Linux 3.15 Kernel Gains EFI Mixed Mode Support
Landing into the latest Linux kernel code for version 3.15 is EFI mixed mode support that will allow 64-bit kernels to run from 32-bit EFI firmware.
One of the features that was merged into Git this afternoon as part of the EFI pull request for the Linux 3.15 merge window is EFI mixed mode support. The EFI mixed mode support allows 64-bit kernels to be booted from 32-bit EFI firmware as long as the boot-loader supports EFI's handover protocol.
GRUB, EFILinux, and Syslinux are among the boot-loaders that already support the EFI handover protocol so it's now possible to boot the 64-bit Linux kernel on 32-bit EFI when enabling the CONFIG_EFI_MIXED kernel option. The bootloader configurations not yet supported are using Gummiboot or booting directly from the EFI shell.
These EFI mixed mode patches initially debuted in early March by Intel's Matt Fleming. This is great news for those with devices -- particularly laptops / mobile devices -- that have only 32-bit EFI but have been wanting to run Linux. Most 32-bit Linux distributions don't ship with EFI support and up until now it hasn't been clean to get a 64-bit kernel running on the 32-bit firmware... One of the well known devices that have run into this problem is the ASUS Transformer Book T100TA that was one of the early Intel Bay Trail convertible laptops/tablets. Sadly that T100TA of mine is now dead and wouldn't even power up correctly at last attempt after doing some last-ditch attempts to make the device Linux friendly some months ago.
The x86 EFI changes for Linux 3.14 were already merged. Besides the EFI mixed mode support there's also debug code to dump EFI page-tables, runtime memory mapping is now more robust, and there's other enhancements.
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