Re: [PATCH] devicetree: Enable generation of __symbols__ in all dtb files

[Tom Rini]

On Tue, Aug 15, 2017 at 05:36:11PM -0500, Rob Herring wrote:
> On Tue, Aug 15, 2017 at 4:15 PM, Tom Rini <trini@xxxxxxxxxxxx> wrote:
> > With support for stacked overlays being part of libfdt it is now
> > possible and likely that overlays which require __symbols__ will be
> > applied to the dtb files generated by the kernel.  This is done by
> > passing -@ to dtc.  This does increase the filesize (and resident memory
> > usage) based on the number of __symbol__ entries added to match the
> > contents of the dts.
> >
> > Cc: Rob Herring <robh+dt@xxxxxxxxxx>
> > Cc: Frank Rowand <frowand.list@xxxxxxxxx>
> > Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > Cc: Michal Marek <mmarek@xxxxxxxx>
> > Cc: Pantelis Antoniou <pantelis.antoniou@xxxxxxxxxxxx>
> > Cc: devicetree@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > CC: linux-kbuild@xxxxxxxxxxxxxxx
> > Signed-off-by: Tom Rini <trini@xxxxxxxxxxxx>
> > ---
> > In order for a dtb file to be useful with all types of overlays, it
> > needs to be generated with the -@ flag passed to dtc so that __symbols__
> > are generated.  This however is not free, and increases the resulting
> > dtb file by up to approximately 50% today.  In the current worst case
> > this is moving from 88KiB to 133KiB.  In talking with Frank about this,
> 
> Plus some amount for the unflattened tree in memory, too.
> 
> > he outlined 3 possible ways (with the 4th option of something else
> > entirely).
> >
> > 1. Make passing -@ to dtc be dependent upon some CONFIG symbol.
> > 2. In the kernel, if the kernel does not have overlay support, discard
> > the __symbols__ information that we've been passed.
> > 3. Have the bootloader pass in, or not, __symbols__ information.
> >
> > This patch is an attempt to implement something between the 3rd option
> > and a different, 4th option.  Frank was thinking that we might introduce
> > a new symbol to control generation of __symbol__ information for option
> > 1.  I think this gets the usage backwards and will lead to confusion
> > among users and developers.
> >
> > My proposal is that we do not want __symbols__ existence to be dependent
> > on some part of the kernel configuration for a number of reasons.
> > First, this is out of step with the rest of how dtbs are created today
> > and more importantly, thought about.  Today, all dtb content is
> > independent of CONFIG options.  If you build a dtb from a given kernel
> > tree, everyone will agree on the result.  This is part of the "contract"
> > on passing old kernels and new dtb files even.
> 
> Agree completely. I don't even like that building dtbs depends on the ARCH.
> 
> However, option 2 may still be useful. There's no point exposing what
> can't be used. Furthermore, exposing __symbols__ in /proc/device-tree
> at all may be a bad idea. We should consider if it should always be
> hidden. That would also allow storing the __symbols__ data however we
> want internally (i.e. with less memory usage). The complication is
> always kexec which I haven't thought about too much here.

A further patch to the kernel at run-time, OK.  If you give me some
crumbs I'll see if I can figure out the next steps.

> Also, perhaps we need finer grain control of __symbols__ generation.

Here I have to disagree.

> We really don't want userspace to be able to modify anything in the DT
> at any point in time. That's a big can of worms and we don't want to
> start there. The problem is labels are widely used just for
> convenience and weren't part of the ABI. With overlays that changes,
> so we either need to restrict labels usage or define another way. It
> could be as simple as defining some prefix for label names for labels
> to export.

I think there needs to be a difference noted between "here is what
policy the kernel is going to enforce about run time changes" and "here
is what the user is going to assemble a system to look like".  Again,
stemming from the part where the Linux kernel is where dts files reside
and are generated from normally.  If we have it in __symbols__, someone
can make use of it in hardware design (again, think of the SoM + carrier
+ custom) bit, I've seen so many real life products now that would be
simplified in this manner).

Thanks!

-- 
Tom

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