[RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

[Pantelis Antoniou]

Hi Grant,

On Nov 9, 2012, at 11:22 PM, Grant Likely wrote:

> On Fri, Nov 9, 2012 at 5:32 AM, Joel A Fernandes <agnel.joel at gmail.com> wrote:
>> Hi Pantelis,
>> 
>> I hope I'm not too late to reply as I'm traveling.
>> 
>> On Nov 6, 2012, at 5:30 AM, Pantelis Antoniou
>> <panto at antoniou-consulting.com> wrote:
>> 
>>> 
>>>> 
>>>> Joanne has purchased one of Jane's capes and packaged it into a rugged
>>>> case for data logging. As far as Joanne is concerned, the BeagleBone and
>>>> cape together are a single unit and she'd prefer a single monolithic FDT
>>>> instead of using an FDT overlay.
>>>> Option A: Using dtc, she uses the BeagleBone and cape .dts source files
>>>>       to generate a single .dtb for the entire system which is
>>>>       loaded by U-Boot. -or-
>>> 
>>> Unlikely.
>>>> Option B: Joanne uses a tool to merge the BeagleBone and cape .dtb files
>>>>       (instead of .dts files), -or-
>>> Possible but low probability.
>>> 
>>>> Option C: U-Boot loads both the base and overlay FDT files, merges them,
>>>>       and passes the resolved tree to the kernel.
>>>> 
>>> 
>>> Could be made to work. Only really required if Joanne wants the
>>> cape interface to work for u-boot too. For example if the cape has some
>>> kind of network interface that u-boot will use to boot from.
>>> 
>> 
>> I love Grant's hashing idea a lot keeping the phandle problem for
>> compile time and not requiring fixups.
>> 
>> IMO it is still a cleaner approach if u-boot does the simple tree merging for
>> all cases, and not the kernel reasons mentioned below.
> 
> I'm more than sufficiently convinced that making changes at runtime
> from userspace is pretty much required.
> 
> Also consider: it is order of magnitudes easier to modify the kernel
> than it is to change the firmware for end users.
> 

Complete agreement here.

>> (1)
>> From a development standpoint, very little or nothing will
>> have to be changed in kernel (except for scripts/dtc) considering we
>> are moving forward with hashing.
>> 
>> (2)
>> Also this discussed a while back but at some point is going to brought
>> up again-  loading of dt fragment directly from EEPROM and merging at
>> run time. If we were to implement this in kernel, we would have to add
>> cape specific EEPROM reading code, merge the tree before it is
>> unflattened and parse.
> 
> Unless it is required for boot to userspace I'm not considering
> merging before userspace starts. That's well after the tree is
> unflattened into the live form. If it is require to boot then I agree
> that is should be done in firmware. I see zero problem with having a
> beaglebone specific cape driver that knows to read the eeprom and
> request a specific configuration file. Heck, the kernel doesn't even
> need to parse the eeprom data. It can be read from userspace and
> userspace decides which overlay to provide. There's nothing stopping
> userspace from reading the eeprom, looking up the correct dts for the
> board, downloading the file from the Internet, compiling it with dtc
> and installing it.... and yes that is getting a little extreme.
> 

We're trying to come up with the method that will work best for us.

From an ease of use perspective, having a kernel driver doing the 
probing and performing the DT fragment insertion looks the best.
It's especially nice for the manufacturer, since he can make sure
that when he ships a board a single kernel image will contain everything
with no possibility of RMAs. 

For h/w prototyping, where the user is tinkering around with his
own design, the user space approach would be best.

The downloading over the internet DTS file, is a bit extreme for now :)

>> I think doing tree merging in kernel is messy
>> and we should do it in uboot considering we might have to read EEPROM for
>> this use case. Ideally reading the fragment from the EEPROM for all capes
>> and merging without worrying about version detection, Doing the merge and
>> passing the merged blob to the kernel which (kernel) works the same way
>> it does today.
>> 
>>>> It may be sufficient to solve it by making the phandle values less
>>>> volatile. Right now dtc generates phandles linearly. Generated phandles
>>>> could be overridden with explicit phandle properties, but it isn't a
>>>> fantastic solution. Perhaps generating the phandle from a hash of the
>>>> node name would be sufficient.
>>>> 
>>> 
>>> I doubt the hash method will work reliably. We only have 32 bits to work with,
>>> nothing like the SHA hashes of git.
>>> 
>> 
>> I was wondering I have worked with kernel's crypto code in the past to
>> generate 32 bit md5sums of 1000s of dataitems, from what I've seen,
>> collisions are rare and since we are talking about just a few nodes
>> that are being referenced in the base dt. I think the probability is
>> even less (ofcourse such an analysis strongly depends on dataset).
>> this method also takes away a lot of complexity with having it to do
>> runtime fixups and will help us get off the ground quickly.
> 
> It wouldn't be hard to put together a test and run it on all the .dts
> files in the kernel; generating md5 sums for all the full_name paths
> and seeing if we've got any collisions yet.
> 
>> We can also put in a collision handling mechanism if needed.
>> I think it is worthy doing a sample hash of all nodes in all dts we
>> have in a script and see for once if we have collisions and what it
>> looks like.
> 
> Collision handling is a must, but again this is all internal to dtc. I
> don't foresee any problems with it.
> 
>> Alternatively to hashing, reading David Gibson's paper I followed,
>> phandle is supposed to 'uniquely' identity node. I wonder why the node
>> name itself is not sufficient to uniquely identify.
> 
> Simply because the FDT draws upon the existing OFW bindings which use
> a 32bit value to reference other nodes.
> 
>> The code that does
>> the tree walking can then just strcmp the node name while it walks the
>> tree instead of having to find a node with a phandle number. I guess
>> the reason is phandles are small to store as data values. Another
>> approach can be to arrange the string block in alphabetical order
>> (unless it already is), and store phandle as index of the node name
>> referenced relative to the starting of the strong block. This will not
>> affect nodes in dtb being moved around since they will still have the
>> same index value. the problem being adding or removing nodes Changes
>> the index of all other nodes in the string block as well.. Hmm.
> 
> And that still doesn't help find all the phandle locations in the tree
> for doing fixups. It would need to be a table of
> nodes+properties+offsets that contain phandles for fixup to work, but
> I shy away from that approach because I think it will be fragile.
> 
> g.

Maybe, but IMHO it will work. I don't know, we're at the point where
we have to start coding and see what come out.

Regards

-- Pantelis