On Wed, Aug 16, 2017 at 10:57 AM, Tom Rini <trini@xxxxxxxxxxxx> wrote: > 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. IMO, that starts with a connector binding. Stephen Boyd has been working towards that[1]. I guess "gpio nexus" is so obscure that all the masses clamoring for overlays and modular board support haven't noticed. Rob [1] https://www.spinics.net/lists/arm-kernel/msg600125.html -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
