Message-Id: <88F93258-5DD2-40BC-A95B-6B3916FC6518@antoniou-consulting.com>
Date:	Wed, 14 Nov 2012 02:00:45 +0200
From:	Pantelis Antoniou <panto@...oniou-consulting.com>
To:	David Gibson <david@...son.dropbear.id.au>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Stephen Warren <swarren@...dotorg.org>,
	Kevin Hilman <khilman@...com>, Matt Porter <mporter@...com>,
	Koen Kooi <koen@...inion.thruhere.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Felipe Balbi <balbi@...com>,
	Deepak Saxena <dsaxena@...aro.org>,
	Scott Wood <scottwood@...escale.com>,
	Russ Dill <Russ.Dill@...com>, linux-omap@...r.kernel.org,
	devicetree-discuss@...ts.ozlabs.org
Subject: Re: [RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

Hi David,

Since this is getting to be too long-winded I went ahead a coded 
something quick and dirty.
 
On Nov 14, 2012, at 1:30 AM, David Gibson wrote:

> On Tue, Nov 13, 2012 at 10:09:28AM +0200, Pantelis Antoniou wrote:
>> Hi David,
>> 
>> On Nov 13, 2012, at 9:25 AM, David Gibson wrote:
>> 
>>> On Mon, Nov 12, 2012 at 09:52:32AM -0700, Stephen Warren wrote:
>>>> On 11/12/2012 05:10 AM, Pantelis Antoniou wrote:
>>> [snip]
>>>>> Oh yes. In fact if one was to use a single kernel image for beagleboard
>>>>> and beaglebone, for the cape to work for both, it is required for it's
>>>>> dtb to be compatible. 
>>>> 
>>>> Well, as Grant pointed out, it's not actually strictly necessary for the
>>>> .dtb to be compatible; only the .dts /need/ be compatible, and the .dtb
>>>> can be generated at run-time using dtc for example.
>>> 
>>> So, actually, I think a whole bunch of problems with phandle
>>> resolution disappear if we don't try to define an overlay .dtb format,
>>> or at least treat it only as a very shortlived object.  A more precise
>>> proposal below.  Note that this works more or less equally well with
>>> either the original overlay approach or the graft/graft-bundle
>>> proposal I made elsewhere.
>>> 
>>> 1) We annotate the base tree with some extra label information for
>>> nodes which overlays are likely to want to reference by phandle.  e.g.
>>> 
>>> 	beaglebone_pic: interrupt-controller@...XX {
>>> 		...
>>> 		phandle,symbolic-name = "beaglebone_pic";
>>> 	};
>>> 
>>> We could extend dtc to (optionally?) auto-generate those properties
>>> from its existing label syntax.  Not sure if that's a good idea or
>>> not yet.  In any case, we compile this augmented base tree to .dtb as
>>> normal and boot our kernel with it.
>>> 
>> 
>> I'm fine with that. You can auto-generate when there's a label to a node.
>> The cape dt fragment will use the label name to reference a node.
>> More details below...
>> 
>>> 2) The information for the capes/modules/whatever is
>>> distributed/packaged as .dts, never .dtb.  When userspace detects the
>>> new module (or the user explicitly tells it, if it's not probeable) it
>>> picks up the correct dts and runs it through dtc in a special mode.
>>> In this mode dtc takes the existing base tree (from /proc/device-tree,
>>> say) as well as the new dts.  In this mode, dtc allocates phandles for
>>> the new tree fragment so as not to collide with anything from the
>>> supplied base tree (as well as avoiding internal conflicts,
>>> obviously).  It also allows node references to the base tree by using
>>> those label annotations from (1) to match symbolic names to the
>>> phandle values in the base tree.
>>> 
>> 
>> Not good to rely on userspace kicking off dtc and compiling from source.
>> Some capes/expansion boards might have your root fs device, for example
>> there is an eMMC cape coming up, while networking capes are common too.
> 
> So?  dtc can go in an initramfs, just like mdadm or whatever other
> tools are there.
> 

Embedded systems aren't servers. Having an initramfs is one more thing that
can break, or require people to modify their build systems.

>> However I have a compromise. 
>> 
>> I agree that compiling from source can be an option for a runtime definable 
>> cape, but for built-in capes we could do a bit better.
>> 
>> In particular, I don't see any particular need to have a DT fragment
>> reference anything that dependent of the runtime device tree. It should
>> be possible to compile the DT fragment in kernel, against the generated
>> flattened device tree, producing a flattened DT fragment with the phandles
>> already resolved.
> 
> Um..?  Sorry, I can't parse that paragraph.
> 

No need for runtime resolution for common use cases. 

>> So the sequence could be something like this:
>> 
>> $ dtc -O dtb -o am335x-bone.dtb -b 0 am335x-bone.dts -@ ${LAST_PHANDLE_FILE}
>> $ dtc -O dtbf -R am335x-bone.dtb -o weather-cape.dtb -b 0 weather-cape.dts -@ ${LAST_PHANDLE_FILE}
>> $ dtc -O dtbf -R am335x-bone.dtb -o geiger-cape.dtb -b 0 geiger-cape.dts -@ ${LAST_PHANDLE_FILE}
>> 
>> The ${LAST_PHANDLE_FILE} can be updated with the last phandle value generated.
>> 
>> Alternatively we could have a way to statically assign a phandle range 
>> for well known capes. All the others will have to use the runtime compile
>> mechanism.
>> $ dtc -O dtb -o am335x-bone.dtb -b 0 am335x-bone.dts
>> $ dtc -O dtbf -R am335x-bone.dtb -o weather-cape.dtb -b 0 weather-cape.dts
>> $ dtc -O dtbf -R am335x-bone.dtb -o geiger-cape.dtb -b 0 geiger-cape.dts
>> 
>> With the cape dtses having a /phandle-range/ statement at the top.
>> 
>> This can work because the cape dts do not cross-reference each other, and
>> neither the boot dts references the capes.
>> 
>> That way we can use request_firmware() pretty early in the boot sequence
>> and get the DT fragment we need even before user-space starts and root fs
>> has mounted. request_firmware() can locate the fragments in the kernel
>> image before rootfs.
>> 
>> I don't know if this will cover all the cases Grant has in mind though.
>> 
>> So just to make sure I got it right, this could work for our case.
>> 
>> i2c2: i2c@...9c000 {
>> 	compatible = "ti,omap4-i2c";
>>        #address-cells = <1>;
>>        #size-cells = <0>;
>>        ti,hwmods = "i2c3";
>>        reg = <0x4819c000 0x1000>;
>>        interrupt-parent = <&intc>;
>>        interrupts = <30>;
>>        status = "disabled";
>> };
>> 
>> And in the cape definition (when compiled with the special mode I describe
>> below)
>> 
>> / {
>> 	plugin-bundle;
>> 	compatible = "cco,weather-cape";
>> 	version = <00A0>;
>> 
>> 	i2c2-graft = {
>> 		compatible = <dt,graft>;
>> 		graft-point = <&i2c2>;
>> 		
>> 		#address-cells = <1>;
>>                #size-cells = <0>;
>> 
>> 		/* Ambient light sensor */ 
>> 		tsl2550@39 { 
>> 			compatible = "tsl,tsl2550"; 
>> 			reg = <0x39>;
>> 		}; 	
>> 	};
>> };
>> 
>> DTC when compiling in the special fragment mode will pick up that
>> &i2c2 can not be resolved and lookup the phandle on the main dtb.
>> That way, even 'phandle,symbolic-name = "i2c2";' is redundant.
> 
> Well, no, because I'm assuming dtc fragment mode only has access to
> the base dtb, not the base dts, so labels will be gone from it, unless
> we add properties to preserve them especially.  That's what the
> symbolic-name thing is about.
> 

They can be easily included, as the patch shows.

>>> 3) The resulting partial .dtb for the module is highly specific to the
>>> base tree (which if the base tree was generated at runtime by firmware
>>> could even be specific to a particular boot).  But that's ok, because
>>> we just spit it into the kernel, absolute phandle values and all, then
>>> throw it away.  Next time we need the module info, we recompile it
>>> again.
>>> 
>>>> Of course, relying on .dts compatibility rather than .dtb compatibility
>>>> might negatively impact the complexity of an initrd environment if we
>>>> end up loading overlays from there...
>>> 
>>> Well, it does mean we'd need dtc in the initrd.  But dtc has no
>>> library dependencies except libc, so that really shouldn't be too
>>> bad.  In return we entirely avoid inventing a new phandle resolution
>>> protocol.
>> 
>> Not every board boots with initrd; most embedded boards don't use it
>> at all. This way we make initrd a hard requirement.
> 
> Well, not really.  You can use initramfs, or you can assemble all the
> necessary dt fragents for boot into a complete dtb included with the
> kernel.  Doing that involves pretty much the same sorts of things you
> need to do to statically configure a kernel for boot without
> initramfs.
> 


You can't assemble the dtb without runtime probing of what's out there.

> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson

Anyway, here's a small patch that shows that it is possible to do both fixups
and symbol tracking using relatively unmodified DT syntax. The only thing
new is the /plugin/; statement.

Also included two dumps of generated dtbs as well as the plugin dts.

Should be relatively simple to come up with a kernel loader and linker using
something similar like this.

Regards 

-- Pantelis



Download attachment "dtc-symbols-fixups-test.patch" of type "application/octet-stream" (8758 bytes)

Download attachment "geiger-cape.fdtdump" of type "application/octet-stream" (1914 bytes)

Download attachment "geiger-cape.dts" of type "application/octet-stream" (1342 bytes)

Download attachment "am335x-bone.fdtdump" of type "application/octet-stream" (45692 bytes)

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