Re: [PATCH 00/16] fdt: Make OF_BOARD a boolean option

[Simon Glass]

Hi François,

On Thu, 28 Oct 2021 at 02:21, François Ozog <francois.ozog@linaro.org> wrote:
>
> Hi Simon,
>
> Le jeu. 28 oct. 2021 à 04:51, Simon Glass <sjg@chromium.org> a écrit :
>>
>> Hi Ilias,
>>
>> On Tue, 26 Oct 2021 at 00:46, Ilias Apalodimas
>> <ilias.apalodimas@linaro.org> wrote:
>> >
>> > Hi Simon,
>> >
>> > A bit late to the party, sorry!
>>
>> (Did you remember the beer? I am replying to this but I don't think it
>> is all that helpful for me to reply to a lot of things on this thread,
>> since I would not be adding much to my cover letter and patches)
>>
>> >
>> > [...]
>> >
>> > > >
>> > > > I really want to see what the binary case looks like since we could 
>> > > > then
>> > > > kill off rpi_{3,3_b,4}_defconfig and I would need to see if we could
>> > > > then also do a rpi_arm32_defconfig too.
>> > > >
>> > > > I want to see less device trees in U-Boot sources, if they can come
>> > > > functionally correct from the hardware/our caller.
>> > > >
>> > > > And I'm not seeing how we make use of "U-Boot /config" if we also don't
>> > > > use the device tree from build time at run time, ignoring the device
>> > > > tree provided to us at run time by the caller.
>> > >
>> > > Firstly I should say that I find building firmware very messy and
>> > > confusing these days. Lots of things to build and it's hard to find
>> > > the instructions. It doesn't have to be that way, but if we carry on
>> > > as we are, it will continue to be messy and in five years you will
>> > > need a Ph.D and a lucky charm to boot on any modern board. My
>> > > objective here is to simplify things, bringing some consistency to the
>> > > different components. Binman was one effort there. I feel that putting
>> > > at least the U-Boot house in order, in my role as devicetree
>> > > maintainer (and as author of devicetree support in U-Boot back in
>> > > 2011), is the next step.
>> > >
>> > > If we set things up correctly and agree on the bindings, devicetree
>> > > can be the unifying configuration mechanism through the whole of
>> > > firmware (except for very early bits) and into the OS, this will set
>> > > us up very well to deal with the complexity that is coming.
>> > >
>> > > Anyway, here are the mental steps that I've gone through over the past
>> > > two months:
>> > >
>> > > Step 1: At present, some people think U-Boot is not even allowed to
>> > > have its own nodes/properties in the DT. It is an abuse of the
>> > > devicetree standard, like the /chosen node but with less history. We
>> > > should sacrifice efficiency, expedience and expandability on the altar
>> > > of 'devicetree is a hardware description'. How do we get over that
>> > > one? Wel, I just think we need to accept that U-Boot uses devicetree
>> > > for its own purposes, as well as for booting the OS. I am not saying
>> > > it always has to have those properties, but with existing features
>> > > like verified boot, SPL as well as complex firmware images where
>> > > U-Boot needs to be able to find things in the image, it is essential.
>> > > So let's just assume that we need this everywhere, since we certainly
>> > > need it in at least some places.
>> > >
>> > > (stop reading here if you disagree, because nothing below will make
>> > > any sense...you can still use U-Boot v2011.06 which doesn't have
>> > > OF_CONTROL :-)
>> >
>> > Having U-Boot keep it's *internal* config state in DTs is fine.  Adding
>> > that to the DTs that are copied over from linux isn't imho.  There are
>> > various reasons for that.  First of all syncing device trees is a huge pain
>> > and that's probably one of the main reasons our DTs are out of sync for a
>> > large number of boards.
>> > The point is this was fine in 2011 were we had SPL only,  but the reality
>> > today is completely different.  There's previous stage boot loaders (and
>> > enough cases were vendors prefer those over SPL).  If that bootloader needs
>> > to use it's own device tree for whatever reason,  imposing restrictions on
>> > it wrt to the device tree it has to include,  and require them to have
>> > knowledge of U-Boot and it's internal config mechanism makes no sense not
>> > to mention it doesn't scale at all.
>>
>> I think the solution here may be the binman image packer. It works
>> from a description of the image (i.e. is data-driver) and can collect
>> all the pieces together. The U-Boot properties (and the ones required
>> by TF-A, etc.) can be added at package time.
>>
>> If you think about it, it doesn't matter what properties are in the DT
>> that is put into the firmware image. TF-A, for example, is presumably
>> reading a devicetree from flash, so what does it care if it has some
>> U-Boot properties in it?
>
>
> I am going to change my position in all mail threads I participate.
> I was trying to make patches relevant in the future and conceptually clean. 
> That may not be the most effective position: I should just care about Linaro 
> and its members being able to implement SystemReady concepts.
>
>
> If you mandate U-Boot has nodes in the device tree passed to the OS, we can 
> put DT fragment in  the nt_fw_config section of the fip and merge it at boot 
> time. So there is a solution compatible with SystemReady.
>
> If you want to put fake, non future proof, DT sources in the dts for 
> platforms that are organized to provide the authoritative DT to U-Boot at 
> runtime, that's kind of your choice (hopefully representing the rest of 
> U-Boot community). There will be quirk code in U-Boot to redo the adaptations 
> on its non authoritative DT that the platform previous stage firmware does 
> (already saw one in the past month); as Mark said there will be issues over 
> time; and it will confuse people about the role of the DT. But I am fine with 
> it as it does not impair Linaro and its members ability to implement 
> SystemReady way of handling DT.

OK thank you. It doesn't sound like you are very on-board though.
Also, you mischaracterise my intent with in-tree devicetrees.

I would be happy enough for now if you could accept that U-Boot has
nodes/properties of its own in the devicetree. It has been a feature
of U-Boot for 10 years now.

Regards,
Simon


>
>>
>>
>> As to syncing, we have solved this using u-boot.dtsi files in U-Boot,
>> so I think this can be dealt with.
>>
>> >
>> > >
>> > > Step 2: Assume U-Boot has its own nodes/properties. How do they get
>> > > there? Well, we have u-boot.dtsi files for that (the 2016 patch
>> > > "6d427c6b1fa binman: Automatically include a U-Boot .dtsi file"), we
>> > > have binman definitions, etc. So we need a way to overlay those things
>> > > into the DT. We already support this for in-tree DTs, so IMO this is
>> > > easy. Just require every board to have an in-tree DT. It helps with
>> > > discoverability and documentation, anyway. That is this series.
>> > >
>> >
>> > Again, the board might decide for it's own reason to provide it's own DT.
>> > IMHO U-Boot must be able to cope with that and asking DTs to be included in
>> > U-Boot source is not the right way to do that,  not to mention cases were
>> > that's completely unrealistic (e.g QEMU or a board that reads the DTB from
>> > it's flash).
>>
>> I think you are at step 2. See above for my response.
>>
>> >
>> > > (I think most of us are at the beginning of step 2, unsure about it
>> > > and worried about step 3)
>> > >
>> > > Step 3: Ah, but there are flows (i.e. boards that use a particular
>> > > flow only, or boards that sometimes use a flow) which need the DT to
>> > > come from a prior stage. How to handle that? IMO that is only going to
>> > > grow as every man and his dog get into the write-a-bootloader
>> > > business.
>> >
>> > And that's exactly why we have to come up with something that scales,  
>> > without
>> > having to add a bunch of unusable DTs in U-Boot.
>>
>> In what way does this not scale? How are the DTs unusable? If there is
>> a standard binding, we should be fine.
>>
>> >
>> > > We need a way to provide the U-Boot nodes/properties in a
>> > > form that the prior stage can consume and integrate with its build
>> > > system. Is TF-A the only thing being discussed here? If so, let's just
>> > > do it. We have the u-boot.dtsi and we can use binman to put the image
>> > > together, for example. Or we can get clever and create some sort of
>> > > overlay dtb.
>> > >
>> > > Step 3a. But I don't want to do that. a) If U-Boot needs this stuff
>> > > then it will need to build it in and use two devicetrees, one internal
>> > > and one from the prior stage....well that is not very efficient and it
>> > > is going to be confusing for people to figure out what U-Boot is
>> > > actually doing. But we actually already do that in a lot of cases
>> > > where U-Boot passes a DT to the kernel which is different to the one
>> > > it uses. So perhaps we have three devicetrees? OMG.
>> >
>> > No we don't. That's a moot point. If you separate the DTs U-Boot
>> > provides the internal one and inherits one 'generic'.  Linux will be able 
>> > to use
>> > that.  So the only case were you'll need 3 DTs is if the *vendor* breaks 
>> > the
>> > DT across kernel versions,  In which case there's not much you can do to
>> > begin with and that's already a case we have to deal with.
>>
>> Linux actually doesn't care if the U-Boot properties are in the tree,
>> so long as we have proper bindings. My point here is we only need
>> either:
>>
>> a. one devicetree, shared with Linux and U-Boot (and TF-A?)
>> b. two devicetrees, one for use in firmware and one for passing to Linux
>>
>> We don't need to separate out the U-Boot properties into a second (or
>> third) devicetree. There just isn't any point.
>>
>> >
>> > > b) Well then
>> > > U-Boot can have its own small devicetree with its bits and then U-Boot
>> > > can merge the two when it starts. Again that is not very efficient. It
>> > > means that U-Boot cannot be controlled by the prior stage (e.g. to get
>> > > its public key from there or to enable/disable the console), so
>> > > unified firmware config is not possible. It will get very confusing,
>> > > particularly for debugging U-Boot. c) Some other scheme to avoid
>> > > accepting step 3...please stop!
>> > >
>> > > Step 4: Yes, but there is QEMU, which makes the devicetree up out of
>> > > whole cloth. What about that? Well, we are just going to have to deal
>> > > with that. We can easily merge in the U-Boot nodes/properties and
>> > > update the U-Boot CI scripts to do this, as needed, e.g. with
>> > > qemu-riscv64_spl. It's only one use case, although Xen might do
>> > > something similar.
>> > >
>> > > To my mind, that deals with both the build-time and run-time issues.
>> > > We have a discoverable DT in U-Boot, which should be considered the
>> > > source of truth for most boards. We can sync it with Linux
>> > > automatically with the tooling that I hope Rob Herring will come up
>> > > with. We can use an empty one where there really is no default,
>> > > although I'd argue that is making perfect an enemy of the good.
>> > >
>> > > Step 5: If we get clever and want to remove them from the U-Boot tree
>> > > and pick them up from somewhere else, we can do that with sufficient
>> > > tooling. Perhaps we should set a timeline for that? A year? Two? Six?
>> >
>> > We can start slowly migrating boards and see how that works out.
>> > We could either use 2 device trees as you proposed, or have u-boot merge
>> > the 'u-boot' DTB and the inherited DTB before DM comes up.  OTOH I'd prefer
>> > if linux gets handed a clean device tree without the u-boot internals in
>> > it, so I think 2 discrete DTs is cleaner overall.
>>
>> I know you would prefer that, but does it really matter in practice?
>> What is the objection, actually?
>>
>> As I mentioned on the call, I think the prior stage should do any
>> merging or fixing up. Trying to do that sort of thing in 'early' code
>> in U-Boot (or any other program, including Linux) is such a pain. With
>> U-Boot, for example, we don't even have any RAM available to do it
>> with half the time and it would dramatically increase the amount of
>> memory needed prior to relocation. It just isn't a very good idea to
>> try to do this in early code. It is also completely unnecessary, once
>> you get past the philosophical objections.
>>
>> If TF-A wants to be in the picture, let it deal with the implications
>> and responsibility thus incurred. TF-A has no right to tell U-Boot how
>> to handle its config. TF-A is 0.5m LOC, i.e. a lot, almost a quarter
>> of the size of U-Boot. It duplicates loads of things in there. No one
>> will even *notice* an FDT merge function, which is actually only 70
>> LOC:
>>
>> /**
>>  * overlay_apply_node - Merges a node into the base device tree
>>  * @fdt: Base Device Tree blob
>>  * @target: Node offset in the base device tree to apply the fragment to
>>  * @fdto: Device tree overlay blob
>>  * @node: Node offset in the overlay holding the changes to merge
>>  *
>>  * overlay_apply_node() merges a node into a target base device tree
>>  * node pointed.
>>  *
>>  * This is part of the final step in the device tree overlay
>>  * application process, when all the phandles have been adjusted and
>>  * resolved and you just have to merge overlay into the base device
>>  * tree.
>>  *
>>  * returns:
>>  *      0 on success
>>  *      Negative error code on failure
>>  */
>> static int overlay_apply_node(void *fdt, int target,
>>                void *fdto, int node)
>> {
>>    int property;
>>    int subnode;
>>
>>    fdt_for_each_property_offset(property, fdto, node) {
>>       const char *name;
>>       const void *prop;
>>       int prop_len;
>>       int ret;
>>
>>       prop = fdt_getprop_by_offset(fdto, property, &name,
>>                     &prop_len);
>>       if (prop_len == -FDT_ERR_NOTFOUND)
>>          return -FDT_ERR_INTERNAL;
>>       if (prop_len < 0)
>>          return prop_len;
>>
>>       ret = fdt_setprop(fdt, target, name, prop, prop_len);
>>       if (ret)
>>          return ret;
>>    }
>>
>>    fdt_for_each_subnode(subnode, fdto, node) {
>>       const char *name = fdt_get_name(fdto, subnode, NULL);
>>       int nnode;
>>       int ret;
>>
>>       nnode = fdt_add_subnode(fdt, target, name);
>>       if (nnode == -FDT_ERR_EXISTS) {
>>          nnode = fdt_subnode_offset(fdt, target, name);
>>          if (nnode == -FDT_ERR_NOTFOUND)
>>             return -FDT_ERR_INTERNAL;
>>       }
>>
>>       if (nnode < 0)
>>          return nnode;
>>
>>       ret = overlay_apply_node(fdt, nnode, fdto, subnode);
>>       if (ret)
>>          return ret;
>>    }
>>
>>    return 0;
>>
>>
>>
>> }
>>
>>
>> >
>> > Regards
>> > /Ilias
>> > >
>> > > To repeat, if we set things up correctly and agree on the bindings,
>> > > devicetree can be the unifying configuration mechanism through the
>> > > whole of firmware (except for very early bits) and into the OS. I feel
>> > > this will set us up very well to deal with the complexity that is
>> > > coming.
>> > >
>>
>> Regards,
>> Simon