• Lorenzo Pieralisi's avatar
    arm64: kernel: refactor the CPU suspend API for retention states · 714f5992
    Lorenzo Pieralisi authored
    CPU suspend is the standard kernel interface to be used to enter
    low-power states on ARM64 systems. Current cpu_suspend implementation
    by default assumes that all low power states are losing the CPU context,
    so the CPU registers must be saved and cleaned to DRAM upon state
    entry. Furthermore, the current cpu_suspend() implementation assumes
    that if the CPU suspend back-end method returns when called, this has
    to be considered an error regardless of the return code (which can be
    successful) since the CPU was not expected to return from a code path that
    is different from cpu_resume code path - eg returning from the reset vector.
    
    All in all this means that the current API does not cope well with low-power
    states that preserve the CPU context when entered (ie retention states),
    since first of all the context is saved for nothing on state entry for
    those states and a successful state entry can return as a normal function
    return, which is considered an error by the current CPU suspend
    implementation.
    
    This patch refactors the cpu_suspend() API so that it can be split in
    two separate functionalities. The arm64 cpu_suspend API just provides
    a wrapper around CPU suspend operation hook. A new function is
    introduced (for architecture code use only) for states that require
    context saving upon entry:
    
    __cpu_suspend(unsigned long arg, int (*fn)(unsigned long))
    
    __cpu_suspend() saves the context on function entry and calls the
    so called suspend finisher (ie fn) to complete the suspend operation.
    The finisher is not expected to return, unless it fails in which case
    the error is propagated back to the __cpu_suspend caller.
    
    The API refactoring results in the following pseudo code call sequence for a
    suspending CPU, when triggered from a kernel subsystem:
    
    /*
     * int cpu_suspend(unsigned long idx)
     * @idx: idle state index
     */
    {
    -> cpu_suspend(idx)
    	|---> CPU operations suspend hook called, if present
    		|--> if (retention_state)
    			|--> direct suspend back-end call (eg PSCI suspend)
    		     else
    			|--> __cpu_suspend(idx, &back_end_finisher);
    }
    
    By refactoring the cpu_suspend API this way, the CPU operations back-end
    has a chance to detect whether idle states require state saving or not
    and can call the required suspend operations accordingly either through
    simple function call or indirectly through __cpu_suspend() which carries out
    state saving and suspend finisher dispatching to complete idle state entry.
    Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
    Reviewed-by: default avatarHanjun Guo <hanjun.guo@linaro.org>
    Signed-off-by: default avatarLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
    Signed-off-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
    714f5992
suspend.c 4.22 KB