Re: [PATCH v8 06/19] openrisc: start CPU timer early in boot

[Stafford Horne <shorne@gmail.com>]

On Sat, Apr 30, 2022 at 02:24:33PM +0200, Jason A. Donenfeld wrote:
> In order to measure the boot process, the timer should be switched on as
> early in boot as possible. This is necessary so that by the time the
> setup code reaches random_init(), get_cycles() (by way of
> random_get_entropy()) returns non-zero, indicating that it is actually
> capable of counting. So this commit enables the timer immediately upon
> booting up. As well, the commit define the get_cycles macro, like the
> previous patches in this series, so that generic code is aware that it's
> implemented by the platform, as is done on other archs.
> 
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Jonas Bonn <jonas@southpole.se>
> Cc: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
> Cc: Stafford Horne <shorne@gmail.com>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> Changes v7->v8:
> - Rather than doing get_cycles()+1 to handle the early boot case,
>   actually start the timer early in boot. This has the huge advantage of
>   properly measuring the boot sequence timing, which could be a valuable
>   source of entropy.
> 
>  arch/openrisc/include/asm/timex.h | 1 +
>  arch/openrisc/kernel/setup.c      | 7 +++++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/arch/openrisc/include/asm/timex.h b/arch/openrisc/include/asm/timex.h
> index d52b4e536e3f..5487fa93dd9b 100644
> --- a/arch/openrisc/include/asm/timex.h
> +++ b/arch/openrisc/include/asm/timex.h
> @@ -23,6 +23,7 @@ static inline cycles_t get_cycles(void)
>  {
>  	return mfspr(SPR_TTCR);
>  }
> +#define get_cycles get_cycles
>  
>  /* This isn't really used any more */
>  #define CLOCK_TICK_RATE 1000
> diff --git a/arch/openrisc/kernel/setup.c b/arch/openrisc/kernel/setup.c
> index 0cd04d936a7a..1cb7c1770a17 100644
> --- a/arch/openrisc/kernel/setup.c
> +++ b/arch/openrisc/kernel/setup.c
> @@ -220,6 +220,13 @@ void __init setup_cpuinfo(void)
>  
>  void __init or1k_early_setup(void *fdt)
>  {
> +	/* Start the TTCR as early as possible, so that the RNG can make use of
> +	 * measurements of boot time from the earliest opportunity. Especially
> +	 * important is that the TTCR does not return zero by the time we reach
> +	 * rand_initialize().
> +	 */
> +	mtspr(SPR_TTMR, SPR_TTMR_CR);
> +
>  	if (fdt)
>  		pr_info("FDT at %p\n", fdt);
>  	else {

It looks like we cross paths on this one.

I can't think of anything better.  Also, I tested this on SMP systems and it
works fine.

Acked-by: Stafford Horne <shorne@gmail.com>