Doc: dt: arch_timer: discourage clock-frequency use

The ARM Generic Timer (AKA the architected timer, arm_arch_timer)
features a CPU register (CNTFRQ) which firmware is intended to
initialize, and non-secure software can read to determine the frequency
of the timer. On CPUs with secure state, this register cannot be written
from non-secure states.

The firmware of early SoCs featuring the timer did not correctly
initialize CNTFRQ correctly on all CPUs, requiring the frequency to be
described in DT as a workaround. This workaround is not complete however
as it is exposed to all software in a privileged non-secure mode
(including guests running under a hypervisor). The firmware and DTs for
recent SoCs have followed the example set by these early SoCs.

This patch updates the arch timer binding documentation to make it
clearer that the use of the clock-frequency property is a poor
work-around. The MMIO generic timer binding is similarly updated, though
this is less of a concern as there is generally no need to expose the
MMIO timers to guest OSs.

Signed-off-by: Mark Rutland <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Acked-by: Marc Zyngier <[email protected]>
Acked-by: Olof Johansson <[email protected]>
Acked-by: Stephen Boyd <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Rob Herring <[email protected]>

0 files changed, 0 insertions, 0 deletions