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@xxxxxxxxxxxx> 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@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, > >> > 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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