If a Zephyr binary is booted on the Zynq-7000 not via JTAG download,
but via u-boot's ELF boot function instead, Zephyr will have to revert
certain changes made by u-boot in order to boot properly:
- clear the ICache/DCache enable, branch prediction enable and
strict alignment enforcement enable bits in the SCTLR register.
By default, u-boot will also set up the MMU prior to Zephyr
doing so as well, this can be avoided by changing the u-boot
build configuration. Therefore, the MMU enable bit is not changed
at this point.
- set the VBAR register to 0. U-boot moves the interrupt vector
table to a non-standard location using the VBAR register (no
change is made by u-boot for SCTLR.V, only VBAR is changed
to a non-zero memory location).
Without these changes, Zephyr will crash upon the first context
switch at latest, when SVC is invoked and u-boot's vector table
is used rather than the vectors copied to address zero by Zephyr.
In order to perform these changes before coming anwhere near the
MMU / device driver / kernel initialization stages or even the
first context switch, the z_arm_platform_init hook is used, which
is now enabled for the Zynq via the Kconfig.defconfig file.
Signed-off-by: Immo Birnbaum <Immo.Birnbaum@weidmueller.com>
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