On Mon, Jan 22, 2018 at 12:09:18AM -0800, Frank Rowand 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. > > 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. > > 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. I have difficulty imagining any format change for which this is not the case. Altering a blob and not downrevving it to the latest version you understand is definitely a bug. > 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. I'm having trouble imagining how that can be while still being backward compatible with v17. > 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 -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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