On Tue, Jul 12, 2016 at 04:24:10PM +0200, Arnd Bergmann wrote: > On Tuesday, July 12, 2016 10:18:11 AM CEST Vivek Goyal wrote: > > > > > > On Open Firmware, the DT is extracted from running firmware and copied > > > into dynamically allocated data structures. After a kexec, the runtime > > > interface to the firmware is not available, so the flattened DT format > > > was created as a way to pass the same data in a binary blob to the new > > > kernel in a format that can be read from the kernel by walking the > > > directories in /proc/device-tree/*. > > > > So this DT is available inside kernel and running kernel can still > > retrieve it and pass it to second kernel? > > The kernel only uses the flattened DT blob at boot time and converts > it into the runtime data structures (struct device_node). The original > dtb is typically overwritten later. On arm64 we deliberately preserved the DTB, so we can take that and build a new DTB from that kernel-side. > > > - we typically ship devicetree sources for embedded machines with the > > > kernel sources. As more hardware of the system gets enabled, the > > > devicetree gains extra nodes and properties that describe the hardware > > > more completely, so we need to use the latest DT blob to use all > > > the drivers > > > > > > - in some cases, kernels will fail to boot at all with an older version > > > of the DT, or fail to use the devices that were working on the > > > earlier kernel. This is usually considered a bug, but it's not rare > > > > > > - In some cases, the kernel can update its DT at runtime, and the new > > > settings are expected to be available in the new kernel too, though > > > there are cases where you actually don't want the modified contents. > > > > I am assuming that modified DT and unmodifed one both are accessible to > > kernel. And if user space can make decisions which modfied fields to use > > for new kernels and which ones not, then same can be done in kernel too? > > The unmodified DT can typically be found on disk next to the kernel binary. > The option you have is to either read it from /proc/devicetree or to > read it from from /boot/*.dtb. /proc/devicetree (aka /sys/firmware/devicetree) is a filesystem derived from the raw DTB (which is exposed at /sys/firmware/fdt). The blob that was handed to the kernel at boot time is exposed at /sys/firmware/fdt. Thanks, Mark.
