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    Doc: dt: arch_timer: discourage clock-frequency use · 4155fc07
    Mark Rutland authored
    
    
    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: default avatarMark Rutland <mark.rutland@arm.com>
    Acked-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
    Acked-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    Acked-by: default avatarOlof Johansson <olof@lixom.net>
    Acked-by: default avatarStephen Boyd <sboyd@codeaurora.org>
    Cc: Rob Herring <robh+dt@kernel.org>
    Cc: Will Deacon <will.deacon@arm.com>
    Signed-off-by: default avatarRob Herring <robh@kernel.org>
    4155fc07