Re: [PATCH v3 3/3] arm64: Early boot time stamps

[Marc Zyngier <marc.zyngier@arm.com>]

Hi Pavel,

On 26/12/2018 16:45, Pavel Tatashin wrote:
> Allow printk time stamps/sched_clock() to be available from the early
> boot.
> 
> Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> ---
>  arch/arm64/kernel/setup.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 4b0e1231625c..28126facc4ed 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -40,6 +40,7 @@
>  #include <linux/efi.h>
>  #include <linux/psci.h>
>  #include <linux/sched/task.h>
> +#include <linux/sched_clock.h>
>  #include <linux/mm.h>
>  
>  #include <asm/acpi.h>
> @@ -279,8 +280,32 @@ arch_initcall(reserve_memblock_reserved_regions);
>  
>  u64 __cpu_logical_map[NR_CPUS] = { [0 ... NR_CPUS-1] = INVALID_HWID };
>  
> +/*
> + * Get time stamps available early in boot, useful to identify boot time issues
> + * from the early boot.
> + */
> +static __init void sched_clock_early_init(void)
> +{
> +	u64 (*read_time)(void) = arch_counter_get_cntvct;
> +	u64 freq = arch_timer_get_cntfrq();
> +
> +	/*
> +	 * The arm64 boot protocol mandates that CNTFRQ_EL0 reflects
> +	 * the timer frequency. To avoid breakage on misconfigured
> +	 * systems, do not register the early sched_clock if the
> +	 * programmed value if zero. Other random values will just
> +	 * result in random output.
> +	 */
> +	if (!freq)
> +		return;
> +
> +	sched_clock_register(read_time, ARCH_TIMER_NBITS, freq);
> +}
> +
>  void __init setup_arch(char **cmdline_p)
>  {
> +	sched_clock_early_init();
> +
>  	init_mm.start_code = (unsigned long) _text;
>  	init_mm.end_code   = (unsigned long) _etext;
>  	init_mm.end_data   = (unsigned long) _edata;
> 

I still think this approach is flawed. You provide the kernel with a
potentially broken sched_clock that may jump back and forth until the
workaround kicks in. Nobody expects this.

Instead, I'd suggest you allow for a something other than local_clock()
to be used for the time stamping until a properly working sched_clock
gets registered.

This way, you'll only impact the timestamps when running on a broken system.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...