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U-Boot sets up the real mode interrupt handler stubs starting from address 0x1000. In most cases, the first 640K (0x00000 - 0x9ffff) system memory is reported as system RAM in E820 table to the OS. (see install_e820_map() implementation for each platform). So OS can use these memories whatever it wants. If U-Boot is in an S3 resume path, care must be taken not to corrupt these memorie otherwise OS data gets lost. Testing shows that, on Microsoft Windows 10 on Intel Baytrail its wake up vector happens to be installed at the same address 0x1000. While on Linux its wake up vector does not overlap this memory range, but after resume kernel checks low memory range per config option CONFIG_X86_RESERVE_LOW which is 64K by default to see whether a memory corruption occurs during the suspend/resume (it's harmless, but warnings are shown in the kernel dmesg logs). We cannot simply mark the these memory as reserved in E820 table because such configuration makes GRUB complain: unable to allocate real mode page. Hence we choose to back up these memories to the place where we reserved on our stack for our S3 resume work. Before jumping to OS wake up vector, we need restore the original content there. Signed-off-by: Bin Meng <bmeng.cn@gmail.com> Reviewed-by: Simon Glass <sjg@chromium.org> Tested-by: Stefan Roese <sr@denx.de> |
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