On 08/15/17 15:36, 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). 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. > 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. > 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. >> 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 <trini@xxxxxxxxxxxx> >> --- >> 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 >> > . > -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
