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

[Frank Rowand]

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@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.

                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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@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
>>>
>> .
>>
> 
>