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@xxxxxxxxxxxx> 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@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). 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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