Message-Id: <8B507360-1760-47FB-A9B6-CAEAD792E64B@konsulko.com>
Date:   Wed, 16 Aug 2017 12:42:35 +0300
From:   Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Rob Herring <robh@...nel.org>, Tom Rini <trini@...sulko.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Tero Kristo <t-kristo@...com>, Nishanth Menon <nm@...com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        Sekhar Nori <nsekhar@...com>, Rob Herring <robh+dt@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <mmarek@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-kbuild@...r.kernel.org
Subject: Re: [PATCH] devicetree: Enable generation of __symbols__ in all dtb files

Hi Frank,

> On Aug 16, 2017, at 02:57 , Frank Rowand <frowand.list@...il.com> wrote:
> 
> On 08/15/17 15:36, Rob Herring wrote:
>> On Tue, Aug 15, 2017 at 4:15 PM, Tom Rini <trini@...sulko.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@...nel.org>
>>> Cc: Frank Rowand <frowand.list@...il.com>
>>> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
>>> Cc: Michal Marek <mmarek@...e.com>
>>> Cc: Pantelis Antoniou <pantelis.antoniou@...sulko.com>
>>> Cc: devicetree@...r.kernel.org
>>> Cc: linux-kernel@...r.kernel.org
>>> CC: linux-kbuild@...r.kernel.org
>>> Signed-off-by: Tom Rini <trini@...sulko.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.

> 
>> 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 <trini@...sulko.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
>>> 
>> .

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