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

[Jason A. Donenfeld]

On Sun, May 1, 2022 at 12:36 AM Stafford Horne <shorne@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx>
> > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > Cc: Jonas Bonn <jonas@xxxxxxxxxxxx>
> > Cc: Stefan Kristiansson <stefan.kristiansson@xxxxxxxxxxxxx>
> > Cc: Stafford Horne <shorne@xxxxxxxxx>
> > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> > ---
> > 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@xxxxxxxxx>

Oh okay. Do you want me to go with this or with the assembly one?

Jason