On Mon, Jan 22, 2018 at 2:09 AM, Frank Rowand <frowand.list@xxxxxxxxx> wrote: > Hi All, > > I've tried to create a decent distribution list, but I'm sure I've missed > someone or some important list. Please share this with anyone you think > will be affected. > > I have been playing around with some thoughts for some additions to > the devicetree FDT aka blob format. > > I would like to get the affected parties thinking about how additions to > the format could improve whichever pieces of FDT related technology you > work on or care about. In my opinion, the FDT format should change > very infrequently because of the impact on so many projects that have > to work together to create a final solution, plus the many many users > of those projects. A few things discussed before: - Adding type information Even just tagging phandles would be good. - Allow applying overlays by just appending to the blob. The need for this is somewhat gone now that libfdt can just fully apply overlays. - Move to an unflattened format so we don't have to unflatten the tree. Or to put it another way, extend FDT enough that the tree can be walked and manipulated efficiently. I think this would involve storing node offsets for all the nodes. Maybe that could be combined with storage for __symbols__? I also am not happy that labels which were purely source level convenience are now part of the ABI with overlays. > So I would like you guys to consider what I send out in a day or so, > but I don't want to preempt your creativity by laying out the details > of my proposal right now. > > I have not looked at how this would impact the devicetree compilers, > but I have hacked together a tool to convert existing blobs to the > new format. The new format is backward compatible, but transforms > the overlay related metadata into separate blocks and removes the > metadata from nodes and properties. My current proposal leaves > the fragment subtrees intact - it only transforms __symbols__, > __fixups__, and __local_fixups__. > > Some Advantages and disadvantages of my proposal are: > > Con: > - New blob version. > > Pro: > - Backward compatible. Bootloaders and kernels that can process v17 blobs > will continue to work in the same manner with a v18 blob. They will not > be able to use the new v18 features. What does libfdt do with a v18 blob? I'd assume it was written to treat new versions the same as the last known version. > > Pro: > - If a bootloader passes a blob unmodified to a kernel, then the kernel will > be able to use the new v18 features. > > Pro: > - If a bootloader modifies a blob before passing it to a kernel _and_ > downrevs the version to v17, then the kernel will continue to work in the > same manner as it works with a v17 blob. > > Con: > - If a bootloader modifies a blob before passing it to a kernel _and_ fails > to downrev the version to v17, then the kernel will most likely detect an > error and may choose to not boot. > > Pro: > - A trailing magic field allows detection of a partially composed blob, > where the blob is created by a multi-pass tool. > > Pro: > - Validation tools can annotate the blob to indicate validation fail or > validation warning. The bootloader and kernel can choose what to do > with the information. For example, a Linux kernel might choose to > taint itself if the blob is not validated or has a validation error. > > Pro: > - A significant file size reduction for a blob that contains the symbol > information needed by overlays. While nice, I think that is generally secondary. I think the feeling is that FDT is overly space efficient at the cost of walking the tree. > > Pro: > - A significant memory size reduction for a blob that contains the symbol > information needed by overlays. > > Con: > - All blobs will have a small file size increase when the symbol information > needed by overlays is not included: 125 - 140 bytes or 44 bytes less > than that if checksums not added. > > Pro: > - Simplifies implementation of Linux kernel overlay application very early > in the boot, mostly before unflattening the blob. Early boot overlay > application will be complete when the unflattenning is complete. > > This feature _might_ _eventually_ reduce of remove the need for the > bootloader to apply overlays. > > Pro: > - Simplifies implementation of the Linux kernel overlay application to > the live tree, post-boot. > > > Some data would be useful, so here it is. I compiled all of the .dts files > in the Linux kernel source tree at arch/arm/boot/dts/. The following data > is the size of the blob (and for Linux the amount of memory that the > flattened tree consumes and never frees after boot). This is for v4.15-rc7. > The following sizes are in bytes. > > 'dtb no symbols' is the existing blob as currently built in the kernel tree. > > 'delta no_sym' is the extra overhead from convering 'dtb no symbols' to the > new format. Again, this could be reduced by 44 bytes if the checksums > portion of the proposal is not implemented. > > 'delta symbols' is the existing format build in the kernel tree, but with > the "-@" flag provided to dtc, so that the __symbols__ node is populated. > > 'delta new fmt' is the 'delta symbols' blobs converted to the proposed > format. > > 'saved' is the number of bytes saved by converting to the new format, > when symbols metadata is included in the blob. > > The following table is sorted from most bytes saved, to least, with > one entry at approximately every 16 percentile: > > > row dtb no delta delta delta saved > symbols no_sym symbols new fmt > ---- ------- ------- ------- ------- ------- > 99% 90531 134 42721 15766 26955 am57xx-beagle-x15-revb1.dtb > 83% 44302 139 14582 5163 9419 imx6dl-tx6dl-comtft.dtb > 66% 26277 132 11662 4628 7034 sun6i-a31s-sinovoip-bpi-m2.dtb > 49% 21047 130 7328 2754 4574 imx53-qsb.dtb > 33% 12864 137 4305 1705 2600 kirkwood-netgear_readynas_nv+_v2.dtb > 16% 12009 128 2929 1520 1409 bcm911360k.dtb > 0% 1220 133 68 149 -81 xenvm-4.2.dtb > > Note that there are a few blobs with so little symbol metadata that the > proposed format increases the size of the blob. This can be seen in the > tail of the distribution: > > 2537 128 593 384 209 mt6589-aquaris5.dtb > 2966 131 537 355 182 mt8127-moose.dtb > 5173 132 552 372 180 spear300-evb.dtb > 2473 128 485 368 117 mt6592-evb.dtb > 2370 135 454 343 111 cx92755_equinox.dtb > 2105 128 377 304 73 mt6580-evbp1.dtb > 2357 132 346 292 54 hip01-ca9x2.dtb > 1232 137 208 201 7 alphascale-asm9260-devkit.dtb > 2571 126 201 222 -21 alpine-db.dtb > 1220 133 68 149 -81 xenvm-4.2.dtb > > I will reply to this message with the complete table of all arm blobs. > > Please discuss. :-) > > -Frank -- To unsubscribe from this list: send the line "unsubscribe devicetree-spec" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
