On Tue, 8 Nov 2022 at 12:52, Schspa Shi <sch...@gmail.com> wrote: > Alex Bennée <alex.ben...@linaro.org> writes: > > There is a whole comment in boot.c talking about keeping initrd within > > lowmem: > > > > /* > > * We want to put the initrd far enough into RAM that when the > > * kernel is uncompressed it will not clobber the initrd. However > > * on boards without much RAM we must ensure that we still leave > > * enough room for a decent sized initrd, and on boards with large > > * amounts of RAM we must avoid the initrd being so far up in RAM > > * that it is outside lowmem and inaccessible to the kernel. > > * So for boards with less than 256MB of RAM we put the initrd > > * halfway into RAM, and for boards with 256MB of RAM or more we put > > * the initrd at 128MB. > > * We also refuse to put the initrd somewhere that will definitely > > * overlay the kernel we just loaded, though for kernel formats which > > * don't tell us their exact size (eg self-decompressing 32-bit kernels) > > * we might still make a bad choice here. > > */ > > > > I think this lowmem does not mean below 4GB. and it is to make sure > the initrd_start > memblock_start_of_DRAM for Linux address range check.
The wording of this comment pre-dates 64-bit CPU support: it is talking about the requirement in the 32-bit booting doc https://www.kernel.org/doc/Documentation/arm/Booting that says "If an initramfs is in use then, as with the dtb, it must be placed in a region of memory where the kernel decompressor will not overwrite it while also with the region which will be covered by the kernel's low-memory mapping." So it does mean "below 4GB", because you can't boot a 32-bit kernel if you don't put the kernel, initrd, etc below 4GB. thanks -- PMM
