[PATCH 4/4] at91 : implement the standby function for pm/cpuidle

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Thu, Jan 19, 2012 at 09:08:40AM +1100, Ryan Mallon wrote:
> On 18/01/12 10:40, Daniel Lezcano wrote:
> 
> > This patch groups the self-refresh on/cpu_do_idle/self-refresh off into
> > a single 'standby' function.
> > 
> > The standby routine for rm9200 has been turned into an asm routine to have
> > a better control of the self refresh and to prevent a memory access when
> > running this code.
> > 
> > Draining the write buffer is done automatically when switching for the self
> > refresh on sam9, so the instruction is added to the rm9200 only.
> > 
> > Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
> 
> 
> I don't think is an improvement. Russell's suggestion was to convert the
> pm operations into a structure, like:
> 
> struct at91_pm_ops {
> 	u32  (*sdram_selfrefresh_enable)(void);
> 	void (*sdram_selfrefresh_disable)(u32 saved_lpr);
> };

Actually no, that was my first suggestion.  I then went on to a second
suggestion, which is what Daniel has implemented.

The improvement is we've gone from this:

		asm("b 1f; .align 5; 1:");
		asm("mcr p15, 0, r0, c7, c10, 4");	/* drain write buffer */
		cpu_do_idle();
		sdram_selfrefresh_disable(saved_lpr);

which is really undefined, and may break with a later compiler, to:

	u32 lpr = at91_sys_read(AT91_SDRAMC_LPR);

	asm volatile(
		"b    1f\n\t"
		".align    5\n\t"
		"1:  mcr    p15, 0, %0, c7, c10, 4\n\t"
		"    str    %0, [%1, %2]\n\t"
		"    str    %3, [%1, %4]\n\t"
		"    mcr    p15, 0, %0, c7, c0, 4\n\t"
		"    str    %5, [%1, %2]"

which gcc guarantees it won't insert instructions randomly into the
middle of this.

So this is technically a far superior solution.

It also means that the implementation of at91_standby() can preserve
as many registers as are necessary over the standby itself - it seems
some AT91 SoCs require two registers rather than just one.