[U-Boot] [PATCH 08/24] armv8: add simple sdelay implementation

[Siarhei Siamashka]

On Mon, 21 Nov 2016 16:52:47 +0100
Alexander Graf <agraf at suse.de> wrote:

> On 20/11/2016 15:57, Andre Przywara wrote:
> > The sunxi DRAM setup code needs an sdelay() implementation, which
> > wasn't defined for armv8 so far.
> > Shamelessly copy the armv7 version and adjust it to work in AArch64.
> >
> > Signed-off-by: Andre Przywara <andre.przywara at arm.com>  
> 
> I don't think it hurts to write this in C - and I also doubt that 
> inlining has any negative effect.
> 
> Something along the lines of
> 
> static inline void sdelay(...) {
>    for (; loops; loops--)
>      asm volatile("");
> }
> 
> inside a header should do the trick as well and is much more readable.

Unfortunately the performance of the generated C code is very
unpredictable. Depending on the optimization settings, it may
place the counter variable in a register, or keep it on stack.

It would be much nicer to have more predictable timings for
these delays. So I like the assembly version a lot better.
Naturally, when it is implemented correctly.

> 
> 
> Alex
> 
> > ---
> >  arch/arm/cpu/armv8/cpu.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/arch/arm/cpu/armv8/cpu.c b/arch/arm/cpu/armv8/cpu.c
> > index e06c3cc..e82e9cf 100644
> > --- a/arch/arm/cpu/armv8/cpu.c
> > +++ b/arch/arm/cpu/armv8/cpu.c
> > @@ -16,6 +16,19 @@
> >  #include <asm/system.h>
> >  #include <linux/compiler.h>
> >
> > +/************************************************************
> > + * sdelay() - simple spin loop.  Will be constant time as
> > + *  its generally used in bypass conditions only.  This
> > + *  is necessary until timers are accessible.
> > + *
> > + *  not inline to increase chances its in cache when called
> > + *************************************************************/
> > +void sdelay(unsigned long loops)
> > +{
> > +	__asm__ volatile ("1:\n" "subs %0, %1, #1\n"
> > +			  "b.ne 1b":"=r" (loops):"0"(loops));
> > +}
> > +
> >  int cleanup_before_linux(void)
> >  {
> >  	/*
> >  



-- 
Best regards,
Siarhei Siamashka