[alsa-devel] [PATCH] ASoC: prevent compilers from optimising pll calculation into __aeabi__uldivmod

[Barry Song]

2011/4/27 Stephen Warren <swarren at nvidia.com>:
> Barry Song wrote at Wednesday, April 27, 2011 1:28 AM:
>> The newest compiliers can optimize pll calculation in several codecs into
>> __aeabi__uldivmod which doesn't exist in kernel.
>> Then the link will fail:
>> ERROR: "__aeabi_uldivmod" [sound/soc/codecs/snd-soc-wm8974.ko] undefined!
>> ERROR: "__aeabi_uldivmod" [sound/soc/codecs/snd-soc-wm8940.ko] undefined!
>> ERROR: "__aeabi_uldivmod" [sound/soc/codecs/snd-soc-wm8510.ko] undefined!
>> This patch prevent the optimizaton by insert ASM.
>
> Aren't there compiler flags that tell gcc it isn't operating in a hosted(?)
> environment, and hence not to emit calls to these functions? If so, I
> imagine they should be added to the kernel's core (or ARM arch ) makefiles.

not real. it is right for gcc to call __aeabi_uldivmod while it think
it is good. most time, people change kernel codes to avoid this kind
of optimization:

http://comments.gmane.org/gmane.linux.kernel/965262

Another example is time.h:

{
ns += a->tv_nsec;
while(unlikely(ns >= NSEC_PER_SEC)) {
+ /* The following asm() prevents the compiler from
+ * optimising this loop into a modulo operation. */
+ asm("" : "+r"(ns));
+
ns -= NSEC_PER_SEC;
a->tv_sec++;
}

>
> I doubt __aeabi_uldivmod is the only function of this type. How was this
> solved for other such functions; does the kernel implement the AEABI
> functions, and this implementation is simply missing?
>
> But yes either way, ask on the main kernel or ARM kernel mailing list
> (ARM now CC'd)
>
> diff --git a/sound/soc/codecs/wm8510.c b/sound/soc/codecs/wm8510.c
> index db0dced..a636b15 100644
> --- a/sound/soc/codecs/wm8510.c
> +++ b/sound/soc/codecs/wm8510.c
> @@ -258,6 +258,12 @@ static void pll_factors(unsigned int target, unsigned int source)
>        Nmod = target % source;
>        Kpart = FIXED_PLL_SIZE * (long long)Nmod;
>
> +       /*
> +        * The following asm() prevents the compiler from optimising
> +        * into a standard EABI function __aeabi__uldivmod()
> +        */
> +       asm("" : "+r"(source));
> +
>        do_div(Kpart, source);
>
>        K = Kpart & 0xFFFFFFFF;
>
> --
> nvpublic
>
>