[U-Boot] [PATCH 050/126] x86: timer: Reduce timer code size in TPL on Intel CPUs

[Simon Glass]

Hi Bin,

On Fri, 11 Oct 2019 at 07:19, Bin Meng <bmeng.cn at gmail.com> wrote:
>
> Hi Simon,
>
> On Fri, Oct 11, 2019 at 1:06 AM Simon Glass <sjg at chromium.org> wrote:
> >
> > Hi Bin,
> >
> > On Sat, 5 Oct 2019 at 08:36, Bin Meng <bmeng.cn at gmail.com> wrote:
> > >
> > > Hi Simon,
> > >
> > > On Wed, Sep 25, 2019 at 10:58 PM Simon Glass <sjg at chromium.org> wrote:
> > > >
> > > > Most of the timer-calibration methods are not needed on recent Intel CPUs
> > > > and just increase code size. Add an option to use the known-good way to
> > > > get the clock frequency in TPL. Size reduction is about 700 bytes.
> > > >
> > > > Signed-off-by: Simon Glass <sjg at chromium.org>
> > > > ---
> > > >
> > > >  drivers/timer/Kconfig     | 29 +++++++++++++++++++----------
> > > >  drivers/timer/tsc_timer.c |  7 +++++--
> > > >  2 files changed, 24 insertions(+), 12 deletions(-)
> > > >
> > > > diff --git a/drivers/timer/Kconfig b/drivers/timer/Kconfig
> > > > index 5f4bc6edb67..90bc8ec7c53 100644
> > > > --- a/drivers/timer/Kconfig
> > > > +++ b/drivers/timer/Kconfig
> > > > @@ -117,16 +117,6 @@ config RENESAS_OSTM_TIMER
> > > >           Enables support for the Renesas OSTM Timer driver.
> > > >           This timer is present on Renesas RZ/A1 R7S72100 SoCs.
> > > >
> > > > -config X86_TSC_TIMER_EARLY_FREQ
> > > > -       int "x86 TSC timer frequency in MHz when used as the early timer"
> > > > -       depends on X86_TSC_TIMER
> > > > -       default 1000
> > > > -       help
> > > > -         Sets the estimated CPU frequency in MHz when TSC is used as the
> > > > -         early timer and the frequency can neither be calibrated via some
> > > > -         hardware ways, nor got from device tree at the time when device
> > > > -         tree is not available yet.
> > > > -
> > > >  config OMAP_TIMER
> > > >         bool "Omap timer support"
> > > >         depends on TIMER
> > > > @@ -174,6 +164,25 @@ config X86_TSC_TIMER
> > > >         help
> > > >           Select this to enable Time-Stamp Counter (TSC) timer for x86.
> > > >
> > > > +config X86_TSC_TIMER_EARLY_FREQ
> > > > +       int "x86 TSC timer frequency in MHz when used as the early timer"
> > > > +       depends on X86_TSC_TIMER
> > > > +       default 1000
> > > > +       help
> > > > +         Sets the estimated CPU frequency in MHz when TSC is used as the
> > > > +         early timer and the frequency can neither be calibrated via some
> > > > +         hardware ways, nor got from device tree at the time when device
> > > > +         tree is not available yet.
> > > > +
> > > > +config TPL_X86_TSC_TIMER_NATIVE
> > > > +       bool "x86 TSC timer uses native calibration"
> > > > +       depends on TPL && X86_TSC_TIMER
> > > > +       help
> > > > +         Selects native timer calibration for TPL and don't include the other
> > > > +         methods in the code. This helps to reduce code size in TPL and works
> > > > +         on fairly modern Intel chips. Code-size reductions is about 700
> > > > +         bytes.
> > > > +
> > > >  config MTK_TIMER
> > > >         bool "MediaTek timer support"
> > > >         depends on TIMER
> > > > diff --git a/drivers/timer/tsc_timer.c b/drivers/timer/tsc_timer.c
> > > > index 919caba8a14..9630036bc7f 100644
> > > > --- a/drivers/timer/tsc_timer.c
> > > > +++ b/drivers/timer/tsc_timer.c
> > > > @@ -49,8 +49,7 @@ static unsigned long native_calibrate_tsc(void)
> > > >                 return 0;
> > > >
> > > >         crystal_freq = tsc_info.ecx / 1000;
> > > > -
> > > > -       if (!crystal_freq) {
> > > > +       if (!CONFIG_IS_ENABLED(X86_TSC_TIMER_NATIVE) && !crystal_freq) {
> > > >                 switch (gd->arch.x86_model) {
> > > >                 case INTEL_FAM6_SKYLAKE_MOBILE:
> > > >                 case INTEL_FAM6_SKYLAKE_DESKTOP:
> > > > @@ -405,6 +404,10 @@ static void tsc_timer_ensure_setup(bool early)
> > > >                 if (fast_calibrate)
> > > >                         goto done;
> > > >
> > > > +               /* Reduce code size by dropping other methods */
> > > > +               if (CONFIG_IS_ENABLED(X86_TSC_TIMER_NATIVE))
> > > > +                       panic("no timer");
> > > > +
> > >
> > > I don't get it. How could this reduce the code size? I don't see any
> > > #ifdefs around the other methods we want to drop?
> >
> > The compiler sees that CONFIG_IS_ENABLED(..) is 1, and leaves out the
> > code that follows it.
>
> Why?
>
> if (1)
>     panic("no timer");
>
> then compiler does not generate any codes of the following?
>
> fast_calibrate = cpu_mhz_from_cpuid();
>
> I don't understand.
>

The panic() function is marked as noreturn, so the compiler assume it
doesn't return. You can try this if you like. It reduces the size by
700 bytes which on a 22KB image is a lot.

> Besides, I think adding some random Kconfig options to exclude some
> specific parts in one C file is a bad idea. It's unclear to me why we
> should exclude one part versus another part. I'm OK to exclude the
> whole C file for TPL/SPL though, but not part of it for size
> limitation purpose.

My understanding is the most of the code in this function is a
fallback in case an earlier method doesn't work. But on modern CPUs
the first method always works, so this is a waste of time?

Regards,
Simon