Date:   Tue, 15 Aug 2017 17:36:11 -0500
From:   Rob Herring <robh@...nel.org>
To:     Tom Rini <trini@...sulko.com>
Cc:     "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>,
        Frank Rowand <frowand.list@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <mmarek@...e.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.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

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). The complication is
always kexec which I haven't thought about too much here.

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.

> 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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