sunxi: Machine id hack to prevent loading buggy sunxi-3.4 kernels

sunxi: Machine id hack to prevent loading buggy sunxi-3.4 kernels

Right now U-Boot supports the CONFIG_OLD_SUNXI_KERNEL_COMPAT option,
which makes it go out of its way in limiting the selection of PLL clock
frequencies and PMIC voltages in order not to upset outdated buggy
sunxi-3.4 kernel releases. And if the CONFIG_OLD_SUNXI_KERNEL_COMPAT
option is not set, then booting such old kernels exhibits various
failures at runtime. This is very user unfriendly, and there were
already several incidents when people wasted their time being hit
by these runtime failures and trying to debug them.

The right solution is not to add hacks and workarounds to the mainline
U-Boot, but to fix these bugs in the sunxi-3.4 kernel. And in fact,
the updated sunxi-3.4 kernels already exist. Still we need to follow
the 'Principle of Least Surprise' and U-Boot needs to ensure that
the old buggy kernels are not getting happily booted when the
CONFIG_OLD_SUNXI_KERNEL_COMPAT option is not set. And this patch
addresses this particular issue.

This patch makes U-Boot store the 'compatibility revision' number in
the top 4 bits of the machine id and pass it to the kernel. The old
buggy kernels will fail to load with a very much googlable error
message on the serial console (the "r1 = 0x100010bb" part of it):

  "Error: unrecognized/unsupported machine ID (r1 = 0x100010bb)"

This error message can be documented in the linux-sunxi wiki with
proper explanations about how to resolve this situation and where
to get the necessary bugfixes for the sunxi-3.4 kernel.

The fixed sunxi-3.4 kernels implement a revision compatibility check
and clear the top 4 bits of the machine id if everything is alright.
By accepting the machine id with the bits 31:28 set to 1, the sunxi-3.4
kernel effectively certifies that it has the PLL5 clock speed and
AXP209 DCDC3 voltage fixes applied.

It is still possible to set the CONFIG_OLD_SUNXI_KERNEL_COMPAT option
in U-Boot if the user desires to use an outdated unpatched sunxi-3.4
kernel.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>