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

Frank Rowand
2017-08-15T23:59:54Z

On 08/15/17 16:57, Frank Rowand wrote:
> On 08/15/17 15:36, Rob Herring wrote:
>> On Tue, Aug 15, 2017 at 4:15 PM, Tom Rini <trini@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+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,
>>
>> 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.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I meant to say restrict visibility of labels, so that only labels in
connector nodes would 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@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
>>>
>> .
>>
>
>