Peter Maydell <peter.mayd...@linaro.org> writes:
> On Tue, 8 Nov 2022 at 13:54, Peter Maydell <peter.mayd...@linaro.org> wrote: >> >> 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. > > A kernel person corrects me on the meaning of "lowmem" here -- the > kernel means by it "within the first 768MB of RAM". There is also > an implicit requirement that everything be within the bottom 32-bits > of the physical address space. > Thanks for your comment. In this view, initrd shouldn't be placed higher than 4GB ? But it seems the Linux kernel can boot when there is no memory below 4GB. I know that lowmem is needed for SWIOTLB etc. It will be used to make the 32bit IP work without IOMMU. But it seems it's not required to boot. > -- PMM -- BRs Schspa Shi
