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

[Tom Rini <trini@konsulko.com>]

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On Wed, Aug 16, 2017 at 10:22:03AM -0500, Rob Herring wrote:
> On Tue, Aug 15, 2017 at 7:42 PM, Tom Rini <trini@konsulko.com> wrote:
> > On Tue, Aug 15, 2017 at 04:50:40PM -0700, Frank Rowand wrote:
> >> On 08/15/17 14:15, Tom Rini 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@kernel.org>
> >> > Cc: Frank Rowand <frowand.list@gmail.com>
> >> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> >> > Cc: Michal Marek <mmarek@suse.com>
> >> > Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
> >> > Cc: devicetree@vger.kernel.org
> >> > Cc: linux-kernel@vger.kernel.org
> >> > CC: linux-kbuild@vger.kernel.org
> >> > Signed-off-by: Tom Rini <trini@konsulko.com>
> >> > ---
> >> > 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,
> >> > 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.
> >>
> >> I also was hoping that other people might have ideas for additional
> >> approaches.
> >
> > Yes, please.
> 
> A couple of other options come to mind:
> 
> "make DTC_FLAGS='-@' dtbs" should already work. So there's already a
> way to build what you want and the kernel is not setting the policy.

Not ideal since that drops out the -Wno... flags we pass in.  I don't
see off-hand why it's not appending to DTC_FLAGS, but that's a fixable
problem.

> Do like we do for the unittests and make it a per board decision:
> 
> DTC_FLAGS_my-som-board.dtb := -@
> 
> Then boards that actually need it like SoMs can turn it on.

A concern about that of mine is that we'll start to see a 'flood' of
patches growing that list at the end of arch/arm/boot/dts/Makefile.

-- 
Tom

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