in the 
device tree.

The target board has several devices on the processor local bus, as described 
in the following device tree fragment.

        localbus@f0010100 {
                compatible = "fsl,mpc8260-localbus",
                             "fsl,pq2-localbus";
                #address-cells = <2>;
                #size-cells = <1>;
                reg = <f0010100 60>;

                ranges = <0 0 40000000 01000000
                          2 0 f2000000 00100000
                          3 0 f3000000 00100000
                          4 0 f4000000 00100000>;

                flash@0,0 {
                        compatible = "cfi-flash";
                        reg = <0 0 01000000>;
                        bank-width = <2>;
                };

                nvram@2,0 {
                        compatible = "mtd-ram";
                        reg = <2 0 00100000>;
                        bank-width = <2>;
                };

                bcsr@3,0 {
                        device_type = "board-control";
                        reg = <3 0 00000020>;
                };

                fpga@4,0 {
                        reg = <4 0 00010000>;
                };
        };

The fourth device is a FPGA that contains several IP cores such as an 
interrupt controller and a SD/MMC host controller. If I understand things 
correctly, each IP core should have its own node in the device tree to allow 
proper binding with device drivers. As booting-without-of.txt describes the 
localbus node ranges as corresponding to a single chipselect and covering the 
entire chipselect access window, I can't have nodes for each IP core as 
children of the localbus node.

Should I put IP core nodes as children of the FPGA node ? If so, how do I map 
addresses at the FPGA level ? A ranges property in the FPGA node would let me 
map addresses in the FPGA scope to the localbus scope. However, as the 
localbus scope use the chipselect number as its first address cell and 0 as 
its second address cell, I don't see how I could translate offsets in the 
FPGA into an address at the localbus scope.

Could anyone advice me regarding how to properly describe my hardware in the 
device tree ?

Best regards,

-- 
Laurent Pinchart
CSE Semaphore Belgium

Chaussée de Bruxelles, 732A
B-1410 Waterloo
Belgium

T +32 (2) 387 42 59
F +32 (2) 387 42 75

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Re: How to describe FPGA-based devices in the device tree ?

[Scott Wood]

On Mon, Feb 18, 2008 at 01:43:52PM +0100, Laurent Pinchart wrote:
>                 bcsr@3,0 {
>                         device_type = "board-control";
>                         reg = <3 0 00000020>;
>                 };

No device_type.  Needs a compatible.

> 
>                 fpga@4,0 {
>                         reg = <4 0 00010000>;
>                 };
>         };
> 
> The fourth device is a FPGA that contains several IP cores such as an 
> interrupt controller and a SD/MMC host controller. If I understand things 
> correctly, each IP core should have its own node in the device tree to allow 
> proper binding with device drivers. 

Correct.

> As booting-without-of.txt describes the localbus node ranges as
> corresponding to a single chipselect and covering the entire chipselect
> access window, I can't have nodes for each IP core as children of the
> localbus node.

That does not follow.  The ranges entry has to cover the whole chipselect,
but there's no one-to-one correspondence between nodes and ranges entries.

There's nothing wrong with doing this:

fpga@4,0 {
	compatible = "foo,bar";
	reg = <4 0 00010000>;
};

fpga@4,10000 {
	compatible = "foo,baz";
	reg = <4 00010000 00010000>;
};

fpga@4,20000 {
	compatible = "foo,blah";
	reg = <4 00020000 00010000>;
};

> Should I put IP core nodes as children of the FPGA node ?

You could do that as well.

> If so, how do I map addresses at the FPGA level ? A ranges property in the
> FPGA node would let me map addresses in the FPGA scope to the localbus
> scope. However, as the localbus scope use the chipselect number as its
> first address cell and 0 as its second address cell,

The second cell is the address within the chipselect.  If it were always
zero, it wouldn't be there at all.

-Scott

Re: How to describe FPGA-based devices in the device tree ?

[Grant Likely]

On Feb 18, 2008 10:47 AM, Scott Wood <scottwood@freescale.com> wrote:
> On Mon, Feb 18, 2008 at 01:43:52PM +0100, Laurent Pinchart wrote:
> > Should I put IP core nodes as children of the FPGA node ?
>
> You could do that as well.

I'd recommend doing that, then your subnodes are isolated from changes
to the bus attachment (chipselect).  (really an insignificant point,
but I think it is a more logical layout).

So, something like this:
fpga@4,0 {
        #address-cells = <1>;
        #size-cells = <1>;
        ranges = <0 4 0 00100000>;
        /* breakdown of 'ranges' fields: */
        /* "0": start address of internal range */
        /* "4 0": start address of external range (chip select 4, address 0) */
        /* "00100000: size of range */

        iocore@0 {
                compatible = "foo,bar";
                reg = <0 00010000>;
        };

        iocore@10000 {
                compatible = "foo,bar";
                reg = <10000 00010000>;
        };

        iocore@20000 {
                compatible = "foo,bar";
                reg = <20000 00010000>;
        };
};

Cheers,
g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.