In article <3BD52454.218387D9@mvista.com>,
george anzinger <george@mvista.com> wrote:
>
>I am beginning to think that defining a u64 and casting, i.e.:
>
>#define jiffies (unsigned long volitial)jiffies_u64
>
>is the way to go.
..except for gcc being bad at even 64->32-bit casts like the above. It
will usually still load the full 64-bit value, and then only use the low
bits.
The efficient and sane way to do it is:
/*
* The 64-bit value is not volatile - you MUST NOT read it
* without holding the spinlock
*/
u64 jiffies_64;
/*
* Most people don't necessarily care about the full 64-bit
* value, so we can just get the "unstable" low bits without
* holding the lock. For historical reasons we also mark
* it volatile so that busy-waiting doesn't get optimized
* away in old drivers.
*/
#if defined(__LITTLE_ENDIAN) || (BITS_PER_LONG > 32)
#define jiffies (((volatile unsigned long *)&jiffies_64)[0])
#else
#define jiffies (((volatile unsigned long *)&jiffies_64)[1])
#endif
which looks ugly, but the ugliness is confined to that one place, and
none of the users will ever have to care..
Linus
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This archive was generated by hypermail 2b29 : Tue Oct 23 2001 - 21:00:38 EST