Hi Rob, On Thu, 12 Jul 2012 15:34:17 -0500, Rob Herring <robherri...@gmail.com> wrote: > [adding u-boot list] > > On 07/12/2012 01:52 AM, Mitch Bradley wrote: > > On 7/8/2012 6:30 PM, Nicolas Pitre wrote: > >> On Fri, 6 Jul 2012, Mitch Bradley wrote: > >> > >>> On 7/6/2012 3:23 PM, David VomLehn (dvomlehn) wrote: > >>>> The kernel *must* go where it is linked, but the FDT contains only > >>>> relative > >>>> references and is thus free to go anywhere. The same is true of ramdisks, > >>>> which > >>>> are usually placed after the kernel. > >> > >> The kernel must go where it is linked *only* if you are using the > >> 'Image' output. When using 'zImage' you can put the kernel anywhere in > >> memory, or in the first 128MB of RAM if CONFIG_AUTO_ZRELADDR is used. > >> > >>> Right, but the kernel image is compressed, so after decompression it > >>> expands > >>> into the area just after it. Also, the .bss segment is in that vicinity. > >> > >> To be exact, the compressed kernel moves itself out of the region where > >> the decompressed kernel will end up before doing the decompression, but > >> only if necessary. So it is a good idea to load zImage away from the > >> decompressed kernel area to avoid this extra move and save some fraction > >> of a second on boot time. > >> > >>> There's some code in arch/arm/boot/compressed/head.S to relocate > >>> device tree blobs, but it requires CONFIG_ARM_APPENDED_DTB which > >>> is not recommended - arch/arm/Kconfig recommends using the > >>> documented boot protocol istead . > >> > >> This is in case a DTB is appended to zImage. When the DTB is detected, > >> the moving of zImage out of the decompressed area must take care of > >> moving the DTB as well. > >> > >>> Documentation/arm/Booting says > >>> to put the dtb "in a region of memory where the kernel decompressor > >>> will not overwrite it", further recommending the first 16KiB. > >>> > >>> As noted, the first 16KiB loses if the dtb is too large. And > >>> "where the kernel decompressor will not overwrite it" says what > >>> won't work, not what will. It appears that the decompressor works > >>> out its addresses dynamically, so there's no hard prescription even > >>> for what to avoid. > >> > >> A good rule of thumb is to take the size of the decompressed kernel and > >> multiply this by 3. Rounding up is also fine. So for example if your > >> arch/arm/boot/Image is 5MB, then putting anciliary data such as a > >> ramdisk or a large DTB from 16MB into RAM or above should be fine. > >> > >>> For now, I'm putting the initrd at the end of memory and the dtb > >>> below that. That seems to work, but I'm unsure whether or not > >>> I'm just "getting lucky". > >> > >> That's also perfectly fine. > > > > > > Alas, that worked for machines with 512 MiB of main memory, but failed > > on 1 GiB machines. My guess is that, when the initrd and dtb are near > > the top of a 1 GiB memory, the virtual address gets too near the top of > > the kernel's 1 GiB of virtual space (which starts at 0xc0000000), > > perhaps colliding with the VMALLOC space. > > > > Putting them just below the 128 MiB boundary seems to work. > > Interesting. I think this is also a problem on u-boot just waiting to > happen. u-boot locates itself at the end of RAM and likes to copy the > fdt and initrd to just below that. Any machine with 1G+ is going to hit > this. I avoided it because I limited u-boot to 512MB on highbank.
If I'm not mistaken, yes U-Boot loads itself as high as it can, and I don't know about the FDT, but no, U-Boot does not "like" to load initrd "just below that": it loads initrd where the boot commands tell it to, and the boot commands are written by board developers. Nothing in U-Boot forces initrd to be loaded as high as possible. That leaves the question of the FDT, though -- I'm not familiar enough with it (yet) to tell if it is always located just under U-Boot or if its placement is controllable by board commands. Amicalement, -- Albert. _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
