Re: [PATCH] deinline sleep/delay functions

[Denis Vlasenko <vda@ilport.com.ua>]

On Thursday 30 June 2005 14:22, Arjan van de Ven wrote:
> 
> > An if(usec > 2000) { printk(..); dump_stack(); } will do.
> 
> that's runtime not compile time.
> The old situation was a compile time check which is far more powerful.

Ok I like compile checks too, it will stay.

But it won't help on accidental mdelay(some math) == mdelay(-1)
== mdelay(4 000 000 000), so we _also_ will have an if() inside
udelay(), ok?

On Thursday 30 June 2005 14:21, Russell King wrote:
> Yes.  udelay() has overflow issues - if you pass too large a number
> to udelay() you get a randomised delay because you've lost the top
> bits.

Thus [umn]delay may fail in unpredictable ways with non-const
parameter which is too big. And this is good exactly why?

I'm ok with making it fail, but _predictably_. With printk(),
trace, whatever.

> The maximum delay is dependent on the architecture implementation,
> and it depends on bogomips.  There is no one single value for it.
> Architectures have to decide this from the way that they do the
> math and the expected range of bogomips.

In example I posted these limitations are lifted. Granted these
limitations were not critical, but removing them can't do harm,
I guess?

> Please - leave asm-*/delay.h alone.

Let's see what udelay(const) will compile down to on ppc:

asm-ppc/delay.h
===============
extern unsigned long loops_per_jiffy;
extern void __delay(unsigned int loops);
...
#define __MAX_UDELAY    (226050910UL/HZ)        /* maximum udelay argument */
#define __MAX_NDELAY    (4294967295UL/HZ)       /* maximum ndelay argument */

extern __inline__ void __udelay(unsigned int x)
{
        unsigned int loops;

        __asm__("mulhwu %0,%1,%2" : "=r" (loops) :
                "r" (x), "r" (loops_per_jiffy * 226));
        __delay(loops);
}

extern __inline__ void __ndelay(unsigned int x)
{
        unsigned int loops;

        __asm__("mulhwu %0,%1,%2" : "=r" (loops) :
                "r" (x), "r" (loops_per_jiffy * 5));
        __delay(loops);
}

extern void __bad_udelay(void);         /* deliberately undefined */
extern void __bad_ndelay(void);         /* deliberately undefined */

#define udelay(n) (__builtin_constant_p(n)? \
        ((n) > __MAX_UDELAY? __bad_udelay(): __udelay((n) * (19 * HZ))) : \
        __udelay((n) * (19 * HZ)))

#define ndelay(n) (__builtin_constant_p(n)? \
        ((n) > __MAX_NDELAY? __bad_ndelay(): __ndelay((n) * HZ)) : \
        __ndelay((n) * HZ))

Thus:

	udelay(const) = loops_per_jiffy * 5; mulhwu thing; call to __delay()

While with proposed code:

	udelay(const) = call to udelay()

Which is smaller.
--
vda

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