Date:   Wed, 16 Aug 2017 11:57:48 -0400
From:   Tom Rini <trini@...sulko.com>
To:     Rob Herring <robh@...nel.org>
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>,
        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 mailing list <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH] devicetree: Enable generation of __symbols__ in all dtb
 files

On Wed, Aug 16, 2017 at 10:43:16AM -0500, Rob Herring wrote:
> On Tue, Aug 15, 2017 at 5:49 PM, Tom Rini <trini@...sulko.com> wrote:
> > On Tue, Aug 15, 2017 at 05:36:11PM -0500, 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). The complication is
> >> always kexec which I haven't thought about too much here.
> >
> > A further patch to the kernel at run-time, OK.  If you give me some
> > crumbs I'll see if I can figure out the next steps.
> >
> >> Also, perhaps we need finer grain control of __symbols__ generation.
> >
> > Here I have to disagree.
> >
> >> 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.
> >
> > I think there needs to be a difference noted between "here is what
> > policy the kernel is going to enforce about run time changes" and "here
> > is what the user is going to assemble a system to look like".  Again,
> > stemming from the part where the Linux kernel is where dts files reside
> > and are generated from normally.  If we have it in __symbols__, someone
> > can make use of it in hardware design (again, think of the SoM + carrier
> > + custom) bit, I've seen so many real life products now that would be
> > simplified in this manner).
> 
> I agree the usecase is an important one and one we should target, but
> I think there are other issues to solve first before we get to the
> trivial change needing to enable __symbols__. Do we have any dts files
> actually structured for the SoM + carrier use case? I guess it's done
> with includes ATM if we do. The run-time restrictions aren't just
> kernel policy. The SoM itself is going to have restrictions defined by
> its pinout. I think those need to be described in DT via a connector
> binding. I worry about leaving things wide open and having overlays
> just be a DT configuration tool with every platform structuring things
> however they want. From what I've looked at on RPi, I'm very concerned
> about having things like CMA overlays to set the CMA size. (On the
> flip side as a user, it was very nice to just apply the RPi 1-wire
> gpio overlay and things just worked.)

I believe the various SoM and EVM and hobbyist cases are all either out
of tree, or glued together (see imx6sx-udoo-neo-* in-tree, RPi or
Hummingboard or TI DRA7 EVM + LCDs) as various groups decided it
wouldn't be accepted to push in N "complete" DTS files for each valid
combination).  Moving forward with an in-kernel policy on how it should
be done, structure-wise would help with consistency and defining what's
really acceptable.

-- 
Tom

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