On 08/16/17 02:42, Pantelis Antoniou wrote: > Hi Frank, > >> On Aug 16, 2017, at 02:57 , Frank Rowand <frowand.l...@gmail.com> wrote: >> >> On 08/15/17 15:36, Rob Herring wrote: >>> On Tue, Aug 15, 2017 at 4:15 PM, Tom Rini <tr...@konsulko.com> 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...@kernel.org> >>>> Cc: Frank Rowand <frowand.l...@gmail.com> >>>> Cc: Masahiro Yamada <yamada.masah...@socionext.com> >>>> Cc: Michal Marek <mma...@suse.com> >>>> Cc: Pantelis Antoniou <pantelis.anton...@konsulko.com> >>>> Cc: devicet...@vger.kernel.org >>>> Cc: linux-kernel@vger.kernel.org >>>> CC: linux-kbu...@vger.kernel.org >>>> Signed-off-by: Tom Rini <tr...@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, >>> >>> 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). >> >> Yes. I would prefer to treat the __symbols__ node as an internal >> representation of information used by the device tree subsystem. >> It is not hardware description. >> >> > > This is correct. This is internal workaround against a serialization format > omission. > >>> The complication is >>> always kexec which I haven't thought about too much here. >> >> That should not be an issue, because the device tree is exposed to kexec >> via /sys/firmware/fdt instead of /sys/firmware/devicetree/base (which >> is what /proc/device-tree links to), according to >> Documentation/ABI/testing/sysfs-firmware-ofw. So the __symbols__ >> node will be exposed to kexec. >> > > Which will have to be recreated if we throw away __symbols__ when converting > to our internal representation of labels.
Nope. /sys/firmware/fdt is the fdt that is passed to the kernel. We are not proposing any changes to that fdt by the kernel. >> >>> Also, perhaps we need finer grain control of __symbols__ generation. >>> 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. >> >> Agreed. We could also restrict labels in connector nodes to be visible. >> > > I would disagree. This is only being considered because runtime device tree > consistency checks currently is minimal (i.e. non existent). When we have > a proper DT syntax and semantic checker (soon I suppose) this restriction > will be useless and make things more complex. > > Regards > > — Pantelis > >> >>>> Second, I think this is out of step with how a lot of overlay usage will >>>> occur. My thinking is that with maximally useful overlays being >>>> available in mainline, lots of use-cases that we have today that result >>>> in a number of DTS files being included can become just overlays. This >>>> is true in terms of not only evaluation kits but also when these systems >>>> are turned into custom hardware. This is even more true for SoM based >>>> systems where a physical widget would be a SoM + carrier overlay + >>>> custom parts overlay. These cases are going to be resolved with >>>> overlays being applied outside of the kernel. >>>> >>>> Signed-off-by: Tom Rini <tr...@konsulko.com> >>>> --- >>>> drivers/of/unittest-data/Makefile | 5 ----- >>>> scripts/Makefile.lib | 3 +++ >>>> 2 files changed, 3 insertions(+), 5 deletions(-) >>>> >>>> diff --git a/drivers/of/unittest-data/Makefile >>>> b/drivers/of/unittest-data/Makefile >>>> index 6e00a9c69e58..70731cfe8900 100644 >>>> --- a/drivers/of/unittest-data/Makefile >>>> +++ b/drivers/of/unittest-data/Makefile >>>> @@ -11,8 +11,3 @@ targets += overlay_base.dtb overlay_base.dtb.S >>>> .PRECIOUS: \ >>>> $(obj)/%.dtb.S \ >>>> $(obj)/%.dtb >>>> - >>>> -# enable creation of __symbols__ node >>>> -DTC_FLAGS_overlay := -@ >>>> -DTC_FLAGS_overlay_bad_phandle := -@ >>>> -DTC_FLAGS_overlay_base := -@ >>>> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib >>>> index 58c05e5d9870..a1f4a6b29d75 100644 >>>> --- a/scripts/Makefile.lib >>>> +++ b/scripts/Makefile.lib >>>> @@ -293,6 +293,9 @@ DTC_FLAGS += -Wnode_name_chars_strict \ >>>> -Wproperty_name_chars_strict >>>> endif >>>> >>>> +# enable creation of __symbols__ node >>>> +DTC_FLAGS += -@ >>>> + >>>> DTC_FLAGS += $(DTC_FLAGS_$(basetarget)) >>>> >>>> # Generate an assembly file to wrap the output of the device tree compiler >>>> -- >>>> 1.9.1 >>>> >>> . > >
