[PATCH] arm: Replace CONFIG_HAS_TLS_REG with HWCAP_TLS and check for it on V6

[Russell King - ARM Linux]

On Fri, Mar 19, 2010 at 08:58:05AM -0700, Tony Lindgren wrote:
> * Russell King - ARM Linux <linux at arm.linux.org.uk> [100319 01:49]:
> > On Thu, Mar 18, 2010 at 06:35:21PM -0700, Tony Lindgren wrote:
> > > -#if defined(CONFIG_HAS_TLS_REG)
> > > -	mcr	p15, 0, r3, c13, c0, 3		@ set TLS register
> > > -#elif !defined(CONFIG_TLS_REG_EMUL)
> > > -	mov	r4, #0xffff0fff
> > > -	str	r3, [r4, #-15]			@ TLS val at 0xffff0ff0
> > > +#if !defined(CONFIG_TLS_REG_EMUL)
> > > +	ldr	r4, =elf_hwcap
> > > +	ldr	r4, [r4, #0]
> > > +	tst	r4, #HWCAP_TLS			@ hardware with TLS?
> > 
> > This is really really inefficient.  Both the second ldr and tst will stall
> > the pipeline because they need to wait for the result of the precending
> > ldr.  Can we do better by re-ordering some instructions?
> 
> Or set ifdef CONFIG_CPU_V6 and test for the cp15 id register every time..

I was suggesting that it might be worth trying to reorder the instructions
here so that we're not immediately using the result of the ldr in the
next instruction.  We have plenty of registers available here (everything
except r0-r2, r6, fp.)

> > Also, the ifndef seems incorrect - if we have TLS_REG_EMUL we seem to omit
> > all this code.
> 
> Is the current ifdef elif wrong? The current code does not seem to
> do anything if TLS_REG_EMUL is set and HAS_TLS_REG is not set.
> HAS_TLS_REG depends !TLS_REG_EMUL.

Now I look back, I don't think so.  Ignore that comment.